Scholars@Duke
Steven A. Cummer

Steven A. Cummer

William H. Younger Distinguished Professor of Engineering
Electrical and Computer Engineering
cummer@duke.edu
Box 90291, Durham, NC 27708-0291
3455 CIEMAS, Durham, NC 27708

Selected Publications

Observation of two-dimensional acoustic bound states in the continuum

Journal Article Communications Physics · December 1, 2024 The design of devices based on acoustic or optical fields requires the fabrication of cavities and structures capable of efficiently trapping these waves. A special type of cavity can be designed to support resonances with a theoretically infinite quality ... Full text Cite

Calculating Radio Emissions of Positive Streamer Phenomena Using 3D Simulations

Journal Article Journal of Geophysical Research: Atmospheres · October 28, 2024 We study radio emissions from positive streamers in air using 3D simulations, from which the radiated electric field is computed by solving Jefimenko’s equations. The simulations are performed at (Formula presented.) using two photoionization methods: the ... Full text Cite

Continuous Initial Breakdown Development of In-Cloud Lightning Flashes

Journal Article Journal of Geophysical Research: Atmospheres · October 28, 2024 Using a 30–250 MHz VHF interferometer, we observed a previously unreported mode of initial lightning development inside thunderclouds. This mode is defined by continuous VHF radiation spanning several km within the first few milliseconds of lightning initi ... Full text Cite

Source Altitude of Energetic In-Cloud Pulses Inside Thunderstorms and Implication for the Intrinsic Brightness of Terrestrial Gamma-Ray Flashes

Journal Article Geophysical Research Letters · October 28, 2024 Upward Terrestrial Gamma-Ray Flashes (TGFs) are mainly produced during the upward propagating negative leaders inside thunderclouds. The exact source position of TGFs, which is crucial to understanding TGF source properties, is still unclear. The link betw ... Full text Cite

Flickering gamma-ray flashes, the missing link between gamma glows and TGFs.

Journal Article Nature · October 2024 Two different hard-radiation phenomena are known to originate from thunderclouds: terrestrial gamma-ray flashes (TGFs)1 and gamma-ray glows2. Both involve an avalanche of electrons accelerated to relativistic energies but are otherwis ... Full text Cite

Highly dynamic gamma-ray emissions are common in tropical thunderclouds.

Journal Article Nature · October 2024 Thunderstorms emit fluxes of gamma rays known as gamma-ray glows1,2, sporadically observed by aircraft1,3-7, balloons8-11 and from the ground12-18. Observations report increased gamma-ray emissions by tens of per ... Full text Cite

Evidence of a New Population of Weak Terrestrial Gamma-Ray Flashes Observed From Aircraft Altitude

Journal Article Geophysical Research Letters · September 16, 2024 Terrestrial Gamma-ray Flashes (TGFs) are ten-to-hundreds of microsecond bursts of gamma-rays produced when electrons in strong electric fields in thunderclouds are accelerated to relativistic energies. Space instruments have observed TGFs with source photo ... Full text Cite

Mountaintop Gamma Ray Observations of Three Terrestrial Gamma-Ray Flashes at the Säntis Tower, Switzerland With Coincident Radio Waveforms

Journal Article Journal of Geophysical Research: Atmospheres · January 28, 2024 We report on the mountain top observation of three terrestrial gamma-ray flashes (TGFs) that occurred during the summer storm season of 2021. To our knowledge, these are the first TGFs observed in a mountaintop environment and the first published European ... Full text Cite

Imaging Step Formation in In-Cloud Lightning Initial Development With VHF Interferometry

Journal Article Geophysical Research Letters · January 16, 2024 We investigate sequential processes underlying the initial development of in-cloud lightning flashes in the form of initial breakdown pulses (IBPs) between 7.4 and 9.0 km altitudes, using a 30–250 MHz VHF interferometer. When resolved, IBPs exhibit typical ... Full text Cite

Inverse design of phononic meta-structured materials

Journal Article Materials Today · January 1, 2024 Flexible manipulation of elastic and acoustic waves through phononic meta-structured materials (PMSMs) has attracted a lot of attention in the last three decades and shows a bright future for potential applications in many fields. Conventional engineering ... Full text Cite

Multi-Layer Evolution of Acoustic-Gravity Waves and Ionospheric Disturbances Over the United States After the 2022 Hunga Tonga Volcano Eruption

Journal Article AGU Advances · December 1, 2023 The Hunga-Tonga Hunga-Ha'apai volcano underwent a series of large-magnitude eruptions that generated broad spectra of mechanical waves in the atmosphere. We investigate the spatial and temporal evolutions of fluctuations driven by atmospheric acoustic-grav ... Full text Cite

Determining a Lower Limit of Luminosity for the First Satellite Observation of a Reverse Beam Terrestrial Gamma Ray Flash Associated With a Cloud to Ground Lightning Leader

Journal Article Journal of Geophysical Research: Atmospheres · September 27, 2023 We provide an updated analysis of the gamma ray signature of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma ray Burst Monitor first reported by Pu et al. (2020, https://doi.org/10.1029/2020GL089427). A TGF produced 3 ms prior to a negative ... Full text Cite

Ultra-broadband low-frequency high-efficiency acoustic energy harvesting with metamaterial-enhanced loudspeakers

Journal Article Applied Physics Letters · August 14, 2023 Acoustic energy harvesters (AEHs) open up opportunities to recycle noise waste and generate electricity. They provide potential power solutions to a wide range of sensors. However, the practicality of AEHs has long been limited by their narrow bandwidths a ... Full text Cite

Unsupervised Clustering and Supervised Machine Learning for Lightning Classification: Application to Identifying EIPs for Ground-Based TGF Detection

Journal Article Journal of Geophysical Research: Atmospheres · May 16, 2023 We developed a framework merging unsupervised and supervised machine learning to classify lightning radio signals, and applied it to the possible detection of terrestrial gamma-ray flashes (TGFs). Recent studies have established a tight connection between ... Full text Cite

Anisotropic Metallic Microlattice Structures for Underwater Operations

Journal Article Advanced Engineering Materials · March 1, 2023 Metamaterials have offered unprecedented potentials for wave manipulations. However, their applications in underwater acoustic wave control have remained largely unexplored. This is because of the limited material choices and the lack of reliable fabricati ... Full text Cite

Ultra-sparse near-perfect sound absorbers.

Journal Article JASA express letters · March 2023 There is a trade-off between the sparseness of an absorber array and its sound absorption imposed by wave physics. Here, near-perfect absorption (99% absorption) is demonstrated when the spatial period of monopole-dipole resonators is close to one working ... Full text Cite

Self-stabilizing three-dimensional particle manipulation via a single-transducer acoustic tweezer

Journal Article Applied Physics Letters · February 27, 2023 This paper investigates the mechanism of self-stabilizing, three-dimensional Mie particle manipulation in water via an acoustic tweezer with a single transducer. A carefully designed acoustic lens is attached to the transducer to form an acoustic vortex, w ... Full text Cite

Emerging topics in nanophononics and elastic, acoustic, and mechanical metamaterials: An overview

Journal Article Nanophotonics · February 2, 2023 This broad review summarizes recent advances and "hot"research topics in nanophononics and elastic, acoustic, and mechanical metamaterials based on results presented by the authors at the EUROMECH 610 Colloquium held on April 25-27, 2022 in Benicássim, Spa ... Full text Cite

Automatic recognition of tweek atmospherics and plasma diagnostics in the lower ionosphere with the machine learning method

Journal Article Earth and Planetary Physics · January 1, 2023 Tweek atmospherics are extremely low frequency and very low frequency pulse signals with frequency dispersion characteristics that originate from lightning discharges and that propagate in the Earth–ionosphere waveguide over long distances. In this study, ... Full text Cite

Quasi-Bound States in the Continuum for Acoustic and Elastic Waves

Conference International Conference on Metamaterials, Photonic Crystals and Plasmonics · January 1, 2023 We study the localization of flexural and acoustic waves in metasurfaces by means of clusters of scatterers. We show that when the scatterers are placed regularly in the perimeter of a circumference the structure forms a resonator which quality factor grow ... Cite

Achromatic metasurfaces by dispersion customization for ultra-broadband acoustic beam engineering.

Journal Article National science review · December 2022 Metasurfaces, the ultra-thin media with extraordinary wavefront modulation ability, have shown great promise for many potential applications. However, most of the existing metasurfaces are limited by narrow-band and strong dispersive modulation, which comp ... Full text Cite

Transition in Optical and Radio Features During the Early Development of Negative Intracloud Leader

Journal Article Geophysical Research Letters · November 28, 2022 What happens when the upward in-cloud (IC) breakdown transitions into horizontally expanding extension? Optical observations of this transition in IC lightning have not previously been reported. We identify the radio and optical signatures of this transiti ... Full text Cite

General Framework of Bound States in the Continuum in an Open Acoustic Resonator

Journal Article Physical Review Applied · November 1, 2022 Bound states in the continuum (BICs) provide a viable way of achieving high-Q resonances in both photonics and acoustics. In this work, we propose a general method of constructing Friedrich-Wintgen (FW) BICs and accidental BICs in a coupled acoustic wavegu ... Full text Cite

Upward propagation of gigantic jets revealed by 3D radio and optical mapping.

Journal Article Science advances · August 2022 Occasionally, lightning will exit the top of a thunderstorm and connect to the lower edge of space, forming a gigantic jet. Here, we report on observations of a negative gigantic jet that transferred an extraordinary amount of charge between the tropospher ... Full text Cite

Quantification of Electric Fields in Fast Breakdown During Lightning Initiation From VHF-UHF Power Spectra

Journal Article Geophysical Research Letters · March 16, 2022 This work applies a new remote sensing approach to indirectly probe the electric fields in the discharge region of fast breakdown (FB) processes during lightning initiation. A VHF-UHF broadband (1–750 MHz) measurement system is used to record the radiation ... Full text Cite

Modeling Low-Frequency Radio Emissions From Terrestrial Gamma Ray Flash Sources

Journal Article Journal of Geophysical Research: Atmospheres · March 16, 2022 Relativistic runaway electron avalanches (RREAs) occur when electrons in electric fields in air reach energies above which they gain more energy from the electric field than they lose to collisions with the surrounding atmosphere. RREAs are known to happen ... Full text Cite

Nonreciprocal coupling in space-time modulated systems at exceptional points

Journal Article Physical Review B · March 1, 2022 Exceptional points (EPs) are critical points in the parameter space of non-Hermitian systems, where two or more eigenvalues and eigenvectors simultaneously coalesce. The remarkable physics and behavior of waves at these EPs have raised considerable attenti ... Full text Cite

Environmental sources of radio frequency noise: potential impacts on magnetoreception.

Journal Article Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology · January 2022 Radio frequency electromagnetic noise (RF) of anthropogenic origin has been shown to disrupt magnetic orientation behavior in some animals. Two sources of natural RF might also have the potential to disturb magnetic orientation behavior under some conditio ... Full text Cite

Three dimensional acoustic tweezers with vortex streaming

Journal Article Communications Physics · December 1, 2021 Acoustic tweezers use ultrasound for contact-free manipulation of particles from millimeter to sub-micrometer scale. Particle trapping is usually associated with either radiation forces or acoustic streaming fields. Acoustic tweezers based on single-beam f ... Full text Cite

Acoustic tweezer with complex boundary-free trapping and transport channel controlled by shadow waveguides.

Journal Article Science advances · August 2021 Acoustic tweezers use ultrasound for contact-free, bio-compatible, and precise manipulation of particles from millimeter to submicrometer scale. In microfluidics, acoustic tweezers typically use an array of sources to create standing wave patterns that can ... Full text Cite

Ground Observation of Negative Sprites Over a Tropical Thunderstorm as the Embryo of Hurricane Harvey (2017)

Journal Article Geophysical Research Letters · July 28, 2021 We report on the observation of six red sprites produced by negative cloud-to-ground (CG) lightning strokes in a tropical thunderstorm that later evolved into Hurricane Harvey (2017). Most of these sprites conformed to the morphological characteristics of ... Full text Cite

A Distinct Class of High Peak-Current Lightning Pulses Over Mountainous Terrain in Thunderstorms

Journal Article Geophysical Research Letters · July 28, 2021 Cloud-to-ground strokes, narrow bipolar events, and energetic in-cloud pulses are known classes of high peak-current lightning processes that occur in thunderstorms. Here, we report one more distinct class of high peak-current events observed exclusively o ... Full text Cite

Examining the Capacity of Hurricane Matthew (2016) in Spawning Halo/Sprite-Producible Lightning Strokes During Its Lifetime

Journal Article Journal of Geophysical Research: Atmospheres · July 27, 2021 This paper evaluated the overall capability of a hurricane in posing the electromagnetic impact on the high-altitude atmosphere. Oceanic thunderstorms are more efficient in producing negative polarity cloud-to-ground (CG) strokes with large impulse charge ... Full text Cite

VHF Radio Spectrum of a Positive Leader and Implications for Electric Fields

Journal Article Geophysical Research Letters · June 16, 2021 Electric fields associated with a developing natural lightning leader are difficult to measure. This work demonstrates a new approach to indirectly probing the electric fields in the streamer zone of a lightning leader. Using a 10–250 MHz broadband lightni ... Full text Cite

Terrestrial Gamma-Ray Flashes Can Be Detected With Radio Measurements of Energetic In-Cloud Pulses During Thunderstorms

Journal Article Geophysical Research Letters · June 16, 2021 Many of the details of how terrestrial gamma-ray flashes (TGFs) are produced, including their association with upward-propagating in-cloud lightning leader channels, remain poorly understood. Measurements of the low-frequency radio emissions associated wit ... Full text Cite

Simultaneous Observations of EIP, TGF, Elve, and Optical Lightning

Journal Article Journal of Geophysical Research: Atmospheres · June 1, 2021 On February 8, 2019, the Atmosphere-Space Interaction Monitor observed a terrestrial gamma-ray flash (TGF) and an Elve from a positive intracloud (+IC) lightning during the initial breakdown stage of a lightning flash north east of Puerto Rico. A second El ... Full text Cite

Comparison Between High-Speed Video Observation of Sprites and Broadband Sferic Measurements

Journal Article Geophysical Research Letters · May 28, 2021 High-speed video observations of two sprites with halo features were analyzed with concurrent measurements of broadband magnetic sferics. Both events were produced by positive cloud-to-ground (CG) strokes. Moreover, the halo features appeared less than 0.5 ... Full text Cite

Electrically Tunable Surface Acoustic Wave Propagation at MHz Frequencies Based on Carbon Nanotube Thin-Film Transistors

Journal Article Advanced Functional Materials · May 1, 2021 Surface acoustic waves (SAWs) that propagate on the surface of a solid at MHz frequencies are widely used in sensing, communication, and acoustic tweezers. However, their properties are difficult to be tuned electrically, and current devices suffer from co ... Full text Cite

On the Terrestrial Gamma-Ray Flashes Preceding Narrow Bipolar Events

Journal Article Geophysical Research Letters · April 28, 2021 Narrow bipolar events (NBEs) are occasionally reported to occur within a few ms after Terrestrial Gamma-ray Flashes (TGFs), while the formation mechanism remains mysterious partially due to the lack of sufficient observations. Here, nine more TGFs of this ... Full text Cite

Lightning Initiation From Fast Negative Breakdown is Led by Positive Polarity Dominated Streamers

Journal Article Geophysical Research Letters · April 28, 2021 Evidence of positive polarity dominated streamers preceding fast negative breakdown (FNB) and of simultaneous positive and negative polarity streamer development in lightning initiation is reported. Observations of lightning initiation as FNB have remained ... Full text Cite

Three-dimensional mapping of two coincident flashes - An upward positive flash triggered by the in-cloud activity of a downward negative flash

Journal Article Atmospheric Research · March 1, 2021 Lightning mapping with radio frequency location systems plays a more and more important role in understanding the mechanism of lightning discharges during thunderstorms. Here we present a comprehensive analysis of the electromagnetic field measurements and ... Full text Cite

Characterization of an underwater metamaterial made of aluminum honeycomb panels at low frequencies.

