Satellite Observation of Low-Latitude VLF Radio Noises and Their Association with Thunderstorms

Abstract

VLF data from the low-altitude Ariel 4 satellite are used to investigate the characteristics and generation (propagation) mechanism of low- and equatorial-latitude VLF radio noises. Low-latitude emissions are defined as VLF noises whose mean (running mean during -30 sec) intensity exceeds 40 dB above 4.8×10^-19W m^-2 Hz^-1(free space equivalent) at the frequency of 3.2kHz (Δf=1kHz) in each 28sec sampling interval in the magnetic latitude range less than 30°. Three months' data (December 11, 1971 to March 19, 1972) have been used to obtain a full local time coverage. It is then found that the noises are impulsive at that frequency as based on the high values of the observed mean/minimum and peak/mean ratios, and also that they are localized to the longitudes of thunderstorm centers. Hence, low-latitude VLF emissions, as observed in the southern hemisphere for our season, have been found to be well explained by the propagation of sferics to the lowaltitude satellite as short-fractional-hop whistlers; those observed in the northern hemisphere are attributed to the interhemisphere propagation of those sferics in the whistler mode. Furthermore, diurnal variations of the occurrence rate, intensity, spectral property and occurrence latitude are studied, and many of these characteristics are found to be compatible with the ionospheric D-region absorption of whistler-mode propagation in the southern hemisphere. Finally, the satellite VLF data as presented here are suggested as being useful for the study of the global distribution of lightning activity.