Amplification of compressional MHD waves in systems with forced entropy oscillations

TL;DR

This paper investigates how external entropy oscillations can resonantly amplify compressional MHD waves, revealing mutual wave coupling and conditions for maximum amplification, applicable to various astrophysical and terrestrial plasma systems.

Contribution

It introduces a formalism for analyzing resonant amplification of MHD waves driven by entropy oscillations, highlighting wave coupling near plasma beta of 1.

Findings

Resonant amplification of fast and slow MHD waves occurs under specific conditions.

Waves are mutually coupled due to nonequilibrium background states.

Maximum coupling occurs when plasma beta is approximately 1.

Abstract

The propagation of compressional MHD waves is studied for an externally driven system. It is assumed that the combined action of the external sources and sinks of the entropy results in the harmonic oscillation of the entropy (and temperature) in the system. It is found that with the appropriate resonant conditions fast and slow waves get amplified due to the phenomenon of parametric resonance. Besides, it is shown that the considered waves are mutually coupled as a consequence of the nonequilibrium state of the background medium. The coupling is strongest when the plasma $\beta \approx 1$. The proposed formalism is sufficiently general and can be applied for many dynamical systems, both under terrestrial and astrophysical conditions.