Drift instabilities in the solar corona within the multi-fluid description

Abstract

Recent observations revealed that the solar atmosphere is highly structured in density, temperature and magnetic field. The presence of these gradients may lead to the appearance of currents in the plasma, which in the weakly collisional corona can constitute sources of free energy for driving micro-instabilities. Such instabilities are very important since they represent a possible source of ion-cyclotron waves which have been conjectured to play a prominent role in coronal heating, but whose solar origin remains unclear. Considering a density stratification transverse to the magnetic field, this paper aims at studying the possible occurrence of gradient-induced plasma micro-instabilities under typical conditions of coronal holes. Taking into account the WKB (Wentzel-Kramers-Brillouin) approximation, we perform a Fourier plane waves analysis using the collisionless multi-fluid model. By neglecting the electron inertia, this model allows us to take into account ion-cyclotron wave effects that are absent from the one-fluid model of magnetohydrodynamics (MHD). Realistic models of density and temperature, as well as a 2-D analytical magnetic-field model, are used to define the background plasma in the open-field funnel in a polar coronal hole. The ray-tracing theory is used to compute the ray path of the unstable waves, as well as the evolution of their growth rates during the propagation. We demonstrate that in typical coronal hole conditions, and when assuming typical transverse density length scales taken from radio observations, the current generated by a relative electron-ion drift provides enough free energy for driving the mode unstable. This instability results from a coupling between oppositely propagating slow-mode waves.