The meteorological and electrical structure of TLE-producing convective storms
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 common of the transient luminous events (TLEs)-sprites (often preceded by halos.) As iΔMq values grow larger than 100 C km, +CGs have a rapidly increasing probability of producing mesospheric sprites. If the iΔMq value of a +CG is >300 C km, there is a >75-80% chance this CG stroke initiates a sprite. Curiously, while negative iΔMq values of this size are much less common, they do occur. Yet on only a rare occasions have-CGs been documented to initiate a sprite over continental stroms (the so-called polarity paradox). The total charge moment change required to initiate sprites is believed to be at least ∼500 C km. Also, the great majority of sprite initiations are delayed after the return stroke by much more than the 2 ms time period used in the iΔMq estimates. This suggests that while both positive and negative CGs may have relatively large iΔMq values, due to the relatively low amperage continuing currents in the negative discharges, only +CGs have large enough continuing currents to routinely reach breakdown values and initiate sprites. While both CG polarities can theoretically initiate sprites, perhaps a somewhat higher breakdown threshold may exist for-CGs, and/or reduced streamer development makes them more difficult to detect optically? Preliminary climatologies of iΔM q for the U.S. are presented. The technique employed in the U.S. utilizes the National Lightning Detection Network for geolocation, allowing placement of >80-90% of sprite parent +CGs. Global lightning location systems such as the Worldwide Lightning Location Network (WWLLN) appear to detect approximately 25% of the CGs producing U.S. sprites, suggesting the possibility of employing such systems elsewhere. © Springer Science+Business Media B.V. 2009.
