Growth rates and Poynting flux directions of VLF chorus emissions in their source region as inferred from THEMIS data\n

0465196 - ÚFA 2017 ZA eng A - Abstract
Demekhov, A. G. - Taubenschuss, Ulrich - Santolík, Ondřej
Growth rates and Poynting flux directions of VLF chorus emissions in their source region as inferred from THEMIS data
.
7th VERSIM-RBS: Abstracts. Hermanus: SANSA, 2016. s. 34.
[VERSIM and RadBelt 2016 /7./. 19.09.2016-24.09.2016, Hermanus]
Institutional support: RVO:68378289
Keywords : VLF chorus emissions * magnetospheric physics * plasmasphere * THEMIS spacecraft
Subject RIV: BL - Plasma and Gas Discharge Physics
https://events.sansa.org.za/images/2016/VERSIM/abstracts/VERSIM_2016_abstracts.pdf

We present results of observations of VLF chorus emissions based on several events
detected by THEMIS spacecraft within the source region. Using energetic particle
distributions measured onboard the spacecraft, we calculate the whistler-mode growth
rates and compare them with those for the observed chorus elements. We find that in a
typical situation the measured energetic particle distribution does not explain the
measured growth rates, which may indicate that the actual velocity space gradients of the
energetic electron distribution are larger than the particle detector can resolve.
Using THEMIS multicomponent wave data on parallel propagating chorus elements
detected at some distance from the magnetic field minimum, we show that the elements
propagating equatorward had systematically higher frequencies and smaller amplitudes
compared with simultaneously observed elements propagating away from the equator.
The exponential growth of the elements propagating in both directions had close values.
We propose an explanation of the observed feature on the basis of the evolution of
energetic electron distribution function in the course of VLF wave generation. The motion
of electrons from the equator is accompanied by a decrease in their parallel velocity,
which causes an additional increase in the wave frequency of chorus elements generated
by such electrons and propagating equatorward. The elements propagating in the
opposite direction, i.e., from the equator, are formed by electrons whose parallel
velocities are larger due to the adiabatic mirror force. Correspondingly, such electrons
generate waves with lower frequencies. We present the results of numerical simulations
confirming the proposed mechanism.
Permanent Link: http://hdl.handle.net/11104/0263856