Simulations of Three‐Dimensional Reconnection in the Solar Corona

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Abstract
Using ideal and resistive MHD, we investigate the stability and dynamic evolution of three-dimensional magnetic field configurations, representing stretched arcade structures above a dipolar "photospheric" magnetic field. Two types of configurations are studied that differ by the amount of divergence ("fanning") of the initial field lines as viewed in the horizontal direction perpendicular to the photospheric magnetic neutral line and, correspondingly, by the radial decrease of magnetic field strength and current density. The two sets of configurations are found to differ in their stability behavior. The strongly fanning fields, associated with a rapid radial decrease of the field strength, current density, and plasma pressure, are more stable. A stability difference is found also when the configurations are first subjected to a converging motion of photospheric footpoints toward the neutral line, which leads to the buildup of thin current sheets in the region above. This current sheet formation is more pronounced for the weakly fanning fields. For similar current density enhancements, the occurrence of anomalous dissipation (resistivity) initiates magnetic reconnection in either configuration. However, the effects are much more drastic in magnitude and spread in the weakly diverging field structure. In the unstable cases, a strongly localized electric field parallel to the magnetic field develops, which results in integrated voltages with maximum values of the order of a few hundred MeV, both on open and closed field lines. For comparison, we studied both low-beta, force-free, and high-beta initial states. The weakly fanning high-beta configurations tend to show more drastic instability effects than the corresponding low-beta fields, but the stabilization of the strongly fanning fields pertains to both low-beta and high-beta fields. The three-dimensional reconnection in the unstable cases generates a region of intertwined magnetic flux tubes with different topologies that lie below a region of closed flux ropes not affected by reconnection. The topological changes could be the source of open flux tubes that are occasionally observed within coronal mass ejections, as recently discussed by Gosling et al. The fast outward flow generated in these simulations affects only the regions of changing topology but does not cause the above-lying closed flux ropes to move (within the times considered). This may be seen as an indication that reconnection may be associated with the onset of a flare, initiated after the eruption of a coronal mass ejection, but is not the driver of the coronal mass ejection itself.