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

TL;DR

This paper investigates how thermal instabilities in a preflare current layer can lead to its fragmentation, potentially triggering solar flares, by analyzing heat balance effects and magnetic perturbations.

Contribution

It introduces a model of thermal instability in the preflare current layer considering heat balance, magnetic perturbations, and radiative cooling, explaining layer fragmentation.

Findings

Thermal instability causes transverse fragmentation of the current layer.

Growth time of instability is proportional to radiative cooling time.

Fragmentation scale ranges from 1 to 10 Mm.

Abstract

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.