On the modelling of oblique firehose instabilities in regularised kappa plasmas using ALPS

In situ measurements in space plasmas indicate that the velocity distributions of charged particles are not in thermal equilibrium, deviating from a standard Maxwellian mainly due to anisotropies and suprathermal populations which enhance high-energy tails. Although the standard

-distribution (SKD) is well established in modelling these non-equilibrium distributions, its application is often viewed controversially due to certain unphysical implications, in particular divergent velocity moments. To address these issues, the regularised

kappa κ $\kappa$

-distribution (RKD) was introduced. Such advanced, in general anisotropic RKDs are invoked here for the first time to investigate oblique firehose instabilities, including those induced by the temperature anisotropy of electrons and protons (the dominant species in space plasmas). In weakly collisional plasmas, both of these instabilities are expected to play significant roles in the self-regulation of the macroscopic properties of space plasmas (such as the expanding solar wind) reported by observations. Is not yet possible to resort to a general dispersion tensor related to RKD plasmas (whose derivation is still a challenge due to the complexity of these models), instead the new generation Arbitrary Linear Plasma Solver (ALPS) is exploited here. The unstable solutions obtained for the already established Maxwellian and SKDs successfully validate the capability of ALPS. For RKDs, the instabilities confirm the stimulating effect of suprathermal populations, with lower

kappa κ $\kappa$

values generally enhancing the firehose growth rates. For the oblique electron firehose instability, RKDs substantially modify the competition between periodic and aperiodic branches only at intermediate angles and can sustain significantly increased growth rates of especially aperiodic modes at highly oblique propagation beyond both the Maxwellian unstable regime and the one that is accessible with SKDs.