Journal Article The Journal of the Acoustical Society of America · March 2021 This paper presents a method to characterize the effective properties of inertial acoustic metamaterial unit cells for underwater operation. The method is manifested by a fast and reliable parameter retrieval procedure utilizing both numerical simulations ... Full text Cite

Space-Based Observation of a Negative Sprite With an Unusual Signature of Associated Sprite Current

Journal Article Journal of Geophysical Research: Atmospheres · February 27, 2021 We report the first observation of an unusual red sprite produced by negative cloud-to-ground (CG) lightning from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) in Colombia on August 15, 2012. The impulse charge moment change for this event ... Full text Cite

Efficient scattering-free wavefront transformation with power flow conformal bianisotropic acoustic metasurfaces

Journal Article Applied Physics Letters · February 8, 2021 Bianisotropic metasurfaces have enabled highly efficient wavefront transformation. However, a passive and lossless bianisotropic metasurface must conserve local power at every point over the metasurface, hindering its application in handling complicated wa ... Full text Cite

Radio Frequency Emissions Associated With Multi-Pulsed Terrestrial Gamma-Ray Flashes

Journal Article Journal of Geophysical Research: Space Physics · February 1, 2021 Terrestrial Gamma-ray Flashes (TGFs) with multiple pulses have been observed many times since their discovery in the 1990s. The BATSE sample of TGFs had a large fraction of multi-pulsed TGFs. It was not until later instruments, such as RHESSI, that the sam ... Full text Cite

Correction: Programmable Acoustic Metasurfaces (Advanced Functional Materials, (2019), 29, (1808489), 10.1002/adfm.201808489)

Journal Article Advanced Functional Materials · December 1, 2020 In the originally published version of the article, the “Conflict of Interests” were not declared. The relevant disclosure statement is as follows: “Conflict of Interests: T.J.H. has co-founded a start-up company, Ascent Bio-Nano Technologies Inc., to comm ... Full text Cite

Prima Facie Evidence of the Fast Impact of a Lightning Stroke on the Lower Ionosphere

Journal Article Geophysical Research Letters · November 16, 2020 Studies show that intense lightning discharges can perturb nighttime lower ionosphere through induced electromagnetic fields. We report here new evidence that every lightning stroke, regardless of intensity, has an impact on both daytime and nighttime lowe ... Full text Cite

Acoustofluidic Holography for Micro- to Nanoscale Particle Manipulation.

Journal Article ACS nano · November 2020 Acoustic-based techniques can manipulate particles in a label-free, contact-free, and biocompatible manner. However, most previous work in acoustic manipulation has been constrained by axisymmetric patterns of pressure nodes and antinodes. Acoustic hologra ... Full text Cite

Radio Interferometer Observations of an Energetic in-Cloud Pulse Reveal Large Currents Generated by Relativistic Discharges

Journal Article Journal of Geophysical Research: Atmospheres · October 27, 2020 The production mechanism for terrestrial gamma ray flashes (TGFs) is not entirely understood, and details of the corresponding lightning activity and thunderstorm charge structure have yet to be fully characterized. Here we examine sub-microsecond VHF (14– ... Full text Cite

Bianisotropic Acoustic Metasurface for Surface-Wave-Enhanced Wavefront Transformation

Journal Article Physical Review Applied · October 8, 2020 Recent advances in metasurfaces have shown that efficient wavefront transformation can be obtained by carefully designing the bianisotropic response of the metasurfaces. However, as will be shown in this paper, applying such a scheme for complicated wavefr ... Full text Cite

Sound vortex diffraction via topological charge in phase gradient metagratings.

Journal Article Science advances · October 2020 Wave fields with orbital angular momentum (OAM) have been widely investigated in metasurfaces. By engineering acoustic metasurfaces with phase gradient elements, phase twisting is commonly used to obtain acoustic OAM. However, it has limited ability to man ... Full text Cite

First Measurements of Low-Frequency Sferics Associated With Terrestrial Gamma-Ray Flashes Produced by Equatorial Thunderstorms

Journal Article Geophysical Research Letters · September 16, 2020 The low-frequency (LF) lightning sferics associated with terrestrial gamma-ray flashes (TGFs) detected by Fermi Gamma-ray Burst Monitor over equatorial thunderstorms have been recorded at a station in Melaka, Malaysia, since 2017. We examine the LF sferics ... Full text Cite

A Satellite-Detected Terrestrial Gamma Ray Flash Produced by a Cloud-to-Ground Lightning Leader

Journal Article Geophysical Research Letters · August 16, 2020 We report measurements of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma-ray Burst Monitor that was produced during a negative cloud-to-ground (CG) lightning leader. This is the first report of a downward directed TGF occurring during a CG ... Full text Cite

Switchable directional sound emission with improved field confinement based on topological insulators

Journal Article Applied Physics Letters · July 27, 2020 Directional sound emission and reception are important in many practical applications, such as acoustic imaging, communications, and structural health monitoring. In this Letter, we propose and demonstrate an acoustic antenna for realizing switchable direc ... Full text Cite

Tunable unidirectional compact acoustic amplifier via space-time modulated membranes

Journal Article Physical Review B · July 1, 2020 Space-time modulation has recently attracted considerable attention as it enables new possibilities in wave manipulation and control. Here we propose a tunable unidirectional acoustic amplifier based on a space-time modulated membrane system. The transfer ... Full text Cite

A GPU-Based Grid Traverse Algorithm for Accelerating Lightning Geolocation Process

Journal Article IEEE Transactions on Electromagnetic Compatibility · April 1, 2020 Most lightning location networks are based on real-time analytical solutions of certain simplified models, while the reality is much more complicated. In this paper, we introduce a graphics processing unit (GPU)-based parallel computing algorithm that can ... Full text Cite

Indirectly Measured Ambient Electric Fields for Lightning Initiation in Fast Breakdown Regions

Journal Article Geophysical Research Letters · February 28, 2020 We report a new approach to measure indirectly the ambient thunderstorm electric fields in fast positive breakdown regions. For a given geometry of the discharged fast breakdown region, we show that there is a minimum ambient electric field required to pro ... Full text Cite

Non-reciprocal acoustic transmission via space-time modulated membranes

Journal Article Applied Physics Letters · January 21, 2020 Non-reciprocity has recently attracted considerable attention as it enables new possibilities in wave manipulation and control. Here, we propose and analyze theoretically and numerically a waveguide system consisting of two membranes whose surface tensions ... Full text Cite

Data from: Indirectly measured ambient electric fields for lightning initiation in fast breakdown regions

Dataset · January 8, 2020 Featured Publication We report a new approach to measure indirectly the ambient thunderstorm electric fields in fast positive breakdown regions. For a given geometry of the discharged fast breakdown region, we show that there is a minimum ambient electric field required to pro ... Full text Cite

Needles and Lightning Leader Dynamics Imaged with 100–200 MHz Broadband VHF Interferometry

Journal Article Geophysical Research Letters · November 28, 2019 A new short-baseline VHF interferometer with 200 MHz bandwidth images lightning with significantly higher spatial resolution than our previous system with a 60 MHz maximum frequency. Needles (Hare et al., 2019, https://doi.org/10.1038/s41586-019-1086-6), n ... Full text Cite

Asymmetric absorption in acoustic metamirror based on surface impedance engineering

Journal Article Physical Review Applied · November 20, 2019 Asymmetric wave manipulation has attracted growing interest due to its great importance in practical applications. We design and demonstrate a planar acoustic metamirror for realizing asymmetric sound absorption with a controllable retroreflection coeffici ... Full text Cite

Transfer matrix method for the analysis of space-time-modulated media and systems

Journal Article Physical Review B · October 31, 2019 Space-time modulation adds another powerful degree of freedom to the manipulation of classical wave systems. It opens the door for complex control of wave behavior beyond the reach of stationary systems, such as nonreciprocal wave transport and realization ... Full text Cite

Printed electronic sensor array for mapping tire tread thickness profiles

Journal Article IEEE Sensors Journal · October 1, 2019 Tire tread wear is a significant vehicular safety concern; yet, monitoring tread depth (or thickness) still relies on manual detection, which is rarely done by consumers and is time-consuming for service lane technicians. In this paper, we present a fully ... Full text Cite

Gigantic jet discharges evolve stepwise through the middle atmosphere.

Journal Article Nature communications · September 2019 In 2002 it was discovered that a lightning discharge can rise out of the top of tropical thunderstorms and branch out spectacularly to the base of the ionosphere at 90 km altitude. Several dozens of such gigantic jets have been recorded or photographed sin ... Full text Cite

Advanced control of invisible cavity resonators with bianisotropic metasurfaces

Conference 2019 13th International Congress on Artificial Materials for Novel Wave Phenomena, Metamaterials 2019 · September 1, 2019 Recently, invisible cavities formed by two parallel metasurfaces have demonstrated the ability to support so-called 'driven' bound states in the continuum. The existence of these modes allows excitation of non-scattering resonant cavities with strongly enh ... Full text Cite

On negative Sprites and the Polarity Paradox

Journal Article Geophysical Research Letters · August 28, 2019 Energetic positive and negative cloud-to-ground (CG) flashes are both capable of producing sprites. Negative CGs typically outnumber the positive ones by 10 to 1. However, >99.9 % of reported sprites were found to be initiated by positive CGs—thus the pola ... Full text Cite

Nonreciprocal acoustic transmission in space-time modulated coupled resonators

Journal Article Physical Review B · August 6, 2019 Systems that break reciprocity offer new possibilities for controlling wave propagation. Here we study the scattering properties of coupled resonator systems that are under dynamic modulation. Strong linear nonreciprocal transmission is manifested in the a ... Full text Cite

Low Frequency Radio Pulses Produced by Terrestrial Gamma-Ray Flashes

Journal Article Geophysical Research Letters · June 28, 2019 Do terrestrial gamma-ray flashes (TGFs) produce their own radio signatures? To explore this question, we analyze TGF data from the Fermi Gamma-ray Burst Monitor, independent lightning geolocation data from the National Lightning Detection Network, and low- ... Full text Cite

Fabrication and experimental demonstration of a hybrid resonant acoustic gradient index metasurface at 40 kHz

Journal Article Applied Physics Letters · June 10, 2019 Over the past few years, acoustic gradient index metasurfaces (GIMs) have been actively studied for the numerous wave control capabilities that they facilitate. Previous research, however, has primarily focused on GIMs that operate in the audible frequency ... Full text Cite

Potential of GPU-based grid traverse algorithm for lightning geolocation

Conference 2019 11th Asia-Pacific International Conference on Lightning, APL 2019 · June 1, 2019 Differing from the traditional Time Difference of Arrival (TDOA) lightning location technique, Grid Traverse Algorithm (GTA) is to build up a series of grid points based on numerical discretion of lightning locations in a network region concerned and then ... Full text Cite

Reversal of transmission and reflection based on acoustic metagratings with integer parity design.

Journal Article Nature communications · May 2019 Phase gradient metagratings (PGMs) have provided unprecedented opportunities for wavefront manipulation. However, this approach suffers from fundamental limits on conversion efficiency; in some cases, higher order diffraction caused by the periodicity can ... Full text Cite

Nonreciprocal sound propagation in space-time modulated media

Journal Article Physical Review B · April 25, 2019 Realization of nonreciprocal devices, such as isolators and circulators, is of fundamental importance in microwave and photonic communication systems. This can be achieved by breaking time-reversal symmetry in the system or exploiting nonlinearity and topo ... Full text Cite

Nonreciprocal acoustic transmission in cascaded resonators via spatiotemporal modulation

Journal Article Physical Review B · April 22, 2019 Systems that break transport reciprocity have recently opened exciting possibilities for wave manipulation. Here we report nonreciprocal acoustic transmission in cascaded resonators that are modulated in space and time. An analytic approach is developed an ... Full text Cite

Lightning Initiation Processes Imaged With Very High Frequency Broadband Interferometry

Journal Article Journal of Geophysical Research: Atmospheres · March 27, 2019 Recent measurements of narrow bipolar lightning events (NBEs) by very high frequency (VHF) radio interferometer have resolved the dynamic development of this special lightning process with submicrosecond time resolution, and showed that the fast positive b ... Full text Cite

A Comparative Study of the Ray Theory Model With the Finite Difference Time Domain Model for Lightning Sferic Transmission in Earth-Ionosphere Waveguide

Journal Article Journal of Geophysical Research: Atmospheres · March 27, 2019 The lightning sferic has been shown to be a valuable radio signal for detecting perturbations of the lower ionosphere caused by the lightning activity itself and by other terrestrial and space events (Shao et al., 2012, https://doi.org/10.1038/ngeo1668). A ... Full text Cite

Broadband high-index prism for asymmetric acoustic transmission

Journal Article Applied Physics Letters · March 25, 2019 Narrow bandwidth and wavefront distortion are two shortcomings of some existing asymmetric acoustic transmission devices. In this letter, we propose a high-index prism for realizing broadband asymmetric acoustic transmission with uniform plane wave outputs ... Full text Cite

Programmable Acoustic Metasurfaces.

Journal Article Advanced functional materials · March 2019 Metasurfaces open up unprecedented potential for wave engineering using subwavelength sheets. However, a severe limitation of current acoustic metasurfaces is their poor reconfigurability to achieve distinct functions on demand. Here a programmable acousti ... Full text Cite

Highly Efficient Generation of Angular Momentum with Cylindrical Bianisotropic Metasurfaces

Journal Article Physical Review Applied · February 6, 2019 Recent advances in metasurfaces have shown the importance of controlling the bianisotropic response of the constituent meta-atoms for maximum efficiency wave-front transformation. By carefully designing the bianisotropic response of the metasurface, full c ... Full text Cite

A comparative study on the lightning sferics associated with terrestrial gamma-ray flashes observed in Americas and Asia

Journal Article Journal of Atmospheric and Solar-Terrestrial Physics · February 1, 2019 We examine the broadband (<1 Hz to 400 kHz) electromagnetic lightning signals associated with Terrestrial Gamma-ray Flashes (TGFs) detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Gamma-ray Burst Monitor (GBM) on the Fe ... Full text Cite

Power flow-conformal metamirrors for engineering wave reflections.

Journal Article Science advances · February 2019 Recently, the complexity behind manipulations of reflected fields by metasurfaces has been addressed, showing that, even in the simplest scenarios, nonlocal response and excitation of auxiliary evanescent fields are required for perfect field control. In t ... Full text Cite

On the High-Energy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes

Journal Article Journal of Geophysical Research: Atmospheres · January 1, 2019 Terrestrial gamma ray flashes (TGFs) are very short bursts of gamma radiation associated to thunderstorm activity and are the manifestation of the highest-energy natural particle acceleration phenomena occurring on Earth. Photon energies up to several tens ... Full text Cite

Synthetic exceptional points and unidirectional zero reflection in non-Hermitian acoustic systems

Journal Article Physical Review Materials · December 17, 2018 Exceptional points (EPs) are responsible for a number of interesting phenomena in non-Hermitian wave systems. Here we show, both theoretically and experimentally, that EPs can be synthesized in non-Hermitian acoustic systems with unbalanced loss factors in ... Full text Cite

Observations of Red Sprites Above Hurricane Matthew

Journal Article Geophysical Research Letters · December 16, 2018 More than three dozen red sprites were captured above Hurricane Matthew on the nights of 1 and 2 October 2016 as it passed to the north of Venezuela after undergoing rapid intensification. Analyses using broadband magnetic fields indicate that all of the s ... Full text Cite

Bianisotropic Acoustic Metasurface for Highly Efficient Wavefront Transformation

Conference 2018 12th International Congress on Artificial Materials for Novel Wave Phenomena, METAMATERIALS 2018 · November 13, 2018 A fundamental limit for GSL-based metasurfaces is their power efficiency, especially at large deflection angles. Here we designed and fabricated the bi-anisotropic cells for wavefront transformation acoustic metasurface that overcomes this limit, allowing ... Full text Cite

High-effcient Acoustic Anomalous Reflector Based on Power-flow Conformal Metamirror

Conference 2018 12th International Congress on Artificial Materials for Novel Wave Phenomena, METAMATERIALS 2018 · November 13, 2018 In the last years, metasurfaces have attracted much attention due to the capability of tailoring the response in a sub-wavelength scale and their compact implementations. In the particular case of reflective metasurfaces, also called metamirrors, anomalous ... Full text Cite

Acoustic Imaging with Metamaterial Luneburg Lenses.

