DYNAMICAL FINITE-AMPLITUDE MAGNETIC RECONNECTION AT AN X-TYPE NEUTRALPOINT

The linear theory of magnetic reconnection demonstrates that the rate of energy release at an X-type neutral point is ''fast''-only logarith mically dependent on the plasma resistivity-if the field is strictly t wo-dimensional, gas pressure is absent, and perturbations