Magnetohydrodynamic Spectroscopy of a Non-adiabatic Solar Atmosphere

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.

Multi-Scale Variability of Coronal Loops Set by Thermal Non-Equilibrium and Instability as a Probe for Coronal Heating

“…• Does coronal rain exclusively correspond to unstable thermal modes? In the complex solar atmosphere, it has been shown that unstable modes can have mixed properties, and that all MHD modes may become unstable (Claes and Keppens, 2021;Zavershinskii et al, 2021). This is linked to the observed morphology of clumps, as discussed above.…”

“…Non-adiabatic MHD spectroscopy codes are now available, such as "Legolas" , that take into account thermal misbalance and allow to quantify the stability of the wave modes in onedimensional setups. Using this tool, Claes and Keppens (2021) show that thermal instabilities are unavoidable in the solar atmosphere, and that all modes (thermal, slow and fast) may exhibit unstable behaviour.…”

Legolas: magnetohydrodynamic spectroscopy with viscosity and Hall current

“…those from modern plasma physics textbooks focusing on MHD spectroscopy (Goedbloed, Keppens & Poedts 2019). A typical application can be found in Claes & Keppens (2021), where it was shown that especially the effect of radiative cooling in a realistic, magnetised solar atmosphere model yields MHD eigenspectra dominated by thermal instabilities and magneto-thermal overstabilities.…”

Prevalence of Thermal Nonequilibrium over an Active Region

“…During the heating, chromospheric evaporation occurs, and the loop gets dense and starts to radiate strongly. During the cooling, catastrophic cooling can occur, leading to thermal instability (TI) locally in the corona (Claes & Keppens 2021). TI is thought to generate cool and dense condensations called rain clumps, which subsequently fall as coronal rain toward the surface under the action of various forces (Oliver et al 2014;Fang et al 2015).…”