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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

A variety of nonthermal characteristics like kinetic, e.g., temperature, anisotropies and suprathermal populations (enhancing the high energy tails of the velocity distributions) are revealed by the in-situ observations in the solar wind indicating quasistationary states of plasma particles out of thermal equilibrium. Large deviations from isotropy generate kinetic instabilities and growing fluctuating fields which should be more efficient than collisions in limiting the anisotropy (below the instability threshold) and explain the anisotropy limits reported by the observations. The present paper aims to decode the principal instabilities driven by the temperature anisotropy of electrons and protons in the solar wind, and contrast the instability thresholds with the bounds observed at 1 AU for the temperature anisotropy. The instabilities are characterized using linear kinetic theory to identify the appropriate (fastest) instability in the relaxation of temperature anisotropies A e,p = T e,p,⊥ /T e,p, = 1. The analysis focuses on the electromagnetic instabilities driven by the anisotropic protons (A p ≶ 1) and invokes for the first time a dynamical model to capture the interplay with the anisotropic electrons by correlating the effects of these two species of plasma particles, dominant in the solar wind.

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