Journal Article Scientific reports · November 2018 The Luneburg lens is a spherically symmetrical gradient refractive index (GRIN) device with unique imaging properties. Its wide field-of-view (FoV) and minimal aberration have lead it to be successfully applied in microwave antennas. However, only limited ... Full text Cite

A Study of Consecutive Terrestrial Gamma-ray Flashes Using the Gamma-ray Burst Monitor

Journal Article Journal of Geophysical Research: Space Physics · November 1, 2018 The Fermi Gamma-ray Burst Monitor (GBM) has detected terrestrial gamma-ray flash (TGF) pulses and TGF pairs with separations from submilliseconds to several minutes. Enhancements in the TGF rate are reobserved on successive orbits. We report on the distrib ... Full text Cite

Compact acoustic retroreflector based on a mirrored Luneburg lens

Journal Article Physical Review Materials · October 18, 2018 We propose and demonstrate a compact acoustic retroreflector that reroutes incident signals back toward the source with minimal scattering. Gradient refractive index acoustic metamaterials, based on Archimedean spiral structures, are designed to fulfill th ... Full text Cite

On the Causative Strokes of Halos Observed by ISUAL in the Vicinity of North America

Journal Article Geophysical Research Letters · October 16, 2018 The analysis of impulse charge moment change (iCMC) for the parent lightning strokes of 497 halos observed by the Imager of Sprites and Upper Atmospheric Lightning near North America in 2004–2015 indicates that the majority were produced by negative cloud- ... Full text Cite

Energetic Radio Emissions and Possible Terrestrial Gamma-Ray Flashes Associated With Downward Propagating Negative Leaders

Journal Article Geophysical Research Letters · October 16, 2018 A study of negative polarity energetic in-cloud pulses (−EIPs) during thunderstorms reveals that −EIPs occur during two distinct stages of downward negative leader development. The first group is observed at the beginning of downward negative leaders, with ... Full text Cite

Measurement of continuing charge transfer in rocket-triggered lightning with low-frequency magnetic sensor at close range

Journal Article Journal of Atmospheric and Solar-Terrestrial Physics · October 1, 2018 Based on the magnetic fields recorded with a compact low-frequency (LF) magnetic sensor deployed at 78 m distance from the channel base, we reconstruct the time-resolved current waveform for the continuously discharging processes in classical rocket-and-wi ... Full text Cite

Roadmap on transformation optics

Journal Article Journal of Optics (United Kingdom) · June 1, 2018 Transformation optics asks, using Maxwell's equations, what kind of electromagnetic medium recreates some smooth deformation of space? The guiding principle is Einstein's principle of covariance: that any physical theory must take the same form in any coor ... Full text Cite

A Terrestrial Gamma-Ray Flash inside the Eyewall of Hurricane Patricia

Journal Article Journal of Geophysical Research: Atmospheres · May 27, 2018 On 23 October 2015 at ~1732 UTC, the Airborne Detector for Energetic Lightning Emissions (ADELE) flew through the eyewall of Hurricane Patricia aboard National Oceanic and Atmospheric Administration's Hurricane Hunter WP-3D Orion, observing the first terre ... Full text Cite

Harnessing Multiple Internal Reflections to Design Highly Absorptive Acoustic Metasurfaces

Journal Article Physical Review Applied · May 8, 2018 The rapid development of metasurfaces has enabled numerous intriguing applications with acoustically thin sheets. Here we report the theory and experimental realization of a nonresonant sound-absorbing strategy using metasurfaces by harnessing multiple int ... Full text Cite

A surface impedance-based three-channel acoustic metasurface retroreflector

Journal Article Applied Physics Letters · April 30, 2018 We propose the design and measurement of an acoustic metasurface retroreflector that works at three discrete incident angles. An impedance model is developed such that for acoustic waves impinging at -60°, the reflected wave is defined by the surface imped ... Full text Cite

Systematic design and experimental demonstration of bianisotropic metasurfaces for scattering-free manipulation of acoustic wavefronts.

Journal Article Nature communications · April 2018 Recent advances in gradient metasurfaces have shown that by locally controlling the bianisotropic response of the cells one can ensure full control of refraction, that is, arbitrarily redirect the waves without scattering into unwanted directions. In this ... Full text Cite

Observations of Blue Discharges Associated With Negative Narrow Bipolar Events in Active Deep Convection

Journal Article Geophysical Research Letters · March 28, 2018 On 19 August 2012, the Imager of Sprites and Upper Atmospheric Lightning on board the FORMOSAT-2 satellite captured a sequence of seven blue discharges within 1 min that emanated from a parent thunderstorm over Lake Taihu in East China. The analysis of lig ... Full text Cite

Acoustic metacages for sound shielding with steady air flow

Journal Article Journal of Applied Physics · March 28, 2018 Conventional sound shielding structures typically prevent fluid transport between the exterior and interior. A design of a two-dimensional acoustic metacage with subwavelength thickness which can shield acoustic waves from all directions while allowing ste ... Full text Cite

Very High Frequency Radio Emissions Associated With the Production of Terrestrial Gamma-Ray Flashes

Journal Article Geophysical Research Letters · February 28, 2018 Recent studies of the close association between terrestrial gamma-ray flashes (TGFs) production and simultaneous lightning processes have shown that many TGFs are produced during the initial leader of intracloud flashes and that some low-frequency (LF) rad ... Full text Cite

Systematic design of broadband path-coiling acoustic metamaterials

Journal Article Journal of Applied Physics · January 14, 2018 A design approach for acoustic metamaterial unit cells based on a coiled path with impedance matching layers (IMLs) is proposed in this paper. A theoretical approach is developed to calculate the transmission of the labyrinthine unit cells with different e ... Full text Cite

Spectral Characteristics of VLF Sferics Associated With RHESSI TGFs.

Journal Article Journal of geophysical research. Atmospheres : JGR · January 2018 We compared the modeled energy spectral density of very low frequency (VLF) radio emissions from terrestrial gamma ray flashes (TGFs) with the energy spectral density of VLF radio sferics recorded by Duke VLF receiver simultaneously with those TGFs. In tot ... Full text Cite

Coherent perfect absorption and laser modes in a cylindrical structure of conjugate metamaterials

Journal Article New Journal of Physics · January 1, 2018 In this work, we theoretically find that coherent perfect absorption (CPA) and laser modes can be realized in a two-dimensional cylindrical structure composed of conjugate metamaterials (CMs). The required phase factors of CMs for achieving CPA and laser m ... Full text Cite

Assessment of Unusual Gigantic Jets observed during the Monsoon season: First observations from Indian Subcontinent.

Journal Article Scientific reports · November 2017 Gigantic Jets are electric discharges from thunderstorm cloud tops to the bottom of ionosphere at ~90 km altitude and electrically connect the troposphere and lower ionosphere. Since their first report in 2002, sporadic observations have been reported from ... Full text Cite

Coherent perfect absorber and laser modes in purely imaginary metamaterials

Journal Article Physical Review A · October 16, 2017 Conjugate metamaterials, in which the permittivity and the permeability are complex conjugates of each other, possess the elements of loss and gain simultaneously. By employing a conjugate metamaterial with a purely imaginary form, we propose a mechanism f ... Full text Cite

Gamma Ray Signatures of Neutrons From a Terrestrial Gamma Ray Flash

Journal Article Geophysical Research Letters · October 16, 2017 Following a lightning strike to a wind turbine in Japan, we have observed a large burst of neutrons lasting 100 ms with a ground fluence of ~1,000 n cm−2, thousands of times greater than the peak neutron flux associated with the largest ground level solar ... Full text Cite

Elves Accompanying Terrestrial Gamma Ray Flashes

Journal Article Journal of Geophysical Research: Space Physics · October 1, 2017 This paper reports a modeling study of the optical phenomenon in the lower ionosphere known as elves that may accompany terrestrial gamma ray flashes (TGFs). Recent research has indicated that the in-cloud (IC) discharge processes, termed energetic in-clou ... Full text Cite

Analysis of lightning strokes associated with sprites observed by ISUAL in the vicinity of North America

Journal Article Terrestrial, Atmospheric and Oceanic Sciences · August 1, 2017 We examined the broadband (<1 Hz to 30 kHz) lightning sferics associated with 395 sprites observed near North America by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) onboard the FORMOSAT-2 satellite in a 12-year period from 2004 to 2015. O ... Full text Cite

Tunable Asymmetric Transmission via Lossy Acoustic Metasurfaces.

Journal Article Physical review letters · July 2017 In this study, we show that robust and tunable acoustic asymmetric transmission can be achieved through gradient-index metasurfaces by harnessing judiciously tailored losses. We theoretically prove that the asymmetric wave behavior stems from loss-induced ... Full text Cite

Microwave metamaterials made by fused deposition 3D printing of a highly conductive copper-based filament

Journal Article Applied Physics Letters · May 1, 2017 This work reports a method for fabricating three-dimensional microwave metamaterials by fused deposition modeling 3D printing of a highly conductive polymer composite filament. The conductivity of such a filament is shown to be nearly equivalent to that of ... Full text Cite

Triggered lightning sky waves, return stroke modeling, and ionosphere effective height

Journal Article Journal of Geophysical Research · January 1, 2017 Ground waves and sky waves measured 209 km and 250 km south of six triggered lightning flashes containing 30 strokes that occurred in the half-hour before sunset on 27 August 2015 are presented and analyzed. We use a cross-correlation technique to find the ... Full text Cite

Subwavelength diffractive acoustics and wavefront manipulation with a reflective acoustic metasurface

Journal Article Journal of Applied Physics · November 21, 2016 Acoustic metasurfaces provide useful wavefront shaping capabilities, such as beam steering, acoustic focusing, and asymmetric transmission, in a compact structure. Most acoustic metasurfaces described in the literature are transmissive devices and focus th ... Full text Cite

Acoustic Holographic Rendering with Two-dimensional Metamaterial-based Passive Phased Array.

Journal Article Scientific reports · October 2016 Acoustic holographic rendering in complete analogy with optical holography are useful for various applications, ranging from multi-focal lensing, multiplexed sensing and synthesizing three-dimensional complex sound fields. Conventional approaches rely on a ... Full text Cite

A sound absorbing metasurface with coupled resonators

Journal Article Applied Physics Letters · August 29, 2016 An impedance matched surface is able, in principle, to totally absorb the incident sound and yield no reflection, and this is desired in many acoustic applications. Here we demonstrate a design of impedance matched sound absorbing surface with a simple con ... Full text Cite

Ground detection of terrestrial gamma ray flashes from distant radio signals

Journal Article Geophysical Research Letters · August 28, 2016 Terrestrial gamma ray flashes (TGFs) are brief bursts of energetic gammy-ray photons generated during thunderstorms, which have been detected almost exclusively by satellite-based instruments. Here we present three lines of evidence which includes the thre ... Full text Cite

Transistor-based metamaterials with dynamically tunable nonlinear susceptibility

Journal Article Applied Physics Letters · August 8, 2016 We present the design, analysis, and experimental demonstration of an electromagnetic metamaterial with a dynamically tunable effective nonlinear susceptibility. Split-ring resonators loaded with transistors are shown theoretically and experimentally to ac ... Full text Cite

Reconstructing initial continuous current waveform in rocket-triggered lightning with close magnetic measurement

Conference 2016 Asia-Pacific International Symposium on Electromagnetic Compatibility, APEMC 2016 · July 26, 2016 We present a new method that remotely measures the initial continuous current in classical rocket-and-wire triggered lightning using a compact dB/dt sensor deployed at 78 m distance from the channel base. The current waveform retrieved from the numerical i ... Full text Cite

Radio emissions from double RHESSI TGFs.

Journal Article Journal of geophysical research. Atmospheres : JGR · July 2016 A detailed analysis of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) terrestrial gamma ray flashes (TGFs) is performed in association with World Wide Lightning Location Network (WWLLN) sources and very low frequency (VLF) sferics recorded a ... Full text Cite

Anisotropic acoustic metafluid for underwater operation.

Journal Article The Journal of the Acoustical Society of America · June 2016 The paper presents a method to design and characterize mechanically robust solid acoustic metamaterials suitable for operation in dense fluids such as water. These structures, also called metafluids, behave acoustically as inertial fluids characterized by ... Full text Cite

Asymmetric acoustic transmission through near-zero-index and gradient-index metasurfaces

Journal Article Applied Physics Letters · May 30, 2016 We present a design of acoustic metasurfaces yielding asymmetric transmission within a certain frequency band. The design consists of a layer of gradient-index metasurface and a layer of low refractive index metasurface. Incident waves are controlled in a ... Full text Cite

Preserving omnidirectionality in optimized asymmetric transformation optics designs

Journal Article Journal of Optics (United Kingdom) · April 1, 2016 Optimization techniques are efficient methods to simplify the design of transformation optics devices. Asymmetric devices obtained through these methods typically lose omnidirectionality and perform well only when illuminated from a finite set of direction ... Full text Cite

Observation-constrained modeling of the ionospheric impact of negative sprites

Journal Article Geophysical Research Letters · March 28, 2016 This paper reports observation and modeling of five negative sprites occurring above two Florida thunderstorms. The sprites were triggered by unusual types of negative cloud-to-ground (CG) lightning discharges with impulse charge moment change ranging from ... Full text Cite

Subionospheric propagation and peak currents of preliminary breakdown pulses before negative cloud-to-ground lightning discharges

Journal Article Geophysical Research Letters · February 16, 2016 We analyze broadband electromagnetic measurements of pulse sequences occurring prior to first return strokes of negative cloud-to-ground lightning flashes. Signals generated by lightning discharges were recorded close to the thunderstorm by a magnetic fiel ... Full text Cite

Controlling sound with acoustic metamaterials

Journal Article Nature Reviews Materials · February 16, 2016 Acoustic metamaterials can manipulate and control sound waves in ways that are not possible in conventional materials. Metamaterials with zero, or even negative, refractive index for sound offer new possibilities for acoustic imaging and for the control of ... Full text Cite

An analysis of five negative sprite-parent discharges and their associated thunderstorm charge structures

Journal Article Journal of Geophysical Research · January 27, 2016 In this study we analyze the discharge morphologies of five confirmed negative sprite-parent discharges and the associated charge structures of the thunderstorms that produced them. The negative sprite-parent lightning took place in two thunderstorms that ... Full text Cite

Perfect conformal invisible device with feasible refractive indexes

Journal Article Physical Review B · January 6, 2016 Optical conformal mapping has been used to construct several isotropic devices with novel functionalities. In particular, a conformal cloak could confer omnidirectional invisibility. However, the maximum values of the refractive indexes needed for current ... Full text Cite

Sprite produced by consecutive impulse charge transfers following a negative stroke: Observation and simulation

Journal Article Journal of Geophysical Research · January 1, 2016 On the morning of 5 June 2013, two cameras of the SpriteCam network concurrently captured a red sprite with diffuse halo over a mesoscale convective system (MCS) passing the panhandle area of Oklahoma. This sprite was produced by a negative cloud-to-ground ... Full text Cite

Imaging lightning intracloud initial stepped leaders by low-frequency interferometric lightning mapping array

Journal Article Geophysical Research Letters · January 1, 2016 By comparing the source position of pulses during initial leaders and the paths of subsequent dart leaders from typical bilevel intracloud (IC) flashes, we demonstrate a new method to image the detailed three-dimensional (3-D) structure and stepping dynami ... Full text Cite

Ground-level observation of a terrestrial gamma ray flash initiated by a triggered lightning

Journal Article Journal of Geophysical Research · January 1, 2016 We report on a terrestrial gamma ray flash (TGF) that occurred on 15 August 2014 coincident with an altitude-triggered lightning at the International Center for Lightning Research and Testing (ICLRT) in North Central Florida. The TGF was observed by a grou ... Full text Cite

Observations of two sprite-producing storms in Colorado

Journal Article Journal of Geophysical Research · January 1, 2016 Two sprite-producing thunderstorms were observed on 8 and 25 June 2012 in northeastern Colorado by a combination of low-light cameras, a lightning mapping array, polarimetric and Doppler radars, the National Lightning Detection Network, and charge moment c ... Full text Cite

Bandwidth tuning in transistor embedded metamaterials using variable resistance

Journal Article Progress in Electromagnetics Research · January 1, 2016 Metamaterials have been previously loaded with diodes and other types of passive circuit elements. Transistors offer an alternative to these established loading elements to expand the possible capabilities of metamaterials. With embedded transistors, addit ... Full text Cite

A lightning-based search for nearby observationally dim terrestrial gamma ray flashes

Journal Article Journal of Geophysical Research · December 16, 2015 Current space-based observations of terrestrial gamma ray flashes (TGFs) are capable of identifying only TGFs that exceed a lower brightness threshold. Observationally, dim TGFs that fall below this threshold are consequently difficult to find using photon ... Full text Cite

Design and full characterization of planar active magnetic RF metamaterials

Journal Article IEEE Antennas and Wireless Propagation Letters · December 1, 2015 Applications of metamaterials have been limited by parasitic resistive losses. We present the design and measurement of split ring resonators with embedded active field effect transistor circuits comprising a magnetic metamaterial. We show that the imagina ... Full text Cite

A study of changes in apparent ionospheric reflection height within individual lightning flashes

Journal Article Journal of Atmospheric and Solar-Terrestrial Physics · December 1, 2015 Ionospheric reflection heights estimated using the zero-to-zero and peak-to-peak methods to measure skywave delay relative to the ground wave were compared for 108 first and 124 subsequent strokes at distances greater than 100 km. For either metric there w ... Full text Cite

Broadband Acoustic Hyperbolic Metamaterial.

