Thermal nonequilibrium - A trigger for solar flares

It is suggested that a thermal nonequilibrium or catastrophe may trigger a solar flare, rather than a magnetic instability, and this possibility is demonstrated by approximately solving the energy equation for a loop under a balance between thermal conduction, optically thin radiation, and a heating source. It is found that if one begins with a cool equilibrium at a few times 10,000 K, and gradually increases the heating or decreases the loop pressure, critical metastable conditions are ultimately reached beyond which no equilibrium exists. During thermal flaring in which the plasma heats up explosively, to a new quasi-equilibrium at 10 million K, any magnetic disruption or particle acceleration are secondary. The cool core of an active-region loop heats up and the magnetic plasma tube maintains its position in the case of a compact flare, while a two-ribbon flare has the material of an active region filament heating and expanding along the filament.