Electron kinetic firehose instability

The linear dispersion equation describing electromagnetic waves propagating in a homogeneous electron-proton plasma along arbitrary directions relative to the direction of the background magnetic field is solved numerically for bi-Maxwellian particle distributions. It is found that in the presence of an electron temperature anisotropy T-perpendicular to < T-<parallel> and a sufficiently warm plasma (beta (e) > 2), several purely growing modes (zero real frequency) and a quasi-parallel electron firehose instability develop. While the quasi-parallel mode is unstable for both parallel and oblique propagation, the zero frequency modes are unstable only for oblique propagation. Comparison of these modes further shows that the propagation angle for maximum growth rate and the maximum growth rate are larger for the purely growing modes than the quasi-parallel electron firehose while the threshold is lower. Potential application of the kinetic electron firehose instability to the slow solar wind is briefly discussed.