Journal Article Physical review letters · December 2015 In this Letter, we report on the design and experimental characterization of a broadband acoustic hyperbolic metamaterial. The proposed metamaterial consists of multiple arrays of clamped thin plates facing the y direction and is shown to yield opposite si ... Full text Cite

Propagation of preliminary breakdown pulses preceding cloud-to-ground lightning discharges

Conference 2015 1st URSI Atlantic Radio Science Conference, URSI AT-RASC 2015 · October 21, 2015 A sequence of electromagnetic pulses lasting from a few milliseconds to several tens of milliseconds is usually observed prior to the first return stroke of a cloud-to-ground lightning flash. The sequence is composed of three parts. It begins with a group ... Full text Cite

Lightning leader altitude progression in terrestrial gamma-ray flashes

Journal Article Geophysical Research Letters · September 28, 2015 Radio emissions continue to provide insight into the production of terrestrial gamma ray flashes (TGFs) by thunderstorms, including the critical question of the conditions under which they are generated. We have identified several TGF-associated lightning ... Full text Cite

Insights into high peak current in-cloud lightning events during thunderstorms

Journal Article Geophysical Research Letters · August 28, 2015 We investigated National Lightning Detection Network reports and lightning radio waveforms in a 44 day observation period to analyze the in-cloud (IC) events producing currents above 200 kA. The results show that there are two distinct classes of IC lightn ... Full text Cite

Single-sensor multispeaker listening with acoustic metamaterials.

Journal Article Proceedings of the National Academy of Sciences of the United States of America · August 2015 Featured Publication Designing a "cocktail party listener" that functionally mimics the selective perception of a human auditory system has been pursued over the past decades. By exploiting acoustic metamaterials and compressive sensing, we present here a single-sensor listeni ... Full text Cite

Active acoustic metamaterials reconfigurable in real time

Journal Article Physical Review B - Condensed Matter and Materials Physics · June 30, 2015 A major limitation of current acoustic metamaterials is that their acoustic properties are either locked into place once fabricated or are only modestly tunable, tying them to the particular application for which they are designed. We present a design appr ... Full text Cite

Water-based metamaterials: Negative refraction of sound.

Journal Article Nature materials · April 2015 Full text Cite

Upward electrical discharges observed above Tropical Depression Dorian.

Journal Article Nature communications · January 2015 Observation of upward electrical discharges from thunderstorms has been sporadically reported in the scientific literature. According to their terminal altitudes, they are classified as starters (20-30 km), jets (40-50 km) and gigantic jets (70-90 km). The ... Full text Cite

Active and applied functional rf metamaterials

Journal Article Springer Series in Materials Science · January 1, 2015 The concept of electromagnetic metamaterials, defined here as artificial materials that are designed to obtain specific effective electromagnetic material properties, has revolutionized thinking about wave behavior in materials and the design of complex el ... Full text Cite

Large charge moment change lightning on 31 May to 1 June 2013, including the El Reno tornadic storm

Journal Article Journal of Geophysical Research · January 1, 2015 On 31 May 2013, a line of severe tornadic thunderstorms (the El Reno event) developed during the local afternoon in central Oklahoma, USA. Within range of the Oklahoma Lightning Mapping Array, the evolution of the event can be separated into three distinct ... Full text Cite

The source altitude, electric current, and intrinsic brightness of terrestrial gamma ray flashes

Journal Article Geophysical Research Letters · December 16, 2014 Many details of how thunderstorms generate terrestrial gamma ray flashes (TGFs) and other forms of high-energy radiation remain uncertain, including the basic question of where they are produced. We exploit the association of distinct low-frequency radio e ... Full text Cite

Implications of lightning emissions for terrestrial gamma-ray flashes and associated lightning discharges

Conference 2014 International Conference on Lightning Protection, ICLP 2014 · December 3, 2014 Featured Publication Remote sensing of broadband (<1 Hz to 300 kHz) lightning signals related to terrestrial gamma-ray flashes (TGFs) indicates that gamma-rays in TGFs are typically produced during a slow process that creates a considerable charge moment change within a few mi ... Full text Cite

A low-frequency near-field interferometric-TOA 3-D Lightning Mapping Array

Journal Article Geophysical Research Letters · November 28, 2014 We report on the development of an easily deployable LF near-field interferometric-time of arrival (TOA) 3-D Lightning Mapping Array applied to imaging of entire lightning flashes. An interferometric cross-correlation technique is applied in our system to ... Full text Cite

Unified approach to design linear and nonlinear acoustic metamaterials

Conference 2014 8th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics, METAMATERIALS 2014 · November 5, 2014 We review a new approach to design active acoustic metematerials that results in artificial media covering a large range of linear acoustic responses. We show how this method can be extended to obtain metamaterials having highly non-linear responses. We il ... Full text Cite

Wavefront modulation and subwavelength diffractive acoustics with an acoustic metasurface.

Journal Article Nature communications · November 2014 Metasurfaces are a family of novel wavefront-shaping devices with planar profile and subwavelength thickness. Acoustic metasurfaces with ultralow profile yet extraordinary wave manipulating properties would be highly desirable for improving the performance ... Full text Cite

Mechanisms of sprite initiation, morphology, and lightning polarity asymmetry

Conference 2014 31th URSI General Assembly and Scientific Symposium, URSI GASS 2014 · October 17, 2014 Sprites are spectacular optical emissions in the mesosphere induced by transient lightning electric fields above thunderstorms. Recently, significant efforts have been devoted to the understanding of the inception mechanism of sprite streamers, the origin ... Full text Cite

Charge motion and altitude of terrestrial gamma-ray flashes

Conference 2014 31th URSI General Assembly and Scientific Symposium, URSI GASS 2014 · October 17, 2014 We describe recent coordinated observations of radio emissions and terrestrial gamma ray flashes (TGFs) that provide new insight into the detailed processes that generate this thunderstorm-driven high-energy radiation. These observations include lightning ... Full text Cite

Negative sprites produced by consecutive impulse charge transfers in negative strokes

Conference 2014 31th URSI General Assembly and Scientific Symposium, URSI GASS 2014 · October 17, 2014 On June 5, 2013, the SpriteCam network recorded a relatively weak sprite over a thunderstorm in Oklahoma. The broadband magnetic fields recorded at multiple stations consistently indicate that this event was produced by a negative cloud-to-ground (CG) even ... Full text Cite

Measurement of sensitive current and charge motion using coherent averaging of remote low frequency magnetic field observations

Conference 2014 United States National Committee of URSI National Radio Science Meeting, USNC-URSI NRSM 2014 · October 16, 2014 Measuring lightning charge transfer, especially on time scales longer than several milliseconds, is a challenge. Instrumented towers can measure this quantity precisely but only for a tiny fraction of cloud-to-ground lightning. Electrostatic field measurem ... Full text Cite

Transistor-embedded acousto-optic and nonlinear metamaterials

Conference 2014 United States National Committee of URSI National Radio Science Meeting, USNC-URSI NRSM 2014 · October 16, 2014 We present experimental results showing the application of transistor-embedded metamaterials to realize nonlinear and acousto-optic metamaterials. Due to their nonlinear properties, transistors can be used to realize a large variety of useful radio frequen ... Full text Cite

Design and realization of transistor-embedded active RF metamaterials

Conference 2014 United States National Committee of URSI National Radio Science Meeting, USNC-URSI NRSM 2014 · October 16, 2014 We present several results that use transistors within the context of metamaterials. Transistors are a fundamental device in the construction of several RF systems and applications. The basic device physics of the transistor allow for the convenient constr ... Full text Cite

Roadmap to electrically self-tuning metamaterials: Design and experimental validation

Conference Proceedings - 2014 International Conference on Electromagnetics in Advanced Applications, ICEAA 2014 · September 18, 2014 We present the design and experimental validation of a self-tuning metamaterial. We begin with an active two-state split ring resonator and build up to a self-contained system that includes feedback. We show the ability of the active split ring resonator t ... Full text Cite

Design and demonstration of broadband thin planar diffractive acoustic lenses

Journal Article Applied Physics Letters · September 8, 2014 We present here two diffractive acoustic lenses with subwavelength thickness, planar profile, and broad operation bandwidth. Tapered labyrinthine unit cells with their inherently broadband effective material properties are exploited in our design. Both the ... Full text Cite

Design for simplified materials in transformation electromagnetics

Chapter · July 1, 2014 In this chapter we review some of the common methods used to simplify the design of transformation electromagnetics devices. One of the major challenges is the complexity of the needed material parameters, and a variety of analytical, numerical, and approx ... Full text Cite

Broadband electromagnetic metamaterials with reconfigurable fluid channels

Journal Article Physical Review B - Condensed Matter and Materials Physics · June 25, 2014 We present the design, characterization, and experimental verification of broadband and tunable metamaterials based on reconfigurable fluid channels. We demonstrate through simulation that a finite-height cylinder of water in a background medium behaves as ... Full text Cite

Time-varying transistor-based metamaterial for tunability, mixing, and efficient phase conjugation

Journal Article Journal of Applied Physics · April 14, 2014 We present a transistor-based microwave metamaterial exhibiting tunability over a wide range of time scales. By loading a metamaterial with a transistor, we show through theory and simulation that both the resonant frequency and quality factor of the metam ... Full text Cite

Three-dimensional broadband omnidirectional acoustic ground cloak.

Journal Article Nature materials · April 2014 Featured Publication The control of sound propagation and reflection has always been the goal of engineers involved in the design of acoustic systems. A recent design approach based on coordinate transformations, which is applicable to many physical systems, together with the ... Full text Open Access Cite

Compressive acoustic imaging with metamaterials.

Journal Article The Journal of the Acoustical Society of America · April 2014 Compressive imaging has brought revolutionary design methodologies to imaging systems. By shuffling and multiplexing the object information space, the imaging system compresses data on the physical layer and enables employing fewer sensors and acquiring le ... Full text Cite

Tapered labyrinthine acoustic metamaterials for coherent controlling of acoustic wave.

Journal Article The Journal of the Acoustical Society of America · April 2014 Acoustic metamaterials with their exotic material properties enable unprecedented control over acoustic wave propagation and reflection. Besides utilizing locally resonating structures or non-resonant composite effective media, non-locally resonating spati ... Full text Cite

Non-reciprocal and highly nonlinear active acoustic metamaterials.

Journal Article Nature communications · February 2014 Unidirectional devices that pass acoustic energy in only one direction have numerous applications and, consequently, have recently received significant attention. However, for most practical applications that require unidirectionality at audio and low freq ... Full text Cite

Ground-based observations of the relations between lightning charge-moment-change and the physical and optical properties of column sprites

Journal Article Journal of Atmospheric and Solar-Terrestrial Physics · January 1, 2014 Optical observations of 66 sprites, using a calibrated commercial CCD camera, were conducted in 2009-2010 and 2010-2011 winter seasons as part of the ILAN (Imaging of Lightning And Nocturnal flashes) campaign in the vicinity of Israel and the eastern Medit ... Full text Cite

Applied physics. Selecting the direction of sound transmission.

Journal Article Science (New York, N.Y.) · January 2014 Featured Publication Full text Cite

Regional, seasonal, and diurnal variations of cloud-to-ground lightning with large impulse charge moment changes

Journal Article Monthly Weather Review · January 1, 2014 The use of both total chargemoment change (CMC) and impulse chargemoment change (iCMC) magnitudes to assess the potential of a cloud-to-ground (CG) lightning stroke to induce a mesospheric sprite has been well described in the literature, particularly on a ... Full text Cite

Overview of the Kansas Windfarm 2013 field program

Conference International Conference on Atmospheric Electricity, ICAE 2014 · January 1, 2014 © International Conference on Atmospheric Electricity, ICAE 2014 Initial observations of lightning attachment to wind turbine generators at a Kansas wind farm in 2012 resulted in a number of insights and left several open questions. This led to the plannin ... Cite

Large charge moment change lightning in an Oklahoma mesoscale convective system

Conference International Conference on Atmospheric Electricity, ICAE 2014 · January 1, 2014 © International Conference on Atmospheric Electricity, ICAE 2014 On 31 May 2013, a line of severe thunderstorms developed during the local afternoon in central Oklahoma, USA. One of the supercells produced the El Reno tornado, which caused significant dama ... Cite

Overview of the Kansas Windfarm 2013 field program

Conference International Conference on Atmospheric Electricity, ICAE 2014 · January 1, 2014 Initial observations of lightning attachment to wind turbine generators at a Kansas wind farm in 2012 resulted in a number of insights and left several open questions. This led to the planning and re-deployment for the summer or 2013. Ten groups have colla ... Cite

Large charge moment change lightning in an Oklahoma mesoscale convective system

Conference International Conference on Atmospheric Electricity, ICAE 2014 · January 1, 2014 On 31 May 2013, a line of severe thunderstorms developed during the local afternoon in central Oklahoma, USA. One of the supercells produced the El Reno tornado, which caused significant damage and killed several people. During the 2300 UTC hour (during th ... Cite

Functional metamaterials for wireless phase conjugation

Journal Article CLEO: QELS_Fundamental Science, CLEO:QELS FS 2013 · November 21, 2013 Functional metamaterials provide useful properties for the design of electromagnetic devices. In this work we demonstrate that functions including high nonlinearity and amplification can be included in metamaterials to realize time reversal imaging. © 2013 ... Cite

Tapered labyrinthine acoustic metamaterials for broadband impedance matching

Journal Article Applied Physics Letters · November 11, 2013 We present five kinds of labyrinthine or space-coiling acoustic metamaterials with tapered channels and apertures. These designs exhibit negative index behavior with modest dispersion, and also have substantially improved impedance matching compared to pre ... Full text Cite

Initial breakdown pulses in intracloud lightning flashes and their relation to terrestrial gamma ray flashes

Journal Article Journal of Geophysical Research Atmospheres · October 16, 2013 The initial breakdown stage of 10 intracloud lightning flashes that may have produced terrestrial gamma ray flashes (TGFs) is studied with wideband E-change, multiband B-change, and VHF lightning mapping data; these flashes fit published criteria known to ... Full text Cite

Mechanism of column and carrot sprites derived from optical and radio observations

Journal Article Geophysical Research Letters · September 16, 2013 The lightning current waveforms observed simultaneously with high-speed video records of a column and a carrot sprite event are incorporated in a plasma fluid model to provide quantitative explanation of these two distinct morphological classes of transien ... Full text Cite

The meteorology of negative cloud-to-ground lightning strokes with large charge moment changes: Implications for negative sprites

Journal Article Journal of Geophysical Research Atmospheres · July 27, 2013 This study examined the meteorological characteristics of precipitation systems that produced 38 "sprite-class" negative cloud-to-ground (CG) strokes (i.e., peak currents in excess of 100 kA and charge moment changes in excess of 800 C km) as well as those ... Full text Cite

Tunable active acoustic metamaterials

Journal Article Physical Review B - Condensed Matter and Materials Physics · July 16, 2013 We describe and demonstrate an architecture for active acoustic metamaterials whose effective material parameters can be tuned independently over a wide range of values, including negative material parameters. The approach is demonstrated experimentally th ... Full text Cite

Coordinated observations of sprites and in-cloud lightning flash structure

Journal Article Journal of Geophysical Research Atmospheres · June 27, 2013 The temporal and spatial development of sprite-producing lightning flashes is examined with coordinated observations over an asymmetric mesoscale convective system (MCS) on 29 June 2011 near the Oklahoma Lightning Mapping Array (LMA). Sprites produced by a ... Full text Cite

Three years of lightning impulse charge moment change measurements in the United States

Journal Article Journal of Geophysical Research Atmospheres · June 16, 2013 We report and analyze 3 years of lightning impulse charge moment change (iCMC) measurements obtained from an automated, real time lightning charge moment change network (CMCN). The CMCN combines U.S. National Lightning Detection Network (NLDN) lightning ev ... Full text Cite

Radio emissions from terrestrial gamma-ray flashes

Journal Article Journal of Geophysical Research: Space Physics · June 1, 2013 The theory of radio frequency (RF) emissions by terrestrial gamma-ray flashes (TGFs) is developed. These radio emissions, which are separate from the emission caused by lightning, are produced by the electric currents generated by runaway electrons and res ... Full text Cite

Simultaneous observations of optical lightning and terrestrial gamma ray flash from space

Journal Article Geophysical Research Letters · May 28, 2013 We present the very first simultaneous detection from space of a terrestrial gamma ray flash (TGF) and the optical signal from lightning. By fortuitous coincidence, two independent satellites passed less than 300 km from the thunderstorm system that produc ... Full text Cite

Radar and lightning analyses of gigantic jet-producing storms

Journal Article Journal of Geophysical Research Atmospheres · April 16, 2013 An analysis of thunderstorm environment, structure, and evolution associated with six gigantic jets (five negative polarity, one positive) was conducted. Three of these gigantic jets were observed within detection range of very high frequency lightning map ... Full text Cite

Measurement of a broadband negative index with space-coiling acoustic metamaterials.

