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The selfgenerated wave fluctuations are particularly interesting in the solarwind and magnetospheric plasmas, where Coulomb collisions are rare and cannotexplain the observed states of quasi-equilibrium. Linear theory predicts thatthe firehose and the ordinary-mode instabilities can develop under the sameconditions, confusing the role of these instabilities in conditioning thespace-plasma properties. The hierarchy of these two instabilities isreconsidered here for nonstreaming plasmas with an electron temperatureanisotropy $T_\parallel > T_\perp$, where $\parallel$ and $\perp$ denotedirections with respect to the local mean magnetic field. In addition to theprevious comparative analysis, here the entire 3D wave-vector spectrum of thecompeting instabilities is investigated, paying particular attention to theoblique firehose instability and the relatively poorly known ordinary-modeinstability. Results show a dominance of the oblique firehose instability witha threshold lower than the parallel firehose instability and lower than theordinary-mode instability. For larger anisotropies, the ordinary mode can growfaster, with maximum growth rates exceeding the ones of the oblique firehoseinstability. In contrast to previous studies that claimed a possible activityof the ordinary-mode in the small $\beta [< 1]$ regimes, here it is rigorouslyshown that only the large $\beta [> 1]$ regimes are susceptible to theseinstabilities.

The Electron Firehose and Ordinary-Mode Instabilities in Space Plasmas