Ground and Space Signatures of VLF Noise Suppression by Whistlers

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. 2020. Vol. 125. P. 1–14.

Shklyar D.

VLF (VeryLow Frequency) spectrograms registered at Kannuslehto ground station, after

cleaning them from strong sferics, reveal VLF noise suppression by whistlers and whistler echo trains,

which consists in significant reduction in the noise spectral power after a strong whistler event.We have

found similar effect in the VLF data from Van Allen Probe B taken in the equatorial region on L-shell ∼3.

Detailed analysis of the data shows that the whistler echo train and the VLF noise have small wave normal

angles. Based on this observation, we limit our analysis to parallel (ducted) whistler wave propagation. The

persistence of whistler echo train, as well as the VLF noise, suggests that in the events under discussion,

plasma is unstable in the frequency range corresponding to the observed VLF noise band. In an attempt to

explain the effect of VLF noise suppression, we follow up the long-standing idea that relates this effect to

the reduction of free energy in the unstable plasma distribution by whistler echo train. To develop this idea

into qualitative model, we have studied the motion of energetic electrons, responsible for the noise

generation, in the field of ducted whistler echo train.We show that energetic electrons that make the main

contribution to the growth rate of VLF noise, during their bounce oscillations in the magnetosphere, are

subject to multiple resonant impacts from the whistler echo train. These lead to energetic electron

diffusion in the phase space and the corresponding reduction in free energy of the unstable distribution.

Keywords: resonant interaction, резонансное взаимодействие, whistlers, свисты, whistler wave growth rates, VLF noise, particle diffusion in phase space, ОНЧ шум, инкременты волн, диффузия частиц в фазовом пространстве