Elves Accompanying Terrestrial Gamma Ray Flashes

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-cloud pulses (EIPs), associated with some TGFs can produce a current moment waveform with a peak of hundreds of kA km and a duration of 10 μs. Simulations using the source current moment waveform associated with a Fermi TGF indicate that the radiated electric field at ionospheric altitudes reaches a few times the threshold electric field to excite the optical emissions. A bright elve is therefore induced, with the intensity reaching tens of Megarayleigh, comparable to the brightest elves caused by cloud-to-ground lightning. Because of the strong electromagnetic field radiated, significant blue emissions from the second positive band system of N2 and the first negative band system of N⁺2 are excited, besides the dominant red emissions from the first positive band system of N2. The elves caused by EIPs with durations of ∼10 μs are elve doublets. For EIPs of longer durations, for example, 30–40 μs, elve multiplets greater than two can be produced. We conclude that elves can be produced by an IC lightning process previously unconnected to elves and that at least some TGFs should have accompanying optical signatures in the lower ionosphere. In addition, TGFs of short durations are more likely to have accompanying elves, because their source currents vary more rapidly.

Duke Scholars

Author Steven A. Cummer Electrical and Computer Engineering