Parallel and oblique firehose instability thresholds for bi‐kappa distributed protons

Abstract

AbstractThe parallel and the oblique firehose instability are generally accepted as the leading mechanisms shaping the boundaries of the protons' pressure anisotropies observed in the solar wind for p∥>p⊥. However, it is still an open question which instability dominates this process. Only recently, first attempts were made to study the linear growth of the parallel firehose assuming more realistic bi‐kappa velocity distributions instead of traditionally used bi‐Maxwellians. We apply a newly developed, fully kinetic dispersion solver to numerically derive the instability thresholds for both firehose instabilities. In contrast to former findings, we observe that the presence of suprathermal populations yields a growth amplification which lowers the instability threshold of the parallel firehose. This is due to enhanced cyclotron resonance. For the first time, we also look at the oblique firehose threshold and find a contrary picture. Here the presence of suprathermal particles leads to an increase of the instability threshold. The enhancement of the parallel firehose and the suppression of the oblique firehose are expected to be of relevance in the solar wind and may alter the competition between both instabilities. Based on our findings, we propose a method how solar wind data could be used to identify the instability mechanism dominating this competition and shaping the observed anisotropy boundary.