Current Sheet Formation and Reconnection on Separator Field Lines

Abstract

Magnetic reconnection is believed to play a fundamental role in many types of coronal activity such as flaring and transient loop brightenings. Reconnection occurs when flux is transferred between neighboring coronal flux systems. A single flux system consists of coronal magnetic field lines originating in a common pair of photospheric features (i.e. flux tubes). Changing the flux in such a system requires a substantial electric field in the corona, parallel to a magnetic field line in at least one place. Such an electric field is not consistent with the highly conductive nature of the hot corona.

We present a model for the evolution of a complexly structured coronal magnetic field driven by motion of photospheric flux tubes. Barring reconnection, motion of photospheric flux tubes leads to the development of three dimensional current sheets (current ribbons) along particular field lines called separators. A separator is a field line which lies at the boundaries of four distinct flux systems simultaneously. The amount of current in a given ribbon is an increasing function of flux tube displacement. The presence of current implies free magnetic field energy, and reconnection must occur eventually if the separator current and stored energy are not to grow indefinitely. When it occurs, reconnection adjusts the fluxes in each system to temporarily obviate the need for current ribbons, thereby liberating the stored free energy. Such instances of reconnection occur sporadically during a typical evolutionary scenario. The frequency of these events, and the energy released, is consistent with transient brightenings observed by Yohkoh (Shimizu et al., 1992).