Alfven wave coupled with flow-driven fluid instability in interpenetrating plasmas

TL;DR

This paper analyzes how Alfvén waves interact with flow-driven instabilities in interpenetrating plasmas, revealing mode coupling, modifications, and conditions for instability growth, which may influence plasma heating processes.

Contribution

It derives a dispersion relation for Alfvén waves coupled with flow-driven modes in interpenetrating plasmas, highlighting the conditions for instability and mode merging.

Findings

Flow speed exceeding Alfvén speed modifies wave propagation.

Coupled mode becomes unstable and grows at high flow speeds.

Potential implications for plasma heating in astrophysical contexts.

Abstract

The Alfven wave is analyzed in case of one quasineutral plasma propagating with some constant speed $v_0$ through another static quasineutral plasma. A dispersion equation is derived describing the Alfven wave coupled with the flow driven mode $\omega= k v_0$ and solutions are discussed analytically and numerically. The usual solutions for two oppositely propagating Alfv\'en waves are substantially modified due to the flowing plasma. More profound is modification of the solution propagating in the negative direction with respect to the magnetic field and the plasma flow. For a large enough flow speed (exceeding the Alfven speed in the static plasma), this negative solution may become non-propagating, with frequency equal to zero. In this case it represents a spatial variation of the electromagnetic field. For greater flow speed it becomes a forward mode, and it may merge with the positive one. This merging of the two modes represents the starting point for a flow-driven instability, with two complex-conjugate solutions. The Alfven wave in interpenetrating plasmas is thus modified and coupled with the flow-driven mode and this coupled mode is shown to be growing when the flow speed is large enough. The energy for the instability is macroscopic kinetic energy of the flowing plasma. The dynamics of plasma particles caused by such a coupled wave still remains similar to the ordinary Alfven wave. This means that well-known stochastic heating by the Alfv\'en wave may work, and this should additionally support the potential role of the Alfven wave in the coronal heating.