  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 MHD approximation taking into account the viscosity, the
electrical and thermal conductivity, and the radiative cooling. Solution of the
problem allows the formation of an instability of a thermal nature. There is no
external magnetic field inside the current layer in equilibrium state, but it
can penetrate inside when the current layer is disturbed. 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 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 for the primary
energy release in solar flares is discussed.
