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 timescales compared to the dynamics. However, as the solar atmosphere evolves, driving mixing between the condensations and surrounding hot material, intermediate temperatures form, which can become subject to highly efficient radiative losses. The thermal energy lost due to radiation can far exceed the turbulent heating thus the system undergoes mixing-induced cooling. Here, a 3D radiative MHD simulation is performed of the shear-driven Kelvin-Helmholtz Instability (KHI) 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 localised clumps of cool, dense (prominence-like) material that can merge and further fragment. In the presented simulation, 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. By analysing the thermal energy loss due to optically-thin radiation, thermal instabilities are found to account for 15-20\% of all radiative losses in the turbulent plasma.