Early/fast VLF events produced by electron density changes associated with sprite halos

Abstract

[1] The magnitudes of scattered fields produced during early/fast very low frequency (VLF) events observed at 13 closely spaced ($65 km) sites are compared with those expected for sprite halo disturbances using a numerical model of wave propagation within the Earth-ionosphere waveguide. Three different early/fast events of varying magnitudes are analyzed using three different nighttime ambient lower ionospheric electron density profiles. The electron density disturbances associated with sprite-halo events are determined using a full-wave electromagnetic (FWEM) model. Observed scattered field amplitudes of typical (VLF amplitude changes of 0.2 dB < ÁA < 0.8 dB) Early/fast events agree with model calculations within a factor of two when the peak return stroke currents (as recorded by the National Lightning Detection Network, referred to in this work as NLDN) are used to determine the sprite-halo characteristics. Scattered field amplitudes associated with larger early/fast events (ÁA > 1 dB) are found to be within a factor of seven for peak currents of causative lightning based on NLDN. However, in previous studies, some sprite-producing lightning flashes have exhibited large slow-tail components, indicating substantial continuing currents and implying charge removal up to 2-3 times larger than that inferred from the peak current reported by NLDN. For the cases discussed in this paper, scattered field calculations using disturbances caused by 2-3 times larger charge removal are found to be within a factor of two of the measured values. VLF scattering from electron density changes associated with sprite halos thus appear to be the underlying cause of at least some of the VLF perturbations observed as early/fast events.

VLF Studies During TLE Occurrences in Europe: A Summary of New Findings

“…This, in addition to the observed strong forward scattering, suggests that the early VLF perturbations observed on Crete in association with sprites relate to electron density enhancements corresponding to the upper diffuse sprite regions and/or associated halos in line with the findings of Barrington-Leigh et al (2001) and Moore et al (2003).…”

“…within one sampling interval of the receiver) followed by relatively long signal recoveries (10 to more than 100 s). Although the early VLF perturbations have been studied extensively over the last 15 years, mostly by the VLF group at Stanford University (e.g., see Johnson and Inan 2000;Moore et al 2003), and also by University of Otago researchers (e.g., Dowden et al 1996b;Dowden et al 1996a), their complex properties are still not well understood. Early VLF events were reported to occur in association with both negative and positive CG discharges and were observed to accompany a subset of sprites (Inan et al 1995).…”

Subionospheric VLF signatures and their association with sprites observed during EuroSprite-2003

“…At first, the perturbations were categorized as ''early Trimpis'' in order to contrast with the long-delayed classic Trimpis caused by whistler-induced precipitation of radiation belt electrons. The explanation of early VLF events relies on two different, but not necessarily independent, processes: (1) heating of the lower ionosphere by strong quasi-electrostatic fields generated by lightning (Inan et al, 1991(Inan et al, , 1996aPasko et al, 1995), and (2) ionization production during transient luminous events (TLEs), such as sprites, sprite halos and elves (e.g., see Moore et al, 2003;Rodger, 2003 and more references therein).…”

Investigating radiation belt losses though numerical modelling of precipitating fluxes

“…The latter perturbations appear to be caused by much faster processes occurring inside and above thunderstorms, e.g. red sprites, elves, and sprite haloes (Rodger, 1999;Moore et al, 2003).…”