Coronal heating by MHD waves

TL;DR

This paper reviews recent observational and numerical advances in understanding how MHD waves contribute to heating the solar corona, highlighting the progress and remaining challenges in modeling these processes.

Contribution

It provides a comprehensive overview of recent observational discoveries and numerical models of MHD wave heating, emphasizing the current limitations of 3D MHD models in explaining all coronal heating phenomena.

Findings

Multiple wave modes observed with significant energy flux.

3D MHD models with Alfvén waves explain quiet Sun temperatures.

Models with structured density fields struggle to reproduce active region loop temperatures.

Abstract

The heating of the solar chromosphere and corona to the observed high temperatures, imply the presence of ongoing heating that balances the strong radiative and thermal conduction losses expected in the solar atmosphere. It has been theorized for decades that the required heating mechanisms of the chromospheric and coronal parts of the active regions, quiet-Sun, and coronal holes are associated with the solar magnetic fields. However, the exact physical process that transport and dissipate the magnetic energy which ultimately leads to the solar plasma heating are not yet fully understood. The current understanding of coronal heating relies on two main mechanism: reconnection and MHD waves that may have various degrees of importance in different coronal regions. In this review we focus on recent advances in our understanding of MHD wave heating mechanisms. First, we focus on giving an overview of observational results, where we show that different wave modes have been discovered in the corona in the last decade, many of which are associated with a significant energy flux, either generated in situ or pumped from the lower solar atmosphere. Afterwards, we summarise the recent findings of numerical modelling of waves, motivated by the observational results. Despite the advances, only 3D MHD models with Alfv\'en wave heating in an unstructured corona can explain the observed coronal temperatures compatible with the quiet Sun, while 3D MHD wave heating models including cross-field density structuring are not yet able to account for the heating of coronal loops in active regions to their observed temperature.