Mixing-induced thermal instabilities and coronal condensations

ABSTRACT Cool, dense material is frequently observed to permeate the hot, tenuous solar corona in the form of prominences, spicules and coronal rain. Both the cool material and surrounding corona exist at temperatures that are effectively thermally stable, in that their local radiative losses occur on relatively long time-scales compared to the dynamics. However, the intermediate temperatures at the boundaries between cool and hot material are subject to highly efficient radiative losses. The turbulent motions in the solar atmosphere can drive mixing between condensations and the corona, leading to the formation of intermediate temperatures and thus efficient cooling. Here, a 3D radiative magnetohydrodynamics simulation is performed of the shear-driven Kelvin–Helmholtz instability occurring between a cool condensation and the hot solar corona. During the evolution, thermal instabilities form naturally within the mixing layer and grow with time to produce long, narrow structures that extend perpendicular to the magnetic field. The thermal instabilities form self-consistently within the mixing layer as small isolated events, and are then stretched by the background flows to create long structures in relatively narrow planes. The turbulent flows agitate the condensations and cause them to fragment, creating smaller localized clumps of cool, dense (prominence-like) material that can merge and act as seeds for further condensations. The thermal instabilities act to replenish the cool, dense material lost due to mixing, with the total mass of cool material being approximately constant through time and are found to account for 15–20 per cent of all radiative losses in the turbulent plasma. As such, thermal instabilities are dynamically important features of condensation-corona mixing, can self-consistently arise from radiative losses and turbulent motions, and can create additional cool, dense material.