Plasma Heating by Current Sheets in Solar Active Regions

Abstract

The magnetic field of active regions is typically closed, i.e., all field lines are connected to the photosphere. Thus the field lines define a mapping from the photosphere to itself. If a nearly frozen-in magnetic field above the photosphere evolves in response to plasma motions at the photospheric boundary, current sheets are likely to develop at places where the mapping defined by the field lines is discontinuous, namely where field lines either run into a magnetic neutral point or are tangential to the photosphere at one end. Whereas it remains an open question whether the existence of neutral points is typical of active regions, tangential field lines are typically present and form surfaces, which are thus likely places of current sheet formation. A comparison of thresholds and time scales of tearing, coalescence, and kinetic current-driven instabilities shows that current sheets which are continuously driven, e.g. by photospheric plasma motions causing continual energy input into the superphotospheric magnetic field, develop internal dynamics also for stationary (but not static) boundary conditions at high magnetic Reynolds numbers. This may explain the fragmentation of solar flare energy release (frequently observed at radio wavelengths and in hard X-rays). It is expected that the main part of released magnetic energy goes into turbulent motions and finally into heat in the vicinity of the current sheet.