Effects of electrons on the electromagnetic ion cyclotron instability: Solar wind implications

Abstract

In diffuse plasmas in space, particle-particle collisions are rare and inefficient, such that a plausible mechanism for constraining the temperature anisotropy of plasma particles may be provided by the resulting instabilities. The implication of the electromagnetic ion-cyclotron (EMIC) instability in the solar wind is still unclear because this instability is fast enough to relax the proton temperature anisotropy, but the 1 AU measurements do not conform to the instability thresholds predicted by the existing theories, which ignore the kinetic effects of electrons, assuming them to be isotropic. This paper presents a refined analysis of the EMIC instability in the presence of a temperature (T) anisotropy of electron (subscript "e") population, i.e., enabling the identification of two distinct regimes of this instability that correspond to an excess of perpendicular temperature () or an excess of parallel temperature (). The growth rates, real frequencies, and threshold conditions are found to be highly sensitive to the electron temperature anisotropy, and electrons with inhibit the instability, while for the instability growth rates increase with the electron anisotropy. Moreover, the electron-proton temperature ratio becomes an important factor that stimulates the effect of the anisotropic electrons. The potential relevance of the new results in the solar wind is analyzed by contrasting the instability thresholds with the observed limits of the proton temperature anisotropy.