3D coupled tearing-thermal evolution in solar current sheets

Context. The tearing instability plays a major role in the disruption of current sheets, whereas thermal modes can be responsible for condensation phenomena (forming prominences and coronal rain) in the solar atmosphere. However, how current sheets made unstable by combined tearing and thermal instability evolve within the solar atmosphere has received limited attention to date. Aims. We numerically explore a combined tearing and thermal instability that causes the break up of an idealized current sheet in the solar atmosphere. The thermal component leads to the formation of localized, cool condensations within an otherwise 3D reconnecting magnetic topology. Methods. We constructed a 3D resistive magnetohydrodynamic simulation of a force-free current sheet under solar atmospheric conditions that incorporates the non-adiabatic influence of background heating, optically thin radiative energy loss, and magnetic-field-aligned thermal conduction with the open source code MPI-AMRVAC. Multiple levels of adaptive mesh refinement reveal the self-consistent development of finer-scale condensation structures within the evolving system. Results. The instability in the current sheet is triggered by magnetic field perturbations concentrated around the current sheet plane, and subsequent tearing modes develop. This in turn drives thermal runaway associated with the thermal instability of the system. We find subsequent, localized cool plasma condensations that form under the prevailing low plasma-beta conditions, and demonstrate that the density and temperature of these condensed structures are similar to more quiescent coronal condensations. Synthetic counterparts at extreme ultraviolet (EUV) and optical wavelengths show the formation of plasmoids (in EUV) and coronal condensations similar to prominences and coronal rain blobs in the vicinity of the reconnecting sheet. Conclusions. Our simulations imply that 3D reconnection in solar current sheets may well present an almost unavoidable multi-thermal aspect that forms during their coupled tearing-thermal evolution.