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

Abstract

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 < ΔA < 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 (ΔA > 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.