4.4 Distribution of elves throughout large storm systems

Elves are optical signatures of strong energetic coupling of lightning EMP to a narrow altitude range in the $D$ region of the ionosphere. Detecting elves in association with negative lightning discharges implies the ubiquity of this phenomenon, since negative CG discharges are known to be much more common than positive discharges [e.g., Orville, 1994]. Indeed, our results indicate that nearly all discharges with EMP intensity above a certain threshold may trigger elves. Our results further indicate that the spatial extent of the ionospheric disturbance from a single discharge is as large as anticipated in Figure 2.5. While the optical emissions in elves are expected to be strongly dependent on the strength of the causative discharge, transient electron heating should occur for smaller discharges which may not produce detectable optical output.

Figure 4.14: Horizontal extents of optical emissions in 38 elves from one mesoscale convective system.

Moreover, the possibility of the superposition of electron density changes due to successive CG strokes described by Taranenko et al. [1993a] and in Section 2.5.3 seems cogent given the large area ($\sim$ $3\times 10^5$ km$^2$) shown here to be affected by a single EMP. Ionization changes decay over time scales on the order of 10 to 100 s in the $D$ region (see Section 1.3), so that the accumulated effect of successive strong cloud-to-ground strokes occurring at different points in a large storm system may profoundly affect the nighttime $D$ region. As an example, within a 770 km length of the Mexican mesoscale convective system of August 27, NLDN recorded 310 CGs with peak current greater than 45 kA during the period 03:00 to 10:00 UT (an average of one per 80 s), with much more intense local clustering during some periods.