Comparing the counter-beaming and temperature anisotropy driven aperiodic electron firehose instabilities in collisionless plasma environments

Abstract: The electron firehose instabilities are among the most studied kinetic instabilities, especially in the context of space plasmas, whose dynamics is mainly controlled by collisionless wave-particle interactions. This paper undertakes a comparative analysis of the aperiodic electron firehose instabilities excited either by the anisotropic temperature or by the electron counter-beaming populations. Two symmetric counter-beams provide an effective kinetic anisotropy similar to the temperature anisotropy of a single (non-drifting) population, with temperature along the magnetic field direction larger than that in perpendicular direction. Therefore, the counter-beaming plasma is susceptible to firehose-like instabilities (FIs), parallel and oblique branches. Here we focus on the oblique beaming FI, which is also aperiodic when the free energy is provided by symmetric counter-beams. Our results show that, for relative small drifts or beaming speeds ($U$), not exceeding the thermal speed ($\alpha$), the aperiodic FIs exist in the same interval of wave-numbers and the same range of oblique angles (with respect to the magnetic field direction), but the growth rates of counter-beaming FI (CBFI) are always higher than those of temperature anisotropy FI (TAFI). For $U/\alpha > 1$, however, another electrostatic two-stream instability (ETSI) is also predicted, which may have growth rates higher than those of CBFI, and may dominate in that case the dynamics.