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.
Highlights
The coronal heating problem remains one of the major puzzles in solar physics for almost 80 years since the pioneering works [1, 2], which apparently makes it the longest-standing unsolved problem in all plasma astrophysics
We address the link between the coronal heating and MHD waves from the wave dynamics point of view, i.e. focussing on the effects that the coronal heating and cooling processes exert on the evolution of MHD waves
The latter implies that for different coronal plasma conditions and different coronal heating models, the phenomenon of thermal misbalance (TM) may lead to a number of different scenarios for the initial compressive perturbation to evolve in the solar corona, with the acoustic and thermal modes being stable and/or unstable
Summary
The coronal heating problem remains one of the major puzzles in solar physics for almost 80 years since the pioneering works [1, 2], which apparently makes it the longest-standing unsolved problem in all plasma astrophysics. Being abundantly present in the corona (see [7, 39, 40], for reviews), slow waves were shown to experience a back-reaction from the perturbed thermal equilibrium, either losing or gaining energy from the coronal heating source [41], which makes the continuously heated and cooling corona an active medium for MA waves (cf burning gases, lasers and masers).
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