The solar atmosphere is a strongly coupled multi-layered and multi-scale system. Increasing observational, theoretical, and numerical work suggests that a significant fraction of the solar atmosphere is in a state of thermal non-equilibrium induced by various compressive perturbations or small-scale reconnection events. This delicate thermal balance (or the absence of it) often results in the development of various thermal instabilities manifesting in the increased dynamics and filamentary structure of the corona observed in lines forming at 10⁴-10⁶ K, the occurrence of prominences and the pervasive coronal rain and EUV pulsation phenomena observed in the quiet Sun and active regions (quiescent and flaring). The effect of heating/cooling misbalance has further been highlighted due to its feedback on the stability and dynamics of MHD waves, providing a new look at the solar atmosphere as an active plasma medium.

Below the surface, the non-adiabatic effects play a key role in sustaining the amplitude and determining the characteristic frequencies of high-degree helioseismic oscillations confined close to the solar surface, where the acoustic wave dynamic time scales become comparable to the characteristic times of thermal processes. The aim of this RAS SD meeting is to bring together and boost the cross-talk between experts in numerical modelling, observational and theoretical analysis of solar atmospheric and helioseismic waves and oscillations, instabilities, and magnetic reconnection in the context of non-equilibrium thermodynamics and the resulting mass and energy exchange between the solar interior and the multi-layered atmosphere.