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| Titel |
Excitation of obliquely propagating whistler waves by an electron beam |
| VerfasserIn |
Richard Sydora, Konrad Sauer |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043868
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| Zusammenfassung |
| Isotropic electron beams are considered to explain the excitation of whistler waves that have
been observed by STEREO in the Earth’s radiation belt (Catell et al., Geo. Res. Lett., vol. 35,
L01105 (2008)). Besides their large amplitudes (~240mV/m), another main signature is the
strongly inclined propagation direction relative to the ambient magnetic field. Electron
temperature anisotropy with Te-¥ >Te||, which preferentially generates parallel propagating
whistler waves, can be excluded. The instability is considered to arise due to the interaction
of the Doppler-shifted cyclotron mode Ï=-Ωe+kVb with the whistler mode in the
wave number range of kc/Ïe -¤1 (Ïe: electron plasma frequency). Fluid and Vlasov
dispersion analysis have been used to calculate the growth rate of the beam-excited
whistlers with dependence of the most relevant parameters. One of these is the beam
velocity (Vb) which, for instability, has to be larger than about 2VAe, where VAe is
the electron Alfven speed. With increasing VAe the propagation angle (θ) of the
maximally growing whistler waves shifts from θ ~200 for Vb=2VAe to θ ~800 for
Vb=5VAe. The growth rate is reduced by finite electron temperatures and disappears if
the electron plasma beta (βe) exceeds βe ~0.2. As a further step in linear theory,
stationary waves are analyzed that allows for a prediction of the conditions under
which nonlinear stationary waves (whistler oscillitons) may exist. Subsequently,
spatial profiles of whistler oscillitons are calculated using the full nonlinear fluid
approach as well as electromagnetic particle simulations. A remarkable feature of the
analysis is the obtained (nonlinear) wave polarization is quite different from the linear
predictions. The results are in good coincidence with the STEREO observations. |
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