Beam-excited whistler waves at oblique propagation with relation to STEREO radiation belt observations
Isotropic electron beams are considered to explain the excitation of whistler waves which have been observed by the STEREO satellite in the Earth's radiation belt. Aside from their large amplitudes (~240 mV/m), another main signature is the strongly inclined propagation direction relative to...
| Main Authors: | , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2010-06-01
|
| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/28/1317/2010/angeo-28-1317-2010.pdf |
| Summary: | Isotropic electron beams are considered to explain the excitation of
whistler waves which have been observed by the STEREO satellite in the Earth's radiation belt.
Aside from their large amplitudes (~240 mV/m), another main signature is the strongly
inclined propagation direction relative to the ambient magnetic field. Electron
temperature anisotropy with <I>T</I><sub>e⊥</sub>><I>T</I><sub>e||</sub>, which preferentially generates parallel
propagating whistler waves, can be excluded as a free energy source.
The instability
arises due to the interaction of the Doppler-shifted cyclotron mode ω=−Ω<sub>e</sub>+<I>kV</I><sub>b</sub>cosθ with
the whistler mode in the wave number range
of <I>kc</I>/ω<sub>e</sub>≤1 (θ is the propagation angle
with respect to the background magnetic field direction, ω<sub>e</sub> is the electron plasma frequency
and Ω<sub>e</sub> the electron cyclotron frequency).
Fluid and kinetic dispersion analysis have been used to calculate the growth rate of the
beam-excited whistlers including the most important parameter dependencies.
One is the beam
velocity (<I>V</I><sub>b</sub>) which, for instability, has to be larger than about 2<I>V</I><sub>Ae</sub>, where <I>V</I><sub>Ae</sub> is the
electron Alfvén speed. With increasing <I>V</I><sub>Ae</sub> the propagation angle (θ) of the fastest growing
whistler waves shifts from θ~20° for <I>V</I><sub>b</sub>=2<I>V</I><sub>Ae</sub> to θ~80° for <I>V</I><sub>b</sub>=5<I>V</I><sub>Ae</sub>.
The growth rate is
reduced by finite electron temperatures and disappears if the electron plasma beta (β<sub>e</sub>)
exceeds β<sub>e</sub>~0.2.
In addition, Gendrin modes (<I>kc</I>/ω<sub>e</sub>≈1) are analyzed to determine the conditions
under which stationary nonlinear waves (whistler oscillitons) can exist. The corresponding
spatial wave profiles are calculated using the full nonlinear fluid approach. The results
are compared with the STEREO satellite
observations. |
|---|---|
| ISSN: | 0992-7689 1432-0576 |