VLF and Ionospheric D-Region Perturbations Associated With WWLLN-Detected Lightning in the South Pacific Region

The subionospheric early very low frequency (VLF) perturbations observed on NWC (19.8 kHz) navigational transmitter signal monitored at a low-latitude station, Suva (18.1 degrees S, 178.5 degrees E), Fiji, during campaign periods of November 2011, 2012, and 2014 and December 2014, are presented. Early VLF events are associated with D-region conductivity changes mainly produced by lightning-generated transient luminous events (TLEs). Early VLF events occurred both during daytime and nighttime, with a considerably higher occurrence at nighttime. VLF perturbations caused by lightning strokes located up to 100 km off the transmitter-receiver great circle path (TRGCP) are attributed to narrow-angle scattering, while lightning strokes 100-500 km off the TRGCP are considered to cause early VLF events by wide-angle scattering. Using the World Wide Lightning Location Network data, for the first time, we have studied the relationship between the energy of lightning strokes and the level of VLF perturbations. Greater is the energy of lightning, greater would be the strength of the VLF perturbation. However, the low-energy lightning stroke can also produce a comparable level of perturbation to that of strong lightning. The modeling results of scattered amplitude (M) and echo phase (phi(E)) of the unusually long recovery early/fast VLF event showed a better exponential fit (r similar to 0.9) than the logarithmic fit. Long-wavelength propagation capability (LWPC) code modeling of nighttime early VLF events considering causative TLE width of 50 km column indicated a decrease in the D-region reference height (H') by up to 30 km and an increase in the sharpness factor (beta) by 0.25 km(-1).