Frequency Dependence of Very Low Frequency Chorus Poynting Flux in the Source Region: THEMIS Observations and a Model

Using Poynting vector measurements of whistler mode chorus emissions detected by the THEMIS spacecraft within the source region, that is, close to the magnetic field minimum, we found both in individual events and statistically that chorus elements propagating equatorward had systematically higher f...

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Bibliographic Details
Published in:Geophysical research letters 2020-03, Vol.47 (6), p.n/a
Main Authors: Demekhov, A. G., Taubenschuss, U., Hanzelka, M., Santolík, O.
Format: Article
Language:English
Subjects:
Adiabatic
Backward waves
BWO model
Chorus waves
Computer simulation
Distribution functions
Electron distribution
Electrons
Emission measurements
Equator
Frequency dependence
Magnetic field
Magnetic fields
Mathematical models
Nonlinear waves
Numerical simulations
Particle interactions
Spacecraft
Very Low Frequencies
VLF chorus
Wave generation
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Summary:Using Poynting vector measurements of whistler mode chorus emissions detected by the THEMIS spacecraft within the source region, that is, close to the magnetic field minimum, we found both in individual events and statistically that chorus elements propagating equatorward had systematically higher frequencies and smaller amplitudes compared with simultaneously observed elements propagating away from the equator. We demonstrate similar features in the results of numerical simulations based on backward wave oscillator equations. It can be qualitatively explained by the nonlinear evolution of the energetic electron distribution function during wave generation. The motion of electrons from the equator is accompanied by a decrease in their velocity component along the magnetic field line due to both the adiabatic mirror force and nonlinear wave‐particle interactions. Thus, the frequency of the chorus elements generated by such electrons and propagating equatorward is higher compared with the elements propagating away from the equator. Key Points THEMIS multicomponent wave data on VLF chorus in the source region are analyzed Opposite Poynting flux directions in shifted frequency bands are observed Theoretical explanation based on the backward wave oscillator model is proposed
ISSN:0094-8276
1944-8007
DOI:10.1029/2020GL086958