Intense low latitude VLF emissions observed aboard Ariel 4

Abstract

Analysis of the peak intensity of low‐latitude VLF emissions observed aboard the Ariel satellite reveal that some north zone 3.2 kHz VLF emissions are unexpectedly more intense than south zone VLF emissions. In fact, the north zone intensity peaks are 5–10 dB higher than those in the south zone. This discrepancy in intensity peaks can be explained in terms of wave growth during transverse resonant interaction between the wave and counterstreaming energetic electrons. It is argued that VLF signals observed on board the Ariel satellite were generated in the south zone during lightning discharges and propagated to equatorial heights in the whistler mode. At the equator, the emissions are amplified by taking energy from energetic electrons, and then these amplified emissions, again propagating in the whistler mode, arrive at the Ariel satellite in the north zone. Thus the intense signals in the south zone are preamplified, and in the north zone, these are observed after amplification. This explains the discrepancy in the observations of intensity. Analysis of VLF satellite data reveals that VLF emissions have high spectral densities in the longitudinal range of 100°–140°E. These phenomena of VLF emissions are explained through the effect of the South Pacific Magnetic Anomaly (SPMA). It is found that atmospheric scattering, enhancing precipitated fluxes of energetic electrons in the South Atlantic Magnetic Anomaly (SAMA) region, has no role in VLF wave amplification.