The solar corona as an active medium for magnetoacoustic waves

Abstract

The presence and interplay of continuous cooling and heating processes maintaining the corona of the Sun at the observed one million K temperature were recently understood to have crucial effects on the dynamics and stability of magnetoacoustic (MA) waves. These essentially compressive waves perturb the coronal thermal equilibrium, leading to the phenomenon of a wave-induced thermal misbalance (TM). Representing an additional natural mechanism for the exchange of energy between the plasma and the wave, TM makes the corona an active medium for MA waves, so that the wave can not only lose but also gain energy from the coronal heating source (similarly to burning gases, lasers and masers). We review recent achievements in this newly emerging research field, focussing on the effects that slow-mode MA waves experience as a back-reaction of this perturbed coronal thermal equilibrium. The new effects include enhanced frequency-dependent damping or amplification of slow waves, and effective, not associated with the coronal plasma non-uniformity, dispersion. We also discuss the possibility to probe the unknown coronal heating function by observations of slow waves and linear theory of thermal instabilities. The manifold of the new properties that slow waves acquire from a thermodynamically active nature of the solar corona indicate a clear need for accounting for the effects of combined coronal heating/cooling processes not only for traditional problems of the formation and evolution of prominences and coronal rain, but also for an adequate modelling and interpretation of magnetohydrodynamic waves.

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

“…where B = (27/4) (R − 1/3) 2 -1. This critical wavelength may be thought as a more general expression for the "acoustic Field's length" described in Kolotkov et al (2021). In other words, the isobaric instability criterion always applies to the entropy mode and to acoustic modes for λ > λ c .…”

“…The argument is based on the fact that during the cooling stage of the TNE cycle and prior to the catastrophic cooling stage, all flows are subsonic and the loop's evolution is slow enough to be in approximate thermal balance. Formally, this can be analysed in terms of thermal misbalance (due to heating-cooling misbalance) under specific timescales, and the occurrence of thermal instability under thermal misbalance (and its back-reaction on the wave modes) is an active new branch of research with very interesting seismological applications (Kolotkov et al, 2020;Duckenfield et al, 2021;Kolotkov et al, 2021;Zavershinskii et al, 2021). 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.…”

“…Coronal rain observations may therefore serve as a template for ISM/ICM multiphase plasma morphologies and dynamics. Furthermore, the routes to unstable evolution of the full spectrum of linear MHD eigenmodes can now be considered in non-trivial magnetic configurations taking into account non-adiabatic (Claes and Keppens, 2021) and thermal misbalance effects (Kolotkov et al, 2021) with recently developed codes inspired by solar condensations .…”

Dynamics of fast and slow magnetoacoustic waves in plasma slabs with thermal misbalance

“…The most commonly used approach involves applying the infinite magnetic field approximation (see Kolotkov et al 2020;Prasad et al 2021, etc). Encouraging results were obtained using the second or zero order thin flux tube approximation (see, e.g., Belov et al 2021;Kolotkov et al 2021;Duckenfield et al 2021). However, these approaches are applicable for the description of the slow body sausage MA modes only.…”

“…A recent review concerning the problem of thermal misbalance in the solar atmosphere can be found in (Kolotkov et al 2021). It was shown by Zavershinskii et al (2019) and Kolotkov et al (2019) that the thermal misbalance can significantly affect the dispersion properties of slow waves in the solar corona.…”