Thermal Trigger for Solar Flares I: Fragmentation of the Preflare Current Layer

We consider the effects of the heat balance on the structural stability of a preflare current layer. The problem of small perturbations is solved in the piecewise homogeneous magnetohydrodynamic (MHD) approximation taking into account viscosity, electrical and thermal conductivity, and radiative cooling. Solution to the problem allows for the formation of an instability of thermal nature. There is no external magnetic field inside the current layer in the equilibrium state, but it can penetrate inside when the current layer is disturbed. The formation of a magnetic field perturbation inside the layer creates a dedicated frequency in a broadband disturbance subject to thermal instability. In the linear phase, the growth time of the instability is proportional to the characteristic time of radiative cooling of the plasma and depends on the logarithmic derivatives of the radiative cooling function with respect to the plasma parameters. The instability results in transverse fragmentation of the current layer with a spatial period of 1–10 Mm along the layer in a wide range of coronal plasma parameters. The role of that instability in the triggering of the primary energy release in solar flares is discussed.