MHD wave refraction and the acoustic halo effect around solar active regions - a 3D study

TL;DR

This study uses 3D wave modeling to investigate how MHD mode conversion causes the acoustic halo effect around solar active regions, highlighting the role of fast wave reflection at the $a=c$ layer.

Contribution

It demonstrates that the acoustic halo results from fast wave reflection due to mode conversion at the $a=c$ surface, supported by simulations with capped Alfvén speed.

Findings

Halo formation correlates with the $a=c$ surface.

Halos disappear when fast wave refraction is prevented.

Simulated halos reproduce observed dual-frequency enhancements.

Abstract

An enhancement in high-frequency acoustic power is commonly observed in the solar photosphere and chromosphere surrounding magnetic active regions. We perform 3D linear forward wave modelling with a simple wavelet pulse acoustic source to ascertain whether the formation of the acoustic halo is caused by MHD mode conversion through regions of moderate and inclined magnetic fields. This conversion type is most efficient when high frequency waves from below intersect magnetic field lines at a large angle. We find a strong relationship between halo formation and the equipartition surface at which the Alfv\'en speed $a$ matches the sound speed $c$, lending support to the theory that photospheric and chromospheric halo enhancement is due to the creation and subsequent reflection of magnetically dominated fast waves from essentially acoustic waves as they cross $a=c$. In simulations where we have capped $a$ such that waves are not permitted to refract after reaching the $a=c$ height, halos are non-existent, which suggests that the power enhancement is wholly dependent on returning fast waves. We also reproduce some of the observed halo properties, such as a dual 6 and 8 mHz enhancement structure and a spatial spreading of the halo with height.