Mixing of Proton and Electron Scales - Effects of Proton Temperature Anisotropy on the Electron Firehose Instability

Abstract:

We perform kinetic linear theory instability analysis in a non-drifting anisotropic electron-proton plasma to study the effects of proton temperature anisotropies on the electron firehose instability in the collisionless solar wind. We solve the Vlasov linear theory dispersion relation for hot highly anisotropic electron-proton plasma in high-beta regime to study the behavior of the solar wind plasma close to the instability thresholds as observed by different spacecraft at 1 AU. We consider temperature and anisotropy regimes for which the electrons and the protons can interact via the excited electromagnetic fluctuations. For the selected parameters simultaneous electron and proton firehose instabilities can be observed with the growth rate of the electron firehose instability extending towards the proton scales. The co-existance of the proton and the electron firehose and the mixing of scales for the electromagnetic fluctuations excited by the two instabilities depends on the initial temperatures, anisotropies and angle of propagation. In the case of parallel wave propagation both left and right-hand polarized waves are simultaneously excited. As we increase the angle of propagation the electron firehose starts to dominate with excitation of large-amplitude aperiodic fluctuations over a large range of wave-numbers, starting at the protons scales and extending up to the smaller electron scales. We calculate the maximum growth rate of the oblique electron firehose as a function of the proton temperature anisotropy and discuss the implications of the electron-proton scale mixing for the observed plasma properties and instability thresholds in the undisturbed solar wind.