3D simulations of realistic power halos in magneto-hydrostatic sunspot atmospheres: linking theory and observation

TL;DR

This study uses 3D simulations to explore the physical mechanisms behind the acoustic halo phenomenon around sunspots, linking theoretical models with observations and highlighting the roles of wave refraction and mode conversion.

Contribution

It provides the first realistic 3D MHS sunspot simulations that reproduce observed halo features and identifies fast magnetic wave refraction and fast-Alfvén mode conversion as key processes.

Findings

Simulated halos qualitatively match observations.

Refraction and return of fast magnetic waves cause the halo.

Fast-Alfvén mode conversion influences halo structure and dual-ring features.

Abstract

The well-observed acoustic halo is an enhancement in time-averaged Doppler velocity and intensity power with respect to quiet-sun values which is prominent for weak and highly inclined field around the penumbra of sunspots and active regions. We perform 3D linear wave modelling with realistic distributed acoustic sources in a MHS sunspot atmosphere and compare the resultant simulation enhancements with multi-height SDO observations of the phenomenon. We find that simulated halos are in good qualitative agreement with observations. We also provide further proof that the underlying process responsible for the halo is the refraction and return of fast magnetic waves which have undergone mode conversion at the critical $a=c$ atmospheric layer. In addition, we also find strong evidence that fast-Alfv\'en mode conversion plays a significant role in the structure of the halo, taking energy away from photospheric and chromospheric heights in the form of field-aligned Alfv\'en waves. This conversion process may explain the observed "dual-ring" halo structure at higher ($> 8 $ mHz) frequencies.