Comment on 'Alfven Instability in a Compressible Flow' [Phys. Rev. Lett. 101, 245001 (2008)]

TL;DR

This paper critically evaluates a previous claim of a new Alfvén wave instability, demonstrating that the original results stem from an invalid equilibrium assumption and should be disregarded.

Contribution

It provides a critical analysis showing that the claimed instability results are invalid due to an impossible equilibrium in the original study.

Findings

The original instability results are based on an impossible equilibrium.

The claimed exponential amplification of Alfvén modes is not valid.

The paper dismisses the previous claim as incorrect.

Abstract

In a recent paper [Y. Taroyan, Phys. Rev. Lett 101, 245001 (2008) it is claimed that a new MHD instability of the Alfven wave has been found: incompressible Alfven modes propagating in a compressible spatially varying flow were apparently exponentially amplified. We show that the results of the work follow from an impossible equilibrium and therefore should be disregarded.