Quasi-periodic ELF/VLF emissions with atypical time structure inside the plasmasphere

Abstract

Some results of observations on board the Van Allen probe A spacecraft of atypical quasi-periodic VLF electromagnetic emissions inside the plasmasphere are discussed. We study in detail two events (December 6, 2015 and November 24, 2015) of the generation of deeply modulated quasi-periodic VLF emissions (QP) in the frequency band below 2kHz, when separate relatively short (on the order of 20 and 60 s) bursts of hiss without any notable frequency drift repeated with a period of 90 and 240s. The emissions selected for analysis were not accompanied by a hydromagnetic compression wave, which indicates the impossibility of their interpretation in terms of the concept of QP 1 modulated hiss. For the considered events, no clear periodicity of spectral bursts typical for QP 2 emissions take place. The dynamic spectra analysis makes it possible to conclude that the cyclotron generation of electromagnetic emissions occurs in the regions of the electron radiation belts with relatively dense cold plasma. The time course of these emissions can be explained under the assumption of a periodic modification of the conditions for the reflection of whistler wave incident on the ionosphere from above. Based on the available data and calculations, it was concluded that it is possible to interpret the analysed atypical quasi-periodic VLF emissions as a result of the resonant parametric action of ionospheric infrasonic waves with periods close to the repetition period of spectral bursts in quasi-periodic emissions.