Effects of suprathermal electrons on the proton temperature anisotropy in space plasmas: Electromagnetic ion-cyclotron instability

TL;DR

This study investigates how suprathermal electrons influence the electromagnetic ion-cyclotron instability and proton temperature anisotropy regulation in space plasmas, revealing that electron anisotropy can significantly alter instability thresholds.

Contribution

It extends the kinetic theory of EMIC instability by incorporating suprathermal electrons and their anisotropy, providing more accurate thresholds relevant to space plasma observations.

Findings

Suprathermal electrons modify EMIC instability thresholds.

Electron anisotropy can suppress proton anisotropy-driven instabilities.

Results align better with solar wind measurements.

Abstract

In collision-poor plasmas from space, e.g., the solar wind and planetary magnetospheres, the kinetic anisotropy of the plasma particles is expected to be regulated by the kinetic instabilities. Driven by an excess of ion (proton) temperature perpendicular to the magnetic field $(~T_\perp >T_\parallel)$, the electromagnetic ion-cyclotron (EMIC) instability is fast enough to constrain the proton anisotropy, but the observations do not conform to the instability thresholds predicted by the standard theory for bi-Maxwellian models of the plasma particles. This paper presents an extended investigation of the EMIC instability in the presence of suprathermal electrons which are ubiquitous in these environments. The analysis is based on the kinetic (Vlasov-Maxwell) theory assuming that both species, protons and electrons, may be anisotropic, and the EMIC unstable solutions are derived numerically providing an accurate description for conditions typically encountered in space plasmas. The effects of suprathermal populations are triggered by the electron anisotropy and the temperature contrast between electrons and protons. For certain conditions the anisotropy thresholds exceed the limits of the proton anisotropy measured in the solar wind considerably restraining the unstable regimes of the EMIC modes.