Plasmas in a thermodynamically non-equilibrium state, with multiple temperature phases coexisting (from 10^6 K to 10^4 K or below), are ubiquitous in different astrophysical systems such as solar & stellar coronae, galactic and AGN outflows, and the circumgalactic (CGM), interstellar (ISM) and intracluster (ICM) media. The role of these multiphase plasmas has been highlighted in mass and energy cycles at all such scales, from thermal non-equilibrium (TNE) cycles and magnetohydrodynamic (MHD) wave dynamics in the solar atmosphere to feedback cycles that regulate the formation of galaxies. The properties of the plasmas in thermal non-equilibrium are strikingly similar across these multiple scales, intimately linked to the as-yet unclear but fundamental mechanisms of coronal and ICM heating, and instabilities of thermal or other nature.

Due to its close proximity, the solar corona constitutes a unique astrophysical laboratory where we can spatially and temporally resolve the production and removal of such multiphase plasma. A specific solar atmospheric response to the violation of a delicate thermal balance is seen across wide spatial and energetic scales. At large coronal loop scales, TNE cycles manifest through puzzling week-long EUV intensity pulsations that may also be detectable in other stars. At small (~100 km) scales, the effects of perturbed thermal equilibrium come through the generation of cool coronal rain, the seed of prominences, whose properties are similar to multiphase filamentary structure in the ISM and ICM or to molecular loops in the Galactic centre. An increased production of solar coronal rain is seen for stronger heating (extending to flares), whose features seem recurrent in active stars. Furthermore, the prominences formed at a star’s co-rotation radius are now thought to be significant contributors to the wind mass loss rate. The efficiency of these unique wind gauges depends, however, on their formation and destruction mechanisms. Recently, the effects of thermal imbalance in plasmas have been further stressed due to the important feedback on the stability of MHD waves, triggers of coronal rain, and also because of their seismological potential. New spectral codes are now available allowing to probe the complex mixed properties of MHD waves in non-adiabatic plasma conditions, which may explain condensation shattering, splattering, and other morphological and dynamic effects observed in the solar atmosphere and in the ISM/ICM. Lastly, experiments performed in the laboratory can further shed light on the complex routes to instability in magnetically dominated environments.

Despite the exciting cross-disciplinary opportunities offered by multi-scale and multi-energetic manifestations of the effects of perturbed thermal equilibrium in the plasmas across the universe, the research in these various fields has progressed mostly independently. Responding to this, a dedicated session at the National Astronomy Meeting is to be held in July 2022, whose aim is to consolidate our understanding of the physics of such plasmas in thermal non-equilibrium in solar and stellar atmospheres, and in the ISM/ICM, and to stimulate an effective knowledge exchange. This Research Topic is aimed at strengthening this consolidation by bringing together the most recent observational and theoretical achievements in these research fields, including laboratory science. We welcome submissions of original works and review papers on the following specific topics:

• Manifestations of thermal instability and thermal non-equilibrium (TNE) in any astrophysical setup: from stellar atmospheres to the CGM, ISM and ICM. We also welcome insights from laboratory science.

• Thermal evolution and dynamics of solar and stellar prominences and coronal rain, and their role in the chromosphere-corona mass and energy cycle.

• Effects of imbalance between heating and cooling processes on dynamics and stability of MHD waves in astrophysical plasmas.

• Flare-driven coronal rain.

• The role of instabilities and turbulence in the generation of multiphase plasmas.

• Differences and similarities between the effects of perturbed thermal equilibrium in different astrophysical and laboratory plasma systems.

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