Simulations of Three‐Dimensional Reconnection in the Solar Corona

Abstract

Using ideal and resistive MHD, we investigate the stability and dynamic evolution of three- dimensional magnetic —eld con—gurations, representing stretched arcade structures above a dipolar ii photospheric ˇˇ magnetic —eld. Two types of con—gurations are studied that diUer by the amount of divergence (ii fanning ˇˇ) of the initial —eld lines as viewed in the horizontal direction perpendicular to the photospheric magnetic neutral line and, correspondingly, by the radial decrease of magnetic —eld strength and current density. The two sets of con—gurations are found to diUer in their stability behavior. The strongly fanning —elds, associated with a rapid radial decrease of the —eld strength, current density, and plasma pressure, are more stable. A stability diUerence is found also when the con—gurations are —rst 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 —elds. For similar current density enhancements, the occurrence of anomalous dissipation (resistivity) initiates magnetic reconnection in either con—guration. However, the eUects are much more drastic in magnitude and spread in the weakly diverging —eld structure. In the unstable cases, a strongly localized electric —eld parallel to the magnetic —eld develops, which results in integrated voltages with maximum values of the order of a few hundred MeV, both on open and closed —eld lines. For comparison, we studied both low-beta, force-free, and high-beta initial states. The weakly fanning high-beta con—gurations tend to show more drastic instability eUects than the corresponding low-beta —elds, but the stabilization of the strongly fanning —elds pertains to both low-beta and high-beta —elds. The three-dimensional reconnection in the unstable cases generates a region of intertwined magnetic —ux tubes with diUerent topologies that lie below a region of closed —ux ropes not aUected by reconnection. The topological changes could be the source of open —ux tubes that are occasionally observed within coronal mass ejections, as recently discussed by Gosling et al. The fast outward —ow generated in these simulations aUects only the regions of changing topology but does not cause the above-lying closed —ux ropes to move (within the times considered). This may be seen as an indication that reconnection may be associated with the onset of a —are, initiated after the eruption of a coronal mass ejection, but is not the driver of the coronal mass ejection itself. Subject headings: MHDSun: coronaSun: —aresSun: magnetic —elds