Very low frequency radio events with a reduced intensity observed by the low‐altitude DEMETER spacecraft

Abstract

AbstractWe present results of a systematic study of unusual very low frequency (VLF) radio events with a reduced intensity observed in the frequency‐time spectrograms measured by the low‐orbiting Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft. They occur exclusively on the nightside. During these events, the intensity of fractional hop whistlers at specific frequencies is significantly reduced. These frequencies are usually above about 3.4 kHz (second Earth‐ionosphere waveguide cutoff frequency), but about 20% of events extend down to about 1.7 kHz (first Earth‐ionosphere waveguide cutoff frequency). The frequencies of a reduced intensity vary smoothly with time. We have inspected 6.5 years of DEMETER data, and we identified in total 1601 such events. We present a simple model of the event formation based on the wave propagation in the Earth‐ionosphere waveguide. We apply the model to two selected events, and we demonstrate that the model is able to reproduce both the minimum frequencies of the events and their approximate frequency‐time shapes. The overall geographic distribution of the events is shifted by about 3000 km westward and slightly southward with respect to the areas with high long‐term average lightning activity. We demonstrate that this shift is related to the specific DEMETER orbit, and we suggest its qualitative explanation by the east‐west asymmetry of the wave propagation in the Earth‐ionosphere waveguide.