Magnetohydrodynamic spectroscopy of a non-adiabatic solar atmosphere

TL;DR

This paper presents a detailed spectroscopic analysis of the solar atmosphere using a new code to compute the full MHD spectrum, revealing the ubiquity of thermal instabilities and their role in solar phenomena.

Contribution

The study introduces the Legolas code for high-resolution MHD spectrum calculation in realistic solar models, highlighting the prevalence of thermal instabilities and mode behaviors.

Findings

Thermal instabilities are unavoidable in solar atmospheric models.

Regions exist where thermal, slow, and fast modes are all unstable.

Thermal and slow continua can become purely imaginary and merge on the imaginary axis.

Abstract

The quantification of all possible waves and instabilities in a given system is of paramount importance, and knowledge of the full magnetohydrodynamic (MHD) spectrum allows one to predict the (in)stability of a given equilibrium state. This is highly relevant in many (astro)physical disciplines, and when applied to the solar atmosphere it may yield various new insights in processes like prominence formation and coronal loop oscillations. In this work we present a detailed, high-resolution spectroscopic study of the solar atmosphere, where we use our newly developed Legolas code to calculate the full spectrum with corresponding eigenfunctions of equilibrium configurations that are based on fully realistic solar atmospheric models, including gravity, optically thin radiative losses and thermal conduction. Special attention is given to thermal instabilities, known to be responsible for the formation of prominences, together with a new outlook on the thermal and slow continua and how they behave in different chromospheric and coronal regions. We show that thermal instabilities are unavoidable in our solar atmospheric models and that there exist certain regions where both the thermal, slow and fast modes all have unstable wave mode solutions. We also encounter regions where the slow and thermal continua become purely imaginary and merge on the imaginary axis. The spectra discussed in this work illustrate clearly that thermal instabilities (both discrete and continuum modes) and magneto-thermal overstable propagating modes are ubiquitous throughout the solar atmosphere, and may well be responsible for much of the observed fine-structuring and multi-thermal dynamics.