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

摘要

In collision-poor plasmas from space, e.g., the solar wind and planetarymagnetospheres, the kinetic anisotropy of the plasma particles is expected tobe regulated by the kinetic instabilities. Driven by an excess of ion (proton)temperature perpendicular to the magnetic field $(~T_\perp >T_\parallel)$, theelectromagnetic ion-cyclotron (EMIC) instability is fast enough to constrainthe proton anisotropy, but the observations do not conform to the instabilitythresholds predicted by the standard theory for bi-Maxwellian models of theplasma particles. This paper presents an extended investigation of the EMICinstability in the presence of suprathermal electrons which are ubiquitous inthese 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 accuratedescription for conditions typically encountered in space plasmas. The effectsof suprathermal populations are triggered by the electron anisotropy and thetemperature contrast between electrons and protons. For certain conditions theanisotropy thresholds exceed the limits of the proton anisotropy measured inthe solar wind considerably restraining the unstable regimes of the EMIC modes.