The Electron Firehose and Ordinary-Mode Instabilities in Space Plasmas
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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.
KeywordsCorona Flares, dynamics Solar wind Instabilities Waves, plasma
ML acknowledges financial support from the EU Commission and Research Foundation Flanders (FWO) as FWO Pegasus Marie Curie Fellow (grant 1.2.070.13). The authors acknowledge support from the Ruhr-Universität Bochum, the Deutsche Forschungsgemeinschaft (DFG), grant Schl 201/21-1, and by the Katholieke Universiteit Leuven. These results were obtained in the framework of the projects GOA/2009-009 (KU Leuven), G.0729.11 (FWO-Vlaanderen) and C 90347 (ESA Prodex 9). The research leading to these results has also received funding from the European Commission’s Seventh Framework Programme (FP7/2007-2013) under the grant agreements SOLSPANET (project n° 269299, www.solspanet.eu ), SPACECAST (project n° 262468, fp7-spacecast.eu ), eHeroes (project n° 284461, www.eheroes.eu ) and SWIFF (project n° 263340, www.swiff.eu ).
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