Journal Article Physical review letters · April 2013 We report the experimental demonstration of a broadband negative refractive index obtained in a labyrinthine acoustic metamaterial structure. Two different approaches were employed to prove the metamaterial negative index nature: one-dimensional extraction ... Full text Cite

Functional metamaterials for wireless phase conjugation

Journal Article 2013 Conference on Lasers and Electro-Optics, CLEO 2013 · January 1, 2013 Functional metamaterials provide useful properties for the design of electromagnetic devices. In this work we demonstrate that functions including high nonlinearity and amplification can be included in metamaterials to realize time reversal imaging. © 2013 ... Full text Cite

Active nonlinear metamaterials loaded with negative differential resistance elements and circuits

Conference Proceedings of SPIE - The International Society for Optical Engineering · December 1, 2012 Using active loads in circuits that result in negative differential resistances at various nodes in the circuit are used for producing resistance cancellation, gain, oscillations, and other types of effects not seen in passive circuits. We aim to demonstra ... Full text Cite

Nonlinear and active RF metamaterial applications using embedded devices

Conference Proceedings of SPIE - The International Society for Optical Engineering · December 1, 2012 Nonlinear metamaterials have received considerable attention in recent years. The inclusion of nonlinear and active effects in metamaterials expands the possibilities for engineering media with designer properties. We detail our recent efforts to create no ... Full text Cite

Sound manipulation with acoustic metamaterials

Journal Article 41st International Congress and Exposition on Noise Control Engineering 2012, INTER-NOISE 2012 · December 1, 2012 Acoustic metamaterials are engineered materials with properties hard or impossible to find in natural materials (e.g. negative effective density and/or negative bulk modulus). Therefore, a myriad of novel applications could be imagined and some of them hav ... Cite

High-energy atmospheric physics: Terrestrial gamma-ray flashes and related phenomena

Journal Article Space Science Reviews · November 1, 2012 It is now well established that both thunderclouds and lightning routinely emit x-rays and gamma-rays. These emissions appear over wide timescales, ranging from sub-microsecond bursts of x-rays associated with lightning leaders, to sub-millisecond bursts o ... Full text Cite

Design of an acoustic metamaterial lens using genetic algorithms.

Journal Article The Journal of the Acoustical Society of America · October 2012 The present work demonstrates a genetic algorithm approach to optimizing the effective material parameters of an acoustic metamaterial. The target device is an acoustic gradient index (GRIN) lens in air, which ideally possesses a maximized index of refract ... Full text Cite

Nonreciprocal active metamaterials

Journal Article Physical Review B - Condensed Matter and Materials Physics · May 2, 2012 We show experimentally that active electromagnetic metamaterials are suitable for the implementation of a large class of nonreciprocal media characterized by magnetoelectric constitutive parameters that cannot be achieved in passive materials. We design, f ... Full text Cite

Phase conjugation metamaterials: Particle design and imaging experiments

Journal Article Journal of Optics (United Kingdom) · January 1, 2012 We present simulation and experimental results demonstrating the effectiveness of phase conjugation or, equivalently, time reversal metamaterials for imaging applications. Split-ring resonators are loaded with varactor diodes to enable straightforward, eas ... Full text Cite

Charge rearrangement by sprites over a north Texas mesoscale convective system

Journal Article Journal of Geophysical Research Atmospheres · January 1, 2012 Charge rearrangement by sprites is analyzed for a mesoscale convective system (MCS) situated in north Texas and east New Mexico on 15 July 2010. During the thunderstorm, electric field data were recorded by the Langmuir Electric Field Array (LEFA), while m ... Full text Cite

Charge moment change and lightning-driven electric fields associated with negative sprites and halos

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2012 [1] Sprites are structured high altitude optical emissions produced by lightning-driven electric fields. Both strong positive and negative cloud to ground flashes (CGs) are capable of initiating sprites. However, reported sprites are almost exclusively pro ... Full text Cite

New measurements of lightning electric fields in Florida: Waveform characteristics, interaction with the ionosphere, and peak current estimates

Journal Article Journal of Geophysical Research Atmospheres · January 1, 2012 We analyzed wideband electric field waveforms of 265 first and 349 subsequent return strokes in negative natural lightning. The distances ranged from 10 to 330 km. Evolution of first-and subsequent-stroke field waveforms as a function of distance is examin ... Full text Cite

Relationship between sprite streamer behavior and lightning-driven electric fields

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2012 The lightning-driven electric fields not only initiate sprite streamers at high altitudes but also control their propagation to lower altitudes until termination. Thus, the relationship between sprite streamer behavior and their causative lightning-driven ... Full text Cite

Lightning morphology and impulse charge moment change of high peak current negative strokes

Journal Article Journal of Geophysical Research Atmospheres · January 1, 2012 We have analyzed very high frequency lightning mapping observations and remote magnetic field measurements to investigate connections between lightning morphology and impulse charge moment change (iCMC) of negative cloud-to-ground (CG) strokes with high es ... Full text Cite

Transformation optics

Journal Article · January 1, 2012 Transformation optics shows great promise as a new way of designing novel optical and electromagnetic devices based on the idea of using coordinate transformations to generate descriptions of dielectric media. It allows for essentially complete control ove ... Full text Cite

Sound manipulation with acoustic metamaterials

Conference American Society of Mechanical Engineers, Noise Control and Acoustics Division (Publication) NCAD · January 1, 2012 Acoustic metamaterials are engineered materials with properties hard or impossible to find in natural materials (e.g. negative effective density and/or negative bulk modulus). Therefore, a myriad of novel applications could be imagined and some of them hav ... Full text Cite

Complex coordinates in transformation optics

Journal Article Physical Review A - Atomic, Molecular, and Optical Physics · December 16, 2011 We show that complex coordinates used in conjunction with transformation optics offer an extra degree of freedom that allows control over not only the propagation direction of electromagnetic waves but also their amplitude. We illustrate this idea in two a ... Full text Cite

RF limiter metamaterial using p-i-n diodes

Journal Article IEEE Antennas and Wireless Propagation Letters · December 1, 2011 We present the design and experimental implementation of an RF limiter metamaterial using a sheet of nonlinear metamaterials. We demonstrate that complementary electric inductive-capacitive resonators loaded with nonlinear p-i-n diodes can act as RF limite ... Full text Cite

Detailed observations of lightning flashes and processes associated with terrestrial gamma ray flashes

Journal Article 2011 30th URSI General Assembly and Scientific Symposium, URSIGASS 2011 · November 21, 2011 We describe recent coordinated observations of lightning and terrestrial gamma ray flashes (TGFs) that provide new insight into the detailed processes that generate this thunderstorm-driven high-energy radiation. These observations include lightning mappin ... Full text Cite

Powered and active RF metamaterials

Journal Article 2011 30th URSI General Assembly and Scientific Symposium, URSIGASS 2011 · November 21, 2011 We report techniques for the design of active and tunable RF metamaterials and measurements of their novel properties, including frequency agile metamaterials in which each element is individually computer controlled with a nonvolatile state memory, powere ... Full text Cite

Observations of lightning flash development associated with gigantic jets

Journal Article 2011 30th URSI General Assembly and Scientific Symposium, URSIGASS 2011 · November 21, 2011 We report the observations of two gigantic jets, one during daytime over sea and one at nighttime over land, in the context of detailed associated observations of lightning flash and discharge evolution provided by a variety of radio measurements. © 2011 I ... Full text Cite

Powered and nonlinear RF metamaterials

Journal Article Proceedings of SPIE - The International Society for Optical Engineering · October 14, 2011 The electromagnetic properties of metamaterials can be engineered to achieve substantially more flexibility and variety than those available from conventional materials. Adding some degree of external control or power to metamaterials enables another level ... Full text Cite

Design and measurements of a broadband two-dimensional acoustic lens

Journal Article Physical Review B - Condensed Matter and Materials Physics · July 26, 2011 We describe the design, fabrication, and measurement of a 2D broadband gradient index acoustic lens in air. The index of refraction is tuned by controlling the dimensions of acoustic metamaterial unit cell inclusions designed through numerical simulations. ... Full text Cite

The lightning-TGF relationship on microsecond timescales

Journal Article Geophysical Research Letters · July 1, 2011 We analyze the count rates of two terrestrial gamma-ray flashes (TGFs) detected by the Fermi Gamma-ray Burst Monitor (GBM) with the broadband magnetic fields (1 to 300 kHz) produced by the simultaneous lightning processes. The microsecond-scale absolute ti ... Full text Cite

Homogeneous and compact acoustic ground cloaks

Journal Article Physical Review B - Condensed Matter and Materials Physics · June 24, 2011 We present the design, architecture, and detailed performance predictions for a class of ground-plane acoustic cloaking shells. The design begins with a coordinate transformation which, in contrast to a quasiconformal design, yields a homogeneous but aniso ... Full text Cite

Experimental acoustic ground cloak in air.

Journal Article Physical review letters · June 2011 We present the design, fabrication, and performance analysis for a class of two-dimensional acoustic cloaking coatings in air. Our approach takes advantage of transformation acoustics and linear coordinate transformations that result in shells which are ho ... Full text Cite

Lightning development associated with two negative gigantic jets

Journal Article Geophysical Research Letters · June 1, 2011 We report observations of two negative polarity gigantic jets sufficiently near very high-frequency (VHF) lightning mapping networks that the associated lightning characteristics and charge transfer could be investigated. In both cases the gigantic jet-pro ... Full text Cite

The rarity of terrestrial gamma-ray flashes

Journal Article Geophysical Research Letters · April 28, 2011 We report on the first search for Terrestrial Gamma-ray Flashes (TGFs) from altitudes where they are thought to be produced. The Airborne Detector for Energetic Lightning Emissions (ADELE), an array of gamma-ray detectors, was flown near the tops of Florid ... Full text Cite

Design and measurements of a broadband two-dimensional acoustic metamaterial with anisotropic effective mass density

Journal Article Journal of Applied Physics · March 1, 2011 We present the experimental realization and characterization of a broadband acoustic metamaterial with strongly anisotropic effective mass density. The metamaterial is composed of arrays of solid inclusions in an air background, and the anisotropy is contr ... Full text Cite

Generalized transformation optics of linear materials

Journal Article Journal of Optics · January 1, 2011 We continue the development of a manifestly four-dimensional, completely covariant, approach to transformation optics in linear dielectric materials begun in a previous paper. This approach, which generalizes the Plebanski based approach, is systematically ... Full text Cite

Frequency conversion by exploiting time in transformation optics

Journal Article Journal of Optics · January 1, 2011 The fundamental framework of transformation optics describes how coordinate transformations on electromagnetic fields can be physically realized through complex material parameters. This approach includes the possibility of transforming time as well as spa ... Full text Cite

A completely covariant approach to transformation optics

Journal Article Journal of Optics · January 1, 2011 We show that the Plebanski based approach to transformation optics overlooks some subtleties in the electrodynamics of moving dielectrics that restricts its applicability to a certain class of transformations. An alternative, completely covariant, approach ... Full text Cite

The properties of a gigantic jet reflected in a simultaneous sprite: Observations interpreted by a model

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2011 Thunderstorm clouds may discharge directly to the ionosphere in spectacular luminous jets-the largest electric discharges of our planet. The properties of these "giants," such as their polarity, conductivity, and currents, have been predicted by models, bu ... Full text Cite

Magnetospheric radio tomographic imaging with IMAGE and Wind

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2011 Recent theoretical studies have shown the feasibility and potential scientific value of radio tomographic imaging of Earth's magnetosphere by measuring Faraday rotation and phase difference (or group delay) of coherent radio wave signals. On 15 August 2000 ... Full text Cite

A terrestrial gamma ray flash observed from an aircraft

Journal Article Journal of Geophysical Research Atmospheres · January 1, 2011 On 21 August 2009, the Airborne Detector for Energetic Lightning Emissions (ADELE), an array of six gamma-ray detectors, detected a brief burst of gamma rays while flying aboard a Gulfstream V jet near two active thunderstorm cells. The duration and spectr ... Full text Cite

Transient luminous events above two mesoscale convective systems: Charge moment change analysis

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2011 Charge moment change (ΔMQ) data were examined for 41 positive cloud-to-ground (+CG) lightning discharges that were parents of transient luminous events (TLEs; mainly sprites) over two different storms: 9 May (20 parents) and 20 June 2007 (21). Data were br ... Full text Cite

Daytime ionospheric D region sharpness derived from VLF radio atmospherics

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2011 We described and applied a technique to measure the local midlatitude daytime ionospheric D region electron density profile sharpness from the Earth-ionosphere waveguide mode interference pattern in the spectra of radio atmospherics (or sferics for short), ... Full text Cite

Estimation of electric charge in sprites from optical and radio observations

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2011 Measurements of very low frequency radio emissions indicate that substantial electric current flows inside some sprites. This charge motion, with presently unknown location and distribution, is related to the detailed internal microphysics of sprite develo ... Full text Cite

Comparison of sprite initiation altitudes between observations and models

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2011 Simultaneous analyses of measured sprite initiation altitudes with predicted initiation altitudes from simulations enable an examination of our understanding of the sprite initiation mechanism and the modeling techniques to simulate this mesospheric electr ... Full text Cite

Characteristics of broadband lightning emissions associated with terrestrial gamma ray flashes

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2011 To characterize lightning processes that produce terrestrial gamma ray flashes (TGFs), we have analyzed broadband (<1 Hz to 30 kHz) lightning magnetic fields for TGFs detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite i ... Full text Cite

Transformation acoustics

Journal Article 17th International Congress on Sound and Vibration 2010, ICSV 2010 · December 1, 2010 We review the development of the concept of transformation acoustics, through which sound fields can be arbitrarily manipulated by complex acoustic materials. The properties of these materials are themselves defined through the application of a desired coo ... Cite

Design of layered transformation-optics devices of arbitrary shape

Journal Article Physical Review A - Atomic, Molecular, and Optical Physics · September 30, 2010 Transformation-optics devices of arbitrary shapes usually result in material parameters inside the device that feature level sets of different shapes. Consequently, these devices cannot easily be implemented using a layered architecture and thus are, gener ... Full text Open Access Cite

A broadband low-reflection metamaterial absorber

Journal Article Journal of Applied Physics · September 15, 2010 Artificially engineered metamaterials have enabled the creation of electromagnetic materials with properties not found in nature. Recent work has demonstrated the feasibility of developing high performance, narrowband electromagnetic absorbers using such m ... Full text Open Access Cite

Phase conjugation and negative refraction using nonlinear active metamaterials.

