Firehose constraints of the bi-Kappa-distributed electrons: a zero-order approach for the suprathermal electrons in the solar wind

The increase of temperature predicted by the solar wind expansion in the direction parallel to the interplanetary magnetic field is already notorious for not being confirmed by the observations. In hot and dilute plasmas from space, particle-particle collisions are not efficient in constraining large deviations from isotropy, but the resulting firehose instability provides itself plausible limitations for the temperature anisotropy of both the electron and proton species. This paper takes into discussion the suprathermal (halo) electrons, which are ubiquitous in the solar wind, and may be highly anisotropic and susceptible to the firehose instability. Suprathermals enhance the high-energy tails of the velocity distributions making them well described by the Kappa distribution functions, with the advantage that these are power laws suitable to reproduce either the entire distribution or only the suprathermal halo tails. New features of the instability are captured from a linear stability analysis of bi-Kappa-distributed electrons with the temperature depending on the power-index K. This approach enables a realistic interpretation of non-thermal electrons and their effects on the instability: growth rates are systematically stimulated and thresholds are lowered with decreasing the power-index K. In a zero-order limiting approach of the halo component (minimizing the effects of a cooler and less anisotropic core population), the instability thresholds align to the limits of the temperature anisotropy reported by the observations. These results provide new and valuable support for an extended implication of the firehose instability in the relaxation of temperature anisotropy in collisionless plasmas from space.