Elves: Lightning‐induced transient luminous events in the lower ionosphere

Abstract

Observations of optical phenomena at high altitude above thunderstorms using a multichannel high‐speed photometer and image intensified CCD cameras were carried out at Yucca Ridge Field Station (40°40′N, 104°, 56′W), Colorado as part of the SPRITES'95 campaign from 15 June to August 6, 1995. These new measurements indicate that diffuse optical flashes with a duration of < 1 ms and a horizontal scale of ∼100–300 km occur at 75–105 km altitude in the lower ionosphere just after the onset of cloud‐to‐ground lightning discharges, but preceding the onset of sprites. Here we designate these events as “elves” to distinguish them from “red sprites”. This finding is consistent with the production of diffuse optical emissions due to the heating of the lower ionosphere by electromagnetic pulses generated by lightning discharges as suggested by several authors.

Criteria for sprites and elves based on Schumann resonance observations

“…Distinct differences in the lightning source characteristics appear to reflect the differences in optical characteristics noted in other studies [Fukunishi et al, 1996;Lyons, 1996b;Watanabe, 1999].…”

“…33D] went on to discuss conditions at 60 km altitude, which is the typical initiation height for sprites found in recent observations [Fukunishi et al, 1996 While the electric force due to a thundercloud falls off rapidly as r increases, the electric force required to cause sparking (which for a given composition of the air is proportional to density)…”

Elves triggered by positive and negative lightning discharges

“…•1 ms) flashes, or 'elves,' were subsequently observed [Fukunishi et al, 1996] in association with rare, large positive cloud-to-ground (+CG) discharges during nighttime sprite observations, and their spario-temporal development was shown to be consistent with the theorized cause [Inan et al, 1997].…”

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

“…This filtering simplifies modeling of the propagation effects, but also eliminates any information about the lightning current on time scales shorter than --0.5 ms. Since sprites are usually observed at least I ms after the source lightning stroke [Winckler et al, 1996;Fukunishi et al, 1996], this shorter time scale information is relatively unimportant for our purposes. Low-light level video observations were made at Yucca Ridge, Colorado during the same period in order to determine the times of sprite occurrence and associate particular sferics with the observed sprites.…”