Journal Article Physical review letters · September 2010 We present an experimental demonstration of phase conjugation using nonlinear metamaterial elements. Active split-ring resonators loaded with varactor diodes are demonstrated theoretically to act as phase-conjugating or time-reversing discrete elements whe ... Full text Open Access Cite

A lightning discharge producing a beam of relativistic electrons into space

Journal Article Geophysical Research Letters · September 1, 2010 Strong electric fields associated with lightning generate brief (∼1 ms) but intense Terrestrial Gamma-ray Flashes (TGFs), detected by spacecrafts. A few events are thought to be the signature of a relativistic electron beam escaping the atmosphere, which i ... Full text Open Access Cite

Wide angle impedance matching metamaterials for waveguide-fed phased-array antennas

Journal Article IET Microwaves, Antennas and Propagation · August 1, 2010 This work investigates the gains realisable through the use of artificially structured materials, otherwise known as metamaterials, in the wide angle impedance matching (WAIM) of waveguide-fed phased-array antennas. The authors propose that the anisotropic ... Full text Cite

Gigantic jets with negative and positive polarity streamers

Journal Article Journal of Geophysical Research: Space Physics · July 2010 The ISUAL gigantic jets (GJs) are categorized into three types from their generating sequence and spectral properties. Generating sequence of the type I GJs resembles that reported previously; after the fully developed jet (FDJ) established the dis ... Full text Open Access Cite

Lightning mapping observation of a terrestrial gamma-ray flash

Journal Article Geophysical Research Letters · June 1, 2010 We report the observation with the North Alabama Lightning Mapping Array (LMA) related to a terrestrial gamma-ray flash (TGF) detected by RHESSI on 26 July 2008. The LMA data explicitly show the TGF was produced during the initial development of a compact ... Full text Open Access Cite

Reconfigurable reflectarray using addressable metamaterials

Journal Article IEEE Antennas and Wireless Propagation Letters · March 19, 2010 We develop a reconfigurable reflectarray using digitally addressable complementary electric LC (CELC) metamaterials. The surface is composed of addressable CELC resonators, which are tuned to achieve a spatial gradient in the reflection coefficient phase a ... Full text Cite

Polarity and energetics of inner core lightning in three intense North Atlantic hurricanes

Journal Article Journal of Geophysical Research: Space Physics · March 2010 We use the World Wide Lightning Location Network (WWLLN), low‐frequency magnetic fields measured at Duke University, and storm intensity data (winds and central pressure) to examine the polarity and energetics of lightning within 100 km of the cent ... Full text Open Access Cite

Integrated sensing and information processing theme-based redesign of the undergraduate electrical and computer engineering curriculum at Duke University

Journal Article Advances in Engineering Education · 2010 The field of electrical and computer engineering has evolved significantly in the past two decades. This evolution has broadened the field of ECE, and subfields have seen deep penetration into very specialized areas. Remarkable devices and systems arising ... Cite

A survey of ELF and VLF research on lightning-ionosphere interactions and causative discharges

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2010 Extremely low frequency (ELF) and very low frequency (VLF) observations have formed the cornerstone of measurement and interpretation of effects of lightning discharges on the overlying upper atmospheric regions, as well as near-Earth space. ELF (0.3-3 kHz ... Full text Open Access Cite

Transient luminous events above two mesoscale convective systems: Storm structure and evolution

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2010 Two warm-season mesoscale convective systems (MCSs) were analyzed with respect to their production of transient luminous events (TLEs), mainly sprites. The 20 June 2007 symmetric MCS produced 282 observed TLEs over a 4 h period, during which the storms int ... Full text Open Access Cite

Contrasting the efficiency of radiation belt losses caused by ducted and nonducted whistler-mode waves from ground-based transmitters

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2010 It has long been recognized that whistler-mode waves can be trapped in plasmaspheric whistler ducts which guide the waves. For nonguided cases these waves are said to be "nonducted", which is dominant for L < 1.6. Wave-particle interactions are affected by ... Full text Open Access Cite

Multi-instrumental observations of a positive gigantic jet produced by a winter thunderstorm in Europe

Journal Article Journal of Geophysical Research Atmospheres · January 1, 2010 At 2336:56 UTC on 12 December 2009, a bright gigantic jet (GJ) was recorded by an observer in Italy. Forty-nine additional sprites, elves, halos and two cases of upward lightning were observed that night. The location of the GJ corresponded to a distinct c ... Full text Open Access Cite

Observations of prolific transient luminous event production above a mesoscale convective system in Argentina during the Sprite2006 Campaign in Brazil

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2010 On the night of 22-23 February 2006, 444 transient luminous events (TLEs), 86% sprites, were observed above a prolific mesoscale convective system (MCS) over Argentina, as part of the third sprite campaign in Brazil. GOES infrared (IR) cloud top temperatur ... Full text Open Access Cite

Midlatitude daytime D region ionosphere variations measured from radio atmospherics

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2010 We measured the midlatitude daytime ionospheric D region electron density profile height variations in July and August 2005 near Duke University by using radio atmospherics (or sferics for short), which are the high-power, broadband very low frequency (VLF ... Full text Open Access Cite

Midlatitude nighttime D region ionosphere variability on hourly to monthly time scales

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2010 Significant temporal variability of the nighttime D region is well known, but its inaccessibility means that the time scales, magnitudes, and sources for that variability are not well understood. We probed the ionospheric D region by measuring the high-pow ... Full text Open Access Cite

Theory and design of practical metamaterials for acoustic cloaking

Journal Article ASME International Mechanical Engineering Congress and Exposition, Proceedings · January 1, 2010 Full text Cite

Ground-based detection of sprites and their parent lightning flashes over Africa during the 2006 AMMA campaign

Journal Article Quarterly Journal of the Royal Meteorological Society · January 1, 2010 Sprites have been detected in video camera observations from Niger over mesoscale convective systems in Nigeria during the 2006 AMMA (African Monsoon Multidisciplinary Analysis) campaign. The parent lightning flashes have been detected by multiple Extremel ... Full text Cite

The meteorological and electrical structure of TLE-producing convective storms

Journal Article · December 1, 2009 Emerging real-time capabilities using sensitive ULF/ELF/VLF magnetic receivers can monitor the impulse charge moment changes (iΔMq) of cloud-to-ground lightning strokes (CGs) over large regions. This provides a means to detect the parent CGs of the most co ... Full text Cite

Controllable magnetic metamaterial using digitally addressable split-ring resonators

Journal Article IEEE Antennas and Wireless Propagation Letters · December 1, 2009 An ideal metamaterial is composed of identical elements in a uniformly structured array so that the material response is a single magnetic or electric resonance. Variations in the metamaterial particle characteristics or array assembly result in undesirabl ... Full text Cite

Design and characterization of broadband acoustic composite metamaterials

Journal Article Physical Review B - Condensed Matter and Materials Physics · November 25, 2009 We demonstrate a method to design nonresonant acoustic composite metamaterials made of periodic arrangements of highly subwavelength unit cells composed of one or more inclusions embedded in a fluid. The inclusion geometry determines the degree of anisotro ... Full text Cite

A dynamic frequency selective surface using addressable metamaterials

Journal Article IEEE Antennas and Propagation Society, AP-S International Symposium (Digest) · November 16, 2009 Full text Cite

Utilization of metamaterials for wide angle impedance matching (WAIM) in phased array antennas

Journal Article IEEE Antennas and Propagation Society, AP-S International Symposium (Digest) · November 16, 2009 Full text Cite

Electromagnetic surface and line sources under coordinate transformations

Journal Article Physical Review A - Atomic, Molecular, and Optical Physics · September 14, 2009 Although the analysis of electromagnetic sources in the context of coordinate transformations and the implications for transformation optics have been discussed in the literature, a correct formulation that includes surface and line currents has not been r ... Full text Cite

Quantification of the troposphere-to-ionosphere charge transfer in a gigantic jet

Journal Article Nature Geoscience · September 1, 2009 Gigantic jets are the clearest manifestation of direct electrical coupling between tropospheric thunderstorms and the ionosphere. They are leaders that emerge from electrical breakdown near the top of thunderstorms and extend all the way to the lower edge ... Full text Cite

Estimating lightning current moment waveforms from satellite optical measurements

Journal Article Geophysical Research Letters · September 1, 2009 From July 2004 to June 2007, the FORMOSAT-2/ ISUAL spectrophotometer and Duke magnetometer observed clear optical and radio signatures of 12 spriteproducing lightning events. In 10 of these, 777.4-nm luminosity normalized to a distance of 3000 km was almos ... Full text Cite

Is the Martian water table hidden from radar view?

Journal Article Geophysical Research Letters · August 16, 2009 Mars may possess a global sub-surface groundwater table as an integral part of its current hydrological system. However, the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard the Mars Express (MEx) spacecraft has yet to make a de ... Full text Cite

Charge transfer and in-cloud structure of large-charge-moment positive lightning strokes in a mesoscale convective system

Journal Article Geophysical Research Letters · August 16, 2009 Lightning observations in the very high frequency band and measurements of ultra low frequency magnetic fields are analyzed to investigate the charge transfer and in-cloud structure of eight positive cloud-to-ground (+CG) strokes in a mesoscale convective ... Full text Cite

Zero loss magnetic metamaterials using powered active unit cells.

Journal Article Optics express · August 2009 We report the design and experimental measurement of a powered active magnetic metamaterial with tunable permeability. The unit cell is based on the combination of an embedded radiofrequency amplifier and a tunable phase shifter, which together control the ... Full text Cite

Conformal array design with transformation electromagnetics

Journal Article Applied Physics Letters · June 29, 2009 We apply the theory of transformation electromagnetics to source arrays and show that a complex conformal antenna array can be made to behave like a geometrically different array when surrounded with a properly designed transformation electromagnetics medi ... Full text Cite

Electromagnetic source transformations using superellipse equations

Journal Article Applied Physics Letters · June 18, 2009 Transformation optics can be used to design media with unique properties that alter the behavior of electromagnetic waves in passive space and recently in space containing source distributions. We present source transformations where current from a linear ... Full text Cite

Measurement of sprite streamer acceleration and deceleration

Journal Article Geophysical Research Letters · May 28, 2009 [1] Measurements of streamer velocity reflect the detailed internal microphysics of sprite development that is connected to the effects streamers create in the mesosphere. We report intensified high speed video observations of streamer development acquired ... Full text Cite

A rigorous and nonsingular two dimensional cloaking coordinate transformation

Journal Article Journal of Applied Physics · March 24, 2009 We derive an approximate but rigorous two dimensional electromagnetic cloaking shell in which no parameters are singular. The simplicity of the material parameters makes it potentially easier to realize in practice than other cloaking solutions. The analyt ... Full text Cite

Fast tomographic reconstruction strategy for diffuse optical tomography.

Journal Article Optics express · March 2009 Diffuse Optical Tomography (DOT) has been growing significantly in the past two decades as a promising tool for in-vivo and non-invasive imaging of tissues using near-infrared light. It can improve our ability to probe complex biologic interactions dynamic ... Full text Cite

Cloaking with optimized homogeneous anisotropic layers

Journal Article Physical Review A - Atomic, Molecular, and Optical Physics · February 2, 2009 We present a method to reduce the scattering from arbitrary objects by surrounding them with shells composed of several layers of homogeneous anisotropic materials. An optimization procedure is used to find the material parameters for each layer, the start ... Full text Cite

Spectral dependence of terrestrial gamma-ray flashes on source distance

Journal Article Geophysical Research Letters · January 16, 2009 We use lightning sferics from the World Wide Lightning Location Network to identify storms near 362 Terrestrial Gamma-ray Flashes (TGFs). The combined spectrum of TGFs with storms within 300 km of the sub-satellite point is much harder than the spectrum of ... Full text Cite

Discharge processes, electric field, and electron energy in ISUAL recorded gigantic jets

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2009 This article reports the first high time resolution measurements of gigantic jets from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment. The velocity of the upward propagating fully developed jet stage of the gigantic jets was ∼107 ... Full text Cite

Electromagnetic source transformations and applications

Journal Article IEEE Antennas and Propagation Society, AP-S International Symposium (Digest) · January 1, 2009 Full text Cite

Characterization of complementary electric field coupled resonant surfaces

Journal Article Applied Physics Letters · December 8, 2008 We present angle-resolved free-space transmission and reflection measurements of a surface composed of complementary electric inductive-capacitive (CELC) resonators. By measuring the reflection and transmission coefficients of a CELC surface with different ... Full text Cite

Optical source transformations.

Journal Article Optics express · December 2008 Transformation optics is a recently appreciated methodology for the design of complex media that control the propagation of electromagnetic and other types of waves. The transformation optical technique involves the use of coordinate transformations applie ... Full text Cite

Material parameters and vector scaling in transformation acoustics

Journal Article New Journal of Physics · November 27, 2008 The degree to which the coordinate transformation concept first demonstrated for electromagnetic waves can be applied to other classes of waves remains an open question. In this work, we thoroughly examine the coordinate transformation invariance of acoust ... Full text Cite

A dual-resonant terahertz metamaterial based on single-particle electric-field-coupled resonators

Journal Article Applied Physics Letters · November 21, 2008 We report the design, fabrication, and measurement of a terahertz metamaterial composed of single geometry electric field coupled resonators that has two closely spaced electric resonances near 1.0 and 1.5 THz. Due to the mutual coupling between the differ ... Full text Cite

Coordinated analysis of delayed sprites with high-speed images and remote electromagnetic fields

Journal Article Journal of Geophysical Research Atmospheres · October 27, 2008 Simultaneous measurements of high-altitude optical emissions and magnetic fields produced by sprite-associated lightning discharges enable a close examination of the link between low-altitude lightning processes and high-altitude sprite processes. We repor ... Full text Cite

Simultaneous remote electric and magnetic field measurements of lightning continuing currents

Journal Article Journal of Geophysical Research Atmospheres · October 27, 2008 Remote detection and measurement of the slowly varying continuing current that follows some lightning return strokes is difficult. Slow changes in quasi-static electric fields have been most frequently used to remotely measure continuing current. Because o ... Full text Cite

Dual-band planar electric THz metamaterial with resonator yield analysis

Journal Article Conference on Quantum Electronics and Laser Science (QELS) - Technical Digest Series · September 15, 2008 THz radiation transmission through a dual-band electric metamaterial is presented, where we analyze manufacturing defects in the metamaterial. Removing different resonator percentages allows us to quantify the effects of manufacturing defects on the materi ... Full text Cite

Dual-band planar electric THz metamaterial with resonator yield analysis

Journal Article 2008 Conference on Quantum Electronics and Laser Science Conference on Lasers and Electro-Optics, CLEO/QELS · September 15, 2008 THz radiation transmission through a dual-band electric metamaterial is presented, where we analyze manufacturing defects in the metamaterial. Removing different resonator percentages allows us to quantify the effects of manufacturing defects on the materi ... Full text Cite

Lumped element-based, highly sub-wavelength, negative index metamaterials at UHF frequencies

Journal Article Journal of Applied Physics · August 25, 2008 Several lumped element-based epsilon-negative (ENG), mu-negative (MNG), and double-negative (DNG) metamaterial designs have been developed for operation at 400 MHz in the UHF frequency band. The use of lumped elements enabled the creation of highly sub-wav ... Full text Cite

Supercelles and sprites

Journal Article Bulletin of the American Meteorological Society · August 1, 2008 Over a decade of monitoring mesospheric transient luminous events (TLEs) above U.S. high plains storms confirmed sprites are almost exclusively w associated with positive polarity cloud-to-ground lightning (+CGs). Following C. T. R. Wilson's theory propose ... Full text Cite

Rare measurements of a sprite with halo event driven by a negative lightning discharge over Argentina

Journal Article Geophysical Research Letters · July 28, 2008 As part of a collaborative campaign to investigate Transient Lummous Events (TLEs) over South America, coordinated optical, ELF/VLF, and lightning measurements were made of a mesoscale thunderstorm observed on February 22-23, 2006 over northern Argentina t ... Full text Cite

Dual-band planar electric metamaterial in the terahertz regime.

Journal Article Optics express · June 2008 We present the design, fabrication, and measurement of a dual-band planar metamaterial with two distinct electric resonances at 1.0 and 1.2 THz, as a step towards the development of frequency agile or broadband THz materials and devices. A method of defini ... Full text Cite

Compact dielectric particles as a building block for low-loss magnetic metamaterials.

Journal Article Physical review letters · May 2008 We characterize experimentally a compact dielectric particle that can be used to design very low-loss artificial electromagnetic materials (metamaterials). Focusing on magnetic media, we show that the particle can behave almost identically to the well-know ... Full text Cite

MARSIS subsurface radar investigations of the South Polar reentrant Chasma Australe

Journal Article Journal of Geophysical Research: Planets · April 20, 2008 The Mars Express/Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) has conducted the first-ever subsurface probing of the south polar layered deposits (SPLD) revealing internal structure that is rich in detail. We report on subsurface fe ... Full text Cite

Frequency tunable electromagnetic metamaterial using ferroelectric loaded split rings

Journal Article Journal of Applied Physics · April 8, 2008 Measurements of a frequency tunable magnetic metamaterial using metallic split rings loaded with barium strontium titanate thin film capacitors are presented. The resonant frequency of this medium is voltage tunable across a 140 MHz band centered at 1.75 G ... Full text Cite

Design of electromagnetic cloaks and concentrators using form-invariant coordinate transformations of Maxwell's equations

Journal Article Photonics and Nanostructures - Fundamentals and Applications · April 1, 2008 The technique of applying form-invariant, spatial coordinate transformations of Maxwell's equations can facilitate the design of structures with unique electromagnetic or optical functionality. Here, we illustrate the transformation-optical approach in the ... Full text Cite

Optical design of reflectionless complex media by finite embedded coordinate transformations.

