THE ROLE OF MAGNETIC RECONNECTION IN CHROMOSPHERIC ERUPTIONS

We investigate the hypothesis that all chromospheric eruptions are man ifestations of a common magneto-hydrodynamic phenomenon occurring on d ifferent scales: the acceleration of chromospheric plasma driven by lo calized magnetic reconnection. Our approach is to perform 2.5-dimensio nal numerical simulations of shear-induced reconnection in a potential magnetic held with a central X-point above the photosphere, embedded in a model chromosphere with solar gravity and numerical resistivity. Calculations with two values of the foot-point displacement were perfo rmed by applying a localized body-force duration twice as long in one case as in the other; after the shearing was discontinued, the system was allowed to relax for an additional interval. For the stronger shea r, the initial X-point lengthens upward into a current sheet which rec onnects gradually for a while but then begins to undergo multiple tear ing. Thereafter, several magnetic islands develop in sequence, move to ward the ends of the sheet, and disappear through reconnection with th e overlying or underlying field. During the relaxation stage, a new qu asi-equilibrium state arises with a central magnetic island. We also p erformed a reference calculation with the stronger shear but with grea tly reduced numerical resistivity along the boundary where the X-point and subsequent current sheet are located. This simulation confirmed o ur expectations for the system evolution in the ideal limit: the curre nt sheet becomes much longer, without significant reconnection. For th e weaker shear, a much shorter sheet forms initially which then shrink s smoothly through reconnection to yield an X-point relocated above it s original position, quite distinct from the final state of the strong -shear case. After reviewing the dynamics and plasma properties as wel l as the evolving magnetic topology, we conclude that geometry, shear strength, and local resistivity must determine the dynamic signatures of chromospheric eruptions. Our model reproduces such fundamental obse rved features as intermittency and large velocities, as well as the ap proximately concurrent appearance of oppositely directed flows. We als o find that reconnection in the vertical current sheet is more consist ent with Sweet-Parker reconnection theory, while the rapid interaction between the magnetic islands and the background field better approxim ates the Petschek process.