Interplay between Alfv\'en and magnetosonic waves in compressible magnetohydrodynamics turbulence

TL;DR

This study provides direct evidence of wave interactions in compressible magnetohydrodynamic turbulence, revealing how Alfvén and magnetosonic waves are excited and transfer energy in different plasma pressure regimes.

Contribution

It offers the first direct spectral evidence of wave excitation and energy transfer between Alfvén and magnetosonic modes in 3D compressible MHD turbulence across different beta regimes.

Findings

In low beta, energy transfers to Alfvén and magnetosonic waves, with signatures of fast magnetosonic waves.

In high beta, fast and slow magnetosonic waves dominate, with no clear Alfvén wave signatures.

Magnetic field fluctuations show different behaviors parallel and perpendicular to the guide field.

Abstract

Using spatio-temporal spectra we show direct evidence of excitation of magnetosonic and Alfv\'en waves in three-dimensional compressible magnetohydrodynamic turbulence at small Mach numbers. For the plasma pressure dominated regime, or high $\beta$ regime (with $\beta$ the ratio between fluid and magnetic pressure), and for the magnetic pressure dominated regime, or low $\beta$ regime, we study magnetic field fluctuations parallel and perpendicular to a guide magnetic field $\textbf{B}_0$. In the low $\beta$ case we find excitation of compressible and incompressible fluctuations, with a transfer of energy towards Alfv\'enic modes and to a lesser extent towards magnetosonic modes. In particular, we find signatures of the presence of fast magnetosonic waves in a scenario compatible with that of weak turbulence. In the high $\beta$ case, fast and slow magnetosonic waves are present, with no clear trace of Alfv\'en waves, and a significant part of the energy is carried by two-dimensional turbulent eddies.