Journal Article Physical review letters · February 2008 Transformation optics offers an unconventional approach to the control of electromagnetic fields. The transformation optical structures proposed to date, such as electromagnetic "invisibility" cloaks and concentrators, are inherently reflectionless and lea ... Full text Cite

Dual-band planar electric THz metamaterial with resonator yield analysis

Journal Article Optics InfoBase Conference Papers · January 1, 2008 THz radiation transmission through a dual-band electric metamaterial is presented, where we analyze manufacturing defects in the metamaterial. Removing different resonator percentages allows us to quantify the effects of manufacturing defects on the materi ... Cite

Dual-band planar electric THz metamaterial with resonator yield analysis

Journal Article Optics InfoBase Conference Papers · January 1, 2008 THz radiation transmission through a dual-band electric metamaterial is presented, where we analyze manufacturing defects in the metamaterial. Removing different resonator percentages allows us to quantify the effects of manufacturing defects on the materi ... Cite

Experimental demonstration of electromagnetic tunneling through an epsilon-near-zero metamaterial at microwave frequencies

Journal Article Physical Review Letters · January 2008 Silveirinha and Engheta have recently proposed that electromagnetic waves can tunnel through a material with an electric permittivity (epsilon) near zero (ENZ). An ENZ material of arbitrary geometry can thus serve as a perfect coupler between incoming and ... Cite

Scattering theory derivation of a 3D acoustic cloaking shell

Journal Article Physical Review Letters · January 2008 Through acoustic scattering theory we derive the mass density and bulk modulus of a spherical shell that can eliminate scattering from an arbitrary object in the interior of the shell-in other words, a 3D acoustic cloaking shell. Calculations confirm that ... Cite

Experimental demonstration of electromagnetic tunneling through an epsilon-near-zero metamaterial at microwave frequencies.

Journal Article Physical review letters · January 2008 Silveirinha and Engheta have recently proposed that electromagnetic waves can tunnel through a material with an electric permittivity (epsilon) near zero (ENZ). An ENZ material of arbitrary geometry can thus serve as a perfect coupler between incoming and ... Full text Cite

Scattering theory derivation of a 3D acoustic cloaking shell.

Journal Article Physical review letters · January 2008 Through acoustic scattering theory we derive the mass density and bulk modulus of a spherical shell that can eliminate scattering from an arbitrary object in the interior of the shell--in other words, a 3D acoustic cloaking shell. Calculations confirm that ... Full text Cite

Q-based design equations and loss limits for resonant metamaterials and experimental validation

Journal Article IEEE Transactions on Antennas and Propagation · January 1, 2008 Practical design parameters of resonant metamaterials, such as loss tangent, are derived in terms of the quality factor Q of the resonant effective medium permeability or permittivity. Through electromagnetic simulations of loop-based resonant particles, i ... Full text Cite

Active and Tunable RF Metamaterials

Conference PROCEEDINGS OF THE 2008 INTERNATIONAL WORKSHOP ON METAMATERIALS · January 1, 2008 Link to item Cite

Characterization of tunable metamaterial elements using MEMS switches

Journal Article IEEE Antennas and Wireless Propagation Letters · December 1, 2007 An experimental demonstration and analysis of a dual-state switchable magnetic metamaterial element in which the resonance excitation of the particle Is controlled using an RF microelectromechanical (MEMS) switch is presented. By controlling the resonant p ... Full text Cite

Applications of metamaterials in the GHz frequency domain

Journal Article IEEE Antennas and Propagation Society, AP-S International Symposium (Digest) · December 1, 2007 Full text Cite

Design and experimental demonstration of active RF metamaterials

Journal Article IEEE Antennas and Propagation Society, AP-S International Symposium (Digest) · December 1, 2007 Full text Cite

Low frequency lumped element-based negative index metamaterial

Journal Article Applied Physics Letters · November 7, 2007 A lumped element-based negative index metamaterial (NIM) was designed, fabricated, and tested for operation at 400 MHz in the ultrahigh frequency (UHF) band. At 400 MHz the measured real part of the index of refraction of this NIM was nreal =-3.11 with a l ... Full text Cite

Modeling elves observed by FORMOSAT-2 satellite

Journal Article Journal of Geophysical Research: Space Physics · November 1, 2007 The ISUAL experiment on the FORMOSAT-2 satellite has confirmed the existence of ionization and Lyman-Birge-Hopfield (LBH) band emissions in elves. In this paper, an in-depth study of the ISUAL recorded elves was carried out. Numerical simulation results of ... Full text Cite

Characterizing the effects of disorder in metamaterial structures

Journal Article Applied Physics Letters · October 31, 2007 We investigate the effects of disorder on metamaterial samples composed of split ring resonators with randomly introduced variation in their geometrical dimensions. We demonstrate that disorder broadens the negative permeability band and introduces effecti ... Full text Cite

Analysis of the first gigantic jet recorded over continental North America

Journal Article Journal of Geophysical Research Atmospheres · October 27, 2007 Two low-light cameras near Marfa, Texas, recorded a gigantic jet over northern Mexico on 13 May 2005 at approximately 0423:50 UTC. Assuming that the farthest of two candidate storm systems was its source, the bright lower channel ended in a fork at around ... Full text Cite

An architecture for active metamaterial particles and experimental validation at RF

Journal Article Microwave and Optical Technology Letters · October 1, 2007 We describe a class of metamaterial particle that employs active elements to overcome the inherent limitations of their passive counterparts, such as significant losses, high dispersion, and narrow bandwidth. The active unit cell can be designed to have ei ... Full text Cite

Halos generated by negative cloud-to-ground lightning

Journal Article Geophysical Research Letters · September 28, 2007 The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) on the FORMOSAT-2 spacecraft observes Transient Luminous Events (TLE) like sprites, elves, and halos from space. We analyzed halos that were observed in Central America close enough to ELF/VLF ... Full text Cite

Testing sprite initiation theory using lightning measurements and modeled electromagnetic fields

Journal Article Journal of Geophysical Research Atmospheres · July 16, 2007 Previous research has shown that the statistical measurements of charge moment changes in sprite-producing lightning are in general agreement with the predictions based on the conventional breakdown theory for sprite initiation in the mesosphere. Measureme ... Full text Cite

Broadband very low frequency measurement of D region ionospheric perturbations caused by lightning electromagnetic pulses

Journal Article Journal of Geophysical Research: Space Physics · June 1, 2007 Prompt early/fast perturbations on narrowband sub-ionospherically propagating very low frequency (VLF) signals are the primary evidence for the direct coupling of energy released by lightning discharge to the lower ionosphere. Different mechanisms have bee ... Full text Cite

The measured electric field spatial distribution within a metamaterial subwavelength cavity resonator

Journal Article IEEE Transactions on Antennas and Propagation · June 1, 2007 Through experimental investigation, a thin subwavelength cavity resonator was physically realized using a bilayer structure composed of air and a negative permeability metamaterial structure one unit cell in thickness. We designed and built the metamateria ... Full text Cite

One path to acoustic cloaking

Journal Article New Journal of Physics · March 2, 2007 A complete analysis of coordinate transformations in elastic media by Milton et al has shown that, in general, the equations of motion are not form invariant and thus do not admit transformation-type solutions of the type discovered by Pendry et al for ele ... Full text Cite

Design and experimental demonstration of active RE metamaterials

Conference 2007 IEEE ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL SYMPOSIUM, VOLS 1-12 · January 1, 2007 Link to item Cite

Redesign of the core curriculum at Duke University

Conference ASEE Annual Conference and Exposition, Conference Proceedings · December 1, 2006 Cite

A model of the ULF magnetic and electric field generated from a dust devil

Journal Article Journal of Geophysical Research: Planets · November 20, 2006 It has been demonstrated that terrestrial dust devils emit ULF magnetic radiation. On Mars, dust devils may also generate such magnetic emissions, which might be used as a hazard alert for manned missions. Specifically, grains in dust devils become charged ... Full text Cite

Metamaterial electromagnetic cloak at microwave frequencies.

Journal Article Science (New York, N.Y.) · November 2006 A recently published theory has suggested that a cloak of invisibility is in principle possible, at least over a narrow frequency band. We describe here the first practical realization of such a cloak; in our demonstration, a copper cylinder was "hidden" i ... Full text Cite

Simultaneous radio and satellite optical measurements of high-altitude sprite current and lightning continuing current

Journal Article Journal of Geophysical Research: Space Physics · October 1, 2006 We report coordinated measurements of lightning and resulting sprites using ground-level magnetic field sensors (<0.1 Hz to 30 kHz bandwidth) and the ISUAL instrument on the FORMOSAT-2 satellite. These measurements demonstrate two distinct elements of the ... Full text Cite

Derivation of effective parameters of magnetic metamaterials composed of passive resonant LC inclusions

Journal Article · September 28, 2006 We determine rigorously the effective permeability of magnetic metamaterials composed of passive particles exhibiting magnetic resonances. The effective permeability is expressed in terms of circuit parameters characteristic to these particles (L, C, and R ... Link to item Cite

Full-wave simulations of electromagnetic cloaking structures.

Journal Article Physical review. E, Statistical, nonlinear, and soft matter physics · September 2006 Pendry et al. have reported electromagnetically anisotropic and inhomogeneous shells that, in theory, completely shield an interior structure of arbitrary size from electromagnetic fields without perturbing the external fields. Neither the coordinate trans ... Full text Cite

Application of an orbital radar sounder model to detecting Martian polar subsurface features

Journal Article Journal of Geophysical Research: Planets · June 20, 2006 A model to simulate the complete planetary orbital radar sounder problem is presented in this paper. The subsurface features and the ionosphere will not only bring ambiguity to the collected radar sounder data, but also critically affect instrument capabil ... Full text Cite

Quasi-electrostatic field analysis and simulation of Martian and terrestrial dust devils

Journal Article Journal of Geophysical Research: Planets · June 20, 2006 Recent experimental and modeling studies show that large quasi-static electric fields (2-20 kV/m) can be developed in a Martian or terrestrial dust devil as a result of contact electrification and charge separation of dust grains with different sizes and c ... Full text Cite

Oxidant enhancement in martian dust devils and storms: implications for life and habitability.

Journal Article Astrobiology · June 2006 We investigate a new mechanism for producing oxidants, especially hydrogen peroxide (H2O2), on Mars. Large-scale electrostatic fields generated by charged sand and dust in the martian dust devils and storms, as well as during normal saltation, can induce c ... Full text Cite

Oxidant enhancement in martian dust devils and storms: storm electric fields and electron dissociative attachment.

Journal Article Astrobiology · June 2006 Laboratory studies, numerical simulations, and desert field tests indicate that aeolian dust transport can generate atmospheric electricity via contact electrification or "triboelectricity." In convective structures such as dust devils and dust storms, gra ... Full text Cite

An FDTD model for low and high altitude lightning-generated EM fields

Journal Article IEEE Transactions on Antennas and Propagation · May 1, 2006 To explore lightning-generated electromagnetic wave behavior and lightning-related ionospheric phenomena, a full-wave two-dimensional cylindrical finite-difference time-domain (FDTD) model was developed to simulate lightning-generated electromagnetic wave ... Full text Cite

Nighttime D region electron density profiles and variabilities inferred from broadband measurements using VLF radio emissions from lightning

Journal Article Journal of Geophysical Research: Space Physics · May 1, 2006 Lightning discharges radiate most of their electromagnetic energy in the very low frequency (VLF, 3-30 kHz) and extremely low frequency (ELF, 3-3000 Hz) bands and are, consequently, an effective tool for remotely sensing the nighttime ionospheric D region ... Full text Cite

An orthogonal projection and regularization technique for magnetospheric radio tomography

Journal Article Journal of Geophysical Research: Space Physics · March 1, 2006 A challenging problem in ill-posed inverse problems is incorporating prior knowledge of the solution into reconstruction techniques. This problem is particularly important in magnetospheric radio tomography where the path integrated measurements of the tar ... Full text Cite

Integration of electrostatic and fluid dynamics within a dust devil

Journal Article Journal of Geophysical Research: Planets · January 20, 2006 In this study we derive an analytical expression to describe the electrostatic field in a dust cloud with grain trajectories defined by fluid dynamics and grain charging via contact electrification. In order to solve for the electric field, we merge the fl ... Full text Cite

Redesign of the core curriculum at Duke University

Conference ASEE Annual Conference and Exposition, Conference Proceedings · January 1, 2006 Cite

Submillisecond imaging of sprite development and structure

Journal Article Geophysical Research Letters · 2006 We report intensified high speed video observations of two mesospheric transient luminous events acquired at 5000 and 7200 frames per second. Downward streamers appear to initiate either spontaneously or from brightening inhomogeneities at the bottom of a ... Full text Link to item Cite

Direct measurement of evanescent wave enhancement inside passive metamaterials.

Journal Article Physical review. E, Statistical, nonlinear, and soft matter physics · January 2006 Electric-field measurements inside a negative-permeability-positive-permittivity metamaterial composed of arrays of split-ring resonators directly show that the theoretically predicted enhancement of evanescent waves in passive materials is physically real ... Full text Cite

Measurements of lightning parameters from remote electromagnetic fields

Conference SPRITES, ELVES AND INTENSE LIGHTNING DISCHARGES · January 1, 2006 Link to item Cite

Lightning characteristics of the aurora, NE record hailstone-producing supercell of 22-23 June 2003 during BAMEX

Journal Article 85th AMS Annual Meeting, American Meteorological Society - Combined Preprints · December 1, 2005 Cite

Determining the effective electromagnetic properties of negative-refractive-index metamaterials from internal fields

Journal Article Physical Review B - Condensed Matter and Materials Physics · October 15, 2005 Effective electromagnetic properties of negative index of refraction metamaterials (NIMs) can be hard to measure. We show through simulations that electromagnetic fields inside a typical, physically realizable wire-SRR (split ring resonator) NIM are more h ... Full text Cite

Beta-type stepped leader of elve-producing lightning

Journal Article Geophysical Research Letters · July 16, 2005 The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) on the Taiwanese FORMOSAT-2 (formerly ROCSAT-2) spacecraft is the first global observatory of transient luminous events (TLEs). During the first 4 months of operation a large number of elves we ... Full text Cite

A flexible and robust direct reconstruction method for magnetospheric radio tomography

Journal Article Radio Science · May 1, 2005 [1] We present a flexible and robust direct reconstruction method for magnetospheric radio tomography. We show that for a combined reconstruction of plasma density Ne and magnetic field B, the direct reconstruction method performs as well as popular iterat ... Full text Cite

Broadband VLF measurements of lightning-induced ionospheric perturbations

Journal Article Geophysical Research Letters · April 28, 2005 Very low frequency (VLF) electromagnetic pulses radiated by lightning are an effective tool for probing the D region ionosphere. We detect and measure the D region ionospheric disturbances caused by the strong lightning flash by analyzing the broadband VLF ... Full text Cite

Measurements and implications of the relationship between lightning and terrestrial gamma ray flashes

Journal Article Geophysical Research Letters · April 28, 2005 We report observations and analysis of 30 kHz radio emissions (sferics) from lightning discharges associated with 26 terrestrial gamma ray flashes (TGFs) recorded by the RHESSI satellite over the Caribbean and Americas, between 1500 and 4000 km away from t ... Full text Cite

Implications of lightning charge moment changes for sprite initiation

Journal Article Journal of Geophysical Research: Space Physics · January 1, 2005 We report impulse lightning charge moment changes (defined as occurring in the first 2 ms after return stroke onset) in all cloud-to-ground lightning strokes detected by the National Lightning Detection Network in three storms during which above-thundersto ... Full text Cite

Theme-based redesign of the duke university ECE curriculum: The first steps

Conference ASEE Annual Conference and Exposition, Conference Proceedings · 2005 Undergraduates in Electrical and Computer Engineering (ECE) at Duke University have benefited from the combination of curricular flexibility and rigorous coursework. The current curriculum is further limited in that the core courses do not offer a vertical ... Cite

