The Onset of Magnetic Reconnection in the Solar Atmosphere

Abstract:

A fundamental question concerning magnetic energy release on the Sun is why the release occurs only after substantial stresses have been built up in the field. If reconnection were to occur readily, then the released energy would be much less than the energy required for coronal heating, CMEs, flares, jets, spicules, etc. How can we explain this switch-on property? What is the physical nature of the onset conditions? One idea involves the secondary instability of current sheets, which switches on when the rotation of the magnetic field across a current sheet reaches a critical angle. Such conditions would occur at the boundaries of flux tubes that become tangled and twisted by turbulent photospheric convection, for example. Other ideas focus on a critical thickness for the current sheet. We report here on the preliminary results of our investigation of reconnection onset. Unlike our earlier work on the secondary instability (Dahlburg, Klimchuk, and Antiochos 2005), here we treat the coupled chromosphere-corona system. Using the BATS-R-US MHD code (Toth et al. 2012), we simulate a single current sheet in a sheared magnetic field that extends from the chromosphere into the corona. Driver motions are applied at the base of the model. The configuration and chromosphere are both idealized, but capture the essential physics of the problem. The advantage of this unique approach is that it resolves the current sheet to the greatest extent possible while maintaining a realistic solar atmosphere. It thus bridges the gap between reconnection in a box studies and studies of large-scale systems such as active regions. One question we will address is whether onset conditions are met first in the chromosphere or corona.