Current Sheets and Prominence Formation in the Solar Atmosphere

Abstract

If magnetic field lines in opposite directions expand toward each other to squeeze out the field between them, they come into contact to form an abrupt field reversal (tangential discontinuity) which supports a current sheet. Field line expansion can be caused by the combined pressure gradient and gravity force, the magnetic field pressure gradient force, magnetic fields merging toward each other, or a combination of these effects. In this paper we show that in a quadrupolar magnetic field geometry magnetohydrostatic (MHS) equilibria without null point can be deformed into equilibrium field configurations containing current sheets by changes in forces associated with thermodynamic properties or field line footpoint displacement. The shape of current sheets depends on the pressure distributions and footpoint displacement profiles. These MHS equilibria containing current sheets can evolve into different prominence magnetic field configurations via resistive magnetic reconnection processes to support a higher plasma density at the reconnection site than its surrounding. Examples are given to demonstrate these physical processes. If the plasma pressure (or density) on the photospheric boundary is higher in the lower flux tubes than in the higher tubes in each bipolar arcade, the combined pressure gradient and gravity force pushes magnetic field lines outward from the center of each bipolar arcade. When the plasma temperature increases above a critical value, the enhanced expansion of these arcade fields results in the formation of a current sheet between these two arcades along a separatrix line. Similarly, if the field line footpoints undergo a spatially continuous shearing motion so that the outward gradient force of the sheared magnetic field pressure pushes the field lines of each bipolar arcade into contact, a current sheet of the same shape is formed when the amount of shear is above a critical value. This field configuration can be deformed into a configuration with an X-point through resistive magnetic reconnection. It is expected that an inverse polarity prominence can stably reside above the X-point. If the plasma pressure (or density) on the photospheric boundary is lower in the lower flux tubes than in the higher tubes in each bipolar arcade, the combined pressure gradient and gravity force which pushes down magnetic field lines is reduced when the atmosphere is cooled. When the temperature is decreased below a critical value, the subsequent expansion of field lines due to the magnetic tension force creates a new current sheet configuration with a sharp downward-pointing tip hanging at a distance above the bottom boundary. Resistive magnetic reconnection in this type of current sheet results in a Malville-type field configuration with a magnetic island wrapped in dipped field lines. It is expected that an inverse polarity prominence can stably reside within the magnetic island. Our results suggest that the formation of MHS equilibria containing current sheets and their evolution into prominence magnetic field configurations must be a general process in the solar atmosphere.