Perfectly matched layer behavior in negative refractive index materials

Journal Article IEEE Antennas and Wireless Propagation Letters · December 1, 2004 The perfectly matched layer (PML) absorbing boundary condition is shown to be analytically unstable for a material with a bandwidth of negative refractive index. Physically, this is a consequence of the reversed phase and group velocities in such a medium. ... Full text Cite

The nearly perfectly matched layer is a perfectly matched layer

Journal Article IEEE Antennas and Wireless Propagation Letters · December 1, 2004 An absorbing boundary condition for computational electromagnetics called the nearly perfectly matched layer (NPML) was recently introduced. This boundary condition, designed to be simple to implement in complex media, was regarded as not quite a perfectly ... Full text Cite

Event driven adaptive filtering

Journal Article Proceedings of the Mediterranean Electrotechnical Conference - MELECON · November 29, 2004 This paper introduces a new adaptive filter structure for a class of event-based signals corrupted by quasi-periodic noise. The event driven adaptive filter emerges from Widrow's adaptive filter, exploiting specific properties of event-based signals such a ... Cite

Wave fields measured inside a negative refractive index metamaterial

Journal Article Applied Physics Letters · November 15, 2004 Measured spatial variations of field phase and amplitude versus position inside a wire/split ring resonator negative refractive index metamaterial are presented that show that the phase velocity inside a negative index material points toward the source and ... Full text Cite

Electric and magnetic signatures of dust devils from the 2000-2001 MATADOR desert tests

Journal Article Journal of Geophysical Research: Planets · March 25, 2004 Dust devils are significant meteorological phenomena on Mars: They are ubiquitous, continually gardening the Martian surface, and may be the primary atmospheric dust-loading mechanism in nonstorm seasons. Further, dust grains in the swirling dust devils ma ... Full text Cite

Lightning charge moment changes in U.S. High Plains thunderstorms

Journal Article Geophysical Research Letters · March 16, 2004 We report measurements of impulsive (2 ms) lightning charge moment changes in more than 1000 cloud-to-ground (CG) return strokes detected by the National Lightning Detection Network in three United States High Plains storms during the Severe Thunderstorm E ... Full text Cite

Adaptive filter for event-based signal extraction

Journal Article Automatika (Croatia) · 2004 This paper introduces a new adaptive filter structure for a class of event-based signals corrupted by quasi-periodic noise. The event driven adaptive filter emerges from Widrow's adaptive filter, exploiting specific properties of event-based signals such a ... Cite

A simple electrodynamic model of a dust devil

Journal Article Geophysical Research Letters · October 15, 2003 We present an electrodynamic model of a dust devil applying a similar methodology as performed previously for charging in terrestrial thunderstorms. While thunderstorm processes focus on inductive charging between large graupel and smaller ice and water dr ... Full text Cite

Characteristics of sprite-producing positive cloud-to-ground lightning during the 19 July 2000 STEPS mesoscale convective systems

Journal Article Monthly Weather Review · October 1, 2003 During the summer of 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) program deployed a three-dimensional Lightning Mapping Array (LMA) near Goodland, Kansas. Video confirmation of sprites triggered by lightning within storms ... Full text Cite

A new FDTD model for lightning generated EM wave simulation

Journal Article IEEE Antennas and Propagation Society, AP-S International Symposium (Digest) · September 1, 2003 A new full-wave finite-difference time domain method (FDTD) model was developed to simulate lightning-generated electromagnetic wave propagation in the ionosphere. A new PML technique (NPML) is applied in this model, which performs better than the classic ... Cite

Applying the nonuniform FFT to the parabolic equation split-step algorithm for high frequency propagation modeling

Journal Article IEEE Antennas and Propagation Society, AP-S International Symposium (Digest) · September 1, 2003 By using the NUFT and NU-IFFT in split-step PE to simulate high frequency troposphere radio propagation, more points could be put into the regions of interest in order to improve local accuracy. Conversely, points can be removed, and only solutions could b ... Cite

Dynamics of causal beam refraction in negative refractive index materials

Journal Article Applied Physics Letters · March 31, 2003 Dynamics of causal beam refraction in negative refractive index materials was studied. A 10-GHz source frequency was chosen to facilitate comparison with experiments with negative index. The interaction of a causal, finite beam with an interface of a posit ... Full text Cite

Simulated causal subwavelength focusing by a negative refractive index slab

Journal Article Applied Physics Letters · March 10, 2003 A finite-difference simulation and analysis of the Maxwell equations for the predicted perfect focusing geometry was reported. Subwavelength electromagnetic focusing by a negative refractive index slab was shown to occur in a full wave numerical simulation ... Full text Cite

A simple, nearly perfectly matched layer for general electromagnetic media

Journal Article IEEE Microwave and Wireless Components Letters · March 1, 2003 A new implementation of the perfectly matched layer (PML) absorbing boundary condition is presented. This formulation is designed such that the partial differential equations in the PML are identical to those in the regular medium for any linear electromag ... Full text Cite

Current moment in sprite-producing lightning

Journal Article Journal of Atmospheric and Solar-Terrestrial Physics · January 1, 2003 Studies of sprite-producing lightning have revealed much of what we currently know about the mechanisms responsible for this phenomenon. With a combination of new and previous results, we summarize the currently known quantitative characteristics of this s ... Full text Cite

Advances in magnetospheric radio wave analysis and tomography

Journal Article Advances in Space Research · January 1, 2003 Initial theoretical studies of multi-spacecraft radio tomographic imaging of the magnetosphere have shown the potential scientific value of the technique. We report a series of multistatic radio propagation experiments with the goal of testing and verifyin ... Full text Cite

Monitoring the monsoon in the Himalayas: Observations in Central Nepal, June 2001

Journal Article Monthly Weather Review · 2003 The Monsoon Himalayan Precipitation Experiment (MOHPREX) occurred during June 2001 along the south slopes of the Himalayas in central Nepal. Radiosondes were launched around the clock from two sites, one in the Marsyandi River basin on the eastern footslop ... Full text Link to item Cite

Lightning charge moment changes for the initiation of sprites

Journal Article Geophysical Research Letters · April 15, 2002 The transient ELF (~50-5000 Hz) magnetic field radiated by lightning discharges across North America was continously measured at Duke University during the summer of 2000. In total, 881 sprite-associated lightning discharge over 17 days were analyzed. We r ... Cite

Lightning charge moment changes for the initiation of sprites

Journal Article Geophysical Research Letters · April 15, 2002 The transient ELF(∼50-5000 Hz) magnetic field radiated by lightning discharges across North America was continuously measured at Duke University during the summer of 2000. In total, 881 sprite-associated lightning discharges over 17 days were analyzed. We ... Full text Cite

Auroral electron distributions derived from combined UV and X-ray emissions

Journal Article Journal of Geophysical Research: Space Physics · November 1, 2001 The Polar Ionospheric X-ray Imaging Experiment and the Ultraviolet Imager on board the Polar satellite provide the first simultaneous global scale views of the electron precipitation over a wide range of electron energies. By combining the results from the ... Full text Cite

A test magnetospheric radio tomographic imaging with IMAGE and WIND

Journal Article Geophysical Research Letters · March 15, 2001 Theoretical studies have shown the potential scientific value of multi-spacecraft radio tomographic imaging of the magnetosphere. The <10 RE WIND perigee passes during August 2000 afforded a unique opportunity to test and verify the capabilities of radio t ... Full text Cite

Unusually intense continuing current in lightning produces delayed mesospheric breakdown

Journal Article Geophysical Research Letters · February 1, 2001 Ultra low frequency magnetic field measurements made 500-2000 km from positive lightning discharges show a signature that is consistent with unusually high amplitude cloud-to-ground continuing lightning current. The magnitude of this nearly constant curren ... Full text Cite

Ionospheric E region remote sensing with ELF radio atmospherics

Journal Article Radio Science · November 1, 2000 Radio remote sensing of the ionospheric E region can be difficult, particularly in the nighttime E region valley which lies between two regions of higher electron density. We show that extremely low frequency (ELF) electromagnetic waves launched from below ... Full text Cite

Cause of the localized maximum of X-ray emission in the morning sector: A comparison with electron measurements

Journal Article Journal of Geophysical Research: Space Physics · September 1, 2000 The Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the Polar satellite has provided the first global scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung. While other remote sensing tech ... Full text Cite

Modeling electromagnetic propagation in the Earth-ionosphere waveguide

Journal Article IEEE Transactions on Antennas and Propagation · September 1, 2000 The ionosphere plays a role in radio propagation that varies strongly with frequency. At extremely low frequency (ELF: 3-3000 Hz) and very low frequency (VLF: 3-30 kHz), the ground and the ionosphere are good electrical conductors and form a spherical eart ... Full text Cite

Detection of daytime sprites via a unique sprite ELF signature

Journal Article Geophysical Research Letters · March 15, 2000 On August 14, 1998, 3 separate daytime sprite events were detected via a unique extremely low frequency (ELF) sprite signature. The onset of the sprite ELF signatures was delayed by 11.0-13.2 ms from positive cloud-to-ground strokes which had attained exce ... Full text Cite

Global multispectral auroral imaging of an isolated substorm

Journal Article Geophysical Research Letters · March 1, 2000 We analyze simultaneous visible, ultraviolet (UV), and X-ray auroral images of an isolated substorm on January 25, 1998. The total precipitating electron energy flux and mean energy extracted near substorm maximum from each of the imagers are in acceptable ... Full text Cite

Global multispectral auroral imaging of an isolated substorm

Journal Article Geophysical Research Letters · March 1, 2000 We analyze simultaneous visible, ultraviolet (UV), and X-ray auroral images of an isolated substorm on January 25, 1998. The total precipitating electron energy flux and mean energy extracted near substorm maximum from each of the imagers are in acceptable ... Full text Cite

Modeling ELF radio atmospheric propagation and extracting lightning currents from ELF observations

Journal Article Radio Science · March 1, 2000 Observations of extremely low frequency (ELF) radio atmospherics (sferics), the transient electromagnetic fields radiated by lightning discharges, are used to determine the current moment waveforms of vertical lightning discharges. In order to extract this ... Full text Cite

Global X-ray emission during an isolated substorm - A case study

Journal Article Journal of Atmospheric and Solar-Terrestrial Physics · January 1, 2000 The polar ionospheric X-ray imaging experiment (PIXIE) and the UV imager (UVI) onboard the Polar satellite have provided the first simultaneous global scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrah ... Full text Cite

The Harang discontinuity in auroral substorms

Conference · January 1, 2000 We have investigated the electrodynamics of the pre-midnight convection reversal during substorm expansion phase through early recovery phase. This is done with measurements of the magnetic field, electric field, ion drift and electron precipitation made b ... Full text Cite

Auroral surge currents and electrodynamics with FAST and VIS

Conference · January 1, 2000 We analyze data from two FAST satellite passes in the vicinity of auroral surges (as seen in the VIS auroraJ imager on the Polar satellite) to study the large scale ionospheric electrodynamics of the surge, focusing in particular on the configuration of fi ... Full text Cite

Sprites triggered by negative lightning discharges

Journal Article Geophysical Research Letters · December 15, 1999 High altitude air breakdown, manifested as 'red sprites,' is reported in close association with negative cloud-to-ground lightning (-CG) on at least two occasions above an unusual storm on August 29, 1998. Data from high speed photometry, low-light-level v ... Full text Cite

Submillisecond resolution lightning currents and sprite development: Observations and implications

Journal Article Geophysical Research Letters · October 15, 1999 We analyze synchronized high speed video images and ELF-VLF radio emissions from 11 sprite clusters observed on 6 October 1997. Quantitative analysis shows that vertical lightning charge moment changes of 150-1100 C·km occurred before the optical emissions ... Full text Cite

Radio atmospheric propagation on Mars and potential remote sensing applications

Journal Article Journal of Geophysical Research: Planets · June 25, 1999 We theoretically analyze the propagation of very low frequency (VLF) and extremely low frequency (ELF) electromagnetic energy in the spherical waveguide formed by the ground and ionosphere of Mars to investigate the possibility of using such signals to rem ... Full text Cite

Global-scale electron precipitation features seen in UV and X rays during substorms

Journal Article Journal of Geophysical Research: Space Physics · May 1, 1999 The Polar Ionospheric X-ray Imaging Experiment (PIXIE) and the ultraviolet imager (UVI) onboard the Polar satellite have provided the first simultaneous global-scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray b ... Full text Cite

Detecting electrical activity from Martian dust storms

Journal Article Journal of Geophysical Research: Planets · February 25, 1999 We present a model addressing the possible electrification of Martian dust storms based on the effective electrical charging of an individual dust grain. An upper charge bound on a grain can be determined based on the grain capacitance in the low-pressure ... Full text Cite

ELF radiation produced by electrical currents in sprites

Journal Article Geophysical Research Letters · April 15, 1998 Measurements of ELF-radiating currents associated with sprite-producing lightning discharges exhibit a second current peak simultaneous in time with sprite luminosity, suggesting that the observed ELF radiation is produced by intense electrical currents fl ... Full text Cite

Ionospheric D region remote sensing using VLF radio atmospherics

Journal Article Radio Science · January 1, 1998 Lightning discharges radiate the bulk of their electromagnetic energy in the very low frequency (VLF, 3-30 kHz) and extremely low frequency (ELF, 3-3000 Hz) bands. This energy, contained in impulse-like signals called radio atmospherics or sferics, is guid ... Full text Cite

Midlatitude particle and electric field effects at the onset of the November 1993 geomagnetic storm

Journal Article Journal of Geophysical Research: Space Physics · January 1, 1998 Millstone Hill incoherent scatter radar elevation scans across midlatitudes captured the ionospheric response to storm-induced electric field and precipitation-induced changes near the equatorward extent of the expanding auroral region during the early pha ... Full text Cite

An Analysis of New and Existing FDTD methods for isotropie cold plasma and a method for improving their accuracy

Journal Article IEEE Transactions on Antennas and Propagation · December 1, 1997 Over the past few years, a number of different finite-difference time-domain (FDTD) methods for modeling electromagnetic propagation in an isotropic cold plasma have been published. We have analyzed the accuracy and stability of these methods to determine ... Full text Cite

Measurement of charge transfer in sprite-producing lightning using ELF radio atmospherics

Journal Article Geophysical Research Letters · January 1, 1997 Transient high altitude optical emissions referred to as "sprites" are believed to occur as a result of the transfer of large amounts of charge (∼100-300 C) from cloud altitudes of 5-10 km to the ground. Using a general subionospheric ELF propagation model ... Full text Cite

VLF remote sensing of high-energy auroral particle precipitation

Journal Article Journal of Geophysical Research: Space Physics · January 1, 1997 Ground-based measurements of VLF transmitter signals propagating in the Earth-ionosphere waveguide can be used to determine the location of nighttime high-energy (≳100 keV) auroral particle precipitation. When the region of auroral particle precipitation p ... Full text Cite

LF and MF observations of the lightning electromagnetic pulse at ionospheric altitudes

Journal Article Geophysical Research Letters · January 1, 1997 For the first time the full ionospheric signature of the lightning electromagnetic pulse (EMP) was measured up to a frequency of 2 MHz. At altitudes below 225 km, the upward-going whistler wave is found to have a nose-whistler wave shape with the fastest p ... Full text Cite

VLF remote sensing of the auroral electrojet

Journal Article Journal of Geophysical Research: Space Physics · March 1996 We investigate a new potential technique to determine the position of the auroral electrojet from ground‐based VLF amplitude and phase measurements. The chief advantage of this technique over conventional ground magnetometer measurements is that it ... Full text Cite

VLF signatures of ionospheric heating by HIPAS

Journal Article Radio Science · January 1, 1995 The amplitude and phase of subionospherically propagating VLF signals are known to be sensitive indicators of the electron density and temperature at D region altitudes. In this paper we present new observations at a station in Alaska positioned to provide ... Full text Cite

Another new FDTD method for linear dispersive media - but this one's the best yet

Journal Article IEEE Antennas and Propagation Society, AP-S International Symposium (Digest) · January 1, 1995 We introduce a new FDTD method for general linear dispersive media (dielectrics and non-magnetized cold plasmas) based on exponential fitting of the auxiliary difference equation. We show that the recursive convolution method, developed by Luebbers el al. ... Cite

Dispersion Tuning and Route Reconfiguration of Acoustic Waves in Valley Topological Phononic Crystals

Journal Article Nature Communications The valley degree of freedom in crystals offers great potential for manipulating classical waves; however, few studies have investigated valley states with complex wavenumbers, valley states in graded systems, or dispersion tuning for valley states. Here, ... Cite