Comment on "Drift instabilities in the solar corona within the multi-fluid description" [Astron. Astrophys. {\bf 481}, 853 (2008)]

TL;DR

This paper critiques a previous study on plasma instabilities in the solar corona, clarifying the role of drift motions and demonstrating that certain proposed wave excitations are not feasible under correct physical assumptions.

Contribution

It revises the understanding of drift currents in inhomogeneous plasmas, correcting previous assumptions and showing that ion-cyclotron waves cannot be excited as previously suggested.

Findings

Diamagnetic drift does not contribute to flux in the continuity equation.

Ion polarization drift cancels with stress tensor drift contributions.

Ion-cyclotron waves cannot be excited by drift currents as previously claimed.

Abstract

Mecheri and Marsch concluded that drift currents caused by density gradients can serve as an energy source for a plasma instability, in particular for the excitation of ion-cyclotron waves in the solar corona. It is pointed out that these authors overlooked some fundamental properties of the drift motion in inhomogeneous plasmas. The calculation is repeated, taking into account the missing terms. It is shown that the diamagnetic drift, which is essential for the new physical phenomena obtained by Mecheri and Marsch, can not contribute to the flux in the continuity equation. Moreover, the part of the ion polarization drift contribution to the ion cancels out exactly with the contribution of the part of the stress tensor drift to the same flux. The ion-cyclotron waves in the solar corona can thus not be excited in the way suggested by Mecheri and Marsch.