The Electron Firehose and Ordinary-Mode Instabilities in Space Plasmas |
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Authors: | M Lazar S Poedts R Schlickeiser D Ibscher |
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Institution: | 1. Center for Mathematical Plasma Astrophysics, K.U. Leuven, Celestijnenlaan 200B, 3001, Leuven, Belgium 2. Institut für Theoretische Physik, Lehrstuhl IV: Weltraum- und Astrophysik, Ruhr-Universit?t Bochum, 44780, Bochum, Germany
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Abstract: | Self-generated wave fluctuations are particularly interesting in the solar wind and magnetospheric plasmas, where Coulomb collisions are rare and cannot explain the observed states of quasi-equilibrium. Linear theory predicts that firehose and ordinary-mode instabilities can develop under the same conditions, which makes it challenging to separate the role of these instabilities in conditioning the space-plasma properties. The hierarchy of these two instabilities is reconsidered here for nonstreaming plasmas with an electron-temperature anisotropy T ∥>T ⊥, where ∥ and ⊥ denote directions with respect to the local mean magnetic field. In addition to the previously reported comparative analysis, here the entire 3D wave-vector spectrum of the competing instabilities is investigated, with a focus on the oblique firehose instability and the relatively poorly known ordinary-mode instability. Results show a dominance of the oblique firehose instability with a threshold lower than the parallel firehose instability and lower than the ordinary-mode instability. For stronger anisotropies, the ordinary mode can grow faster, with maximum growth rates exceeding those of the oblique firehose instability. In contrast to previous studies that claimed a possible activity of the ordinary-mode in the low β <?1] regimes, here it is rigorously shown that only the high β >?1] regimes are susceptible to these instabilities. |
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