Spontaneous current sheets and stellar x-ray emission

Abstract

The x-ray emission from a star like the sun comes from plasma at 106-107 K and 1010 ions cm-3- trapped in the 100 Gauss bipolar magnetic fields of active regions. The x-ray intensity reaches 107 ergs cm-2s-1, which is a measure of the heat input required to maintain the temperature of the emitting gas. The basic energy input is photospheric convection, which swirls and intermixes the photospheric footpoints of the bipolar magnetic fields, thereby creating an interlaced topology of the field lines in the bipolar field above the photosphere. The basic theorem of magnetostatics asserts that almost all field topologies develop internal tangential discontinuities as the field relaxes to equilibrium. The discontinuities are a necessary and sufficient condition for the Maxwell stresses to achieve static balance. It appears that rapid reconnection across these internal current sheets provides the principal heat source for the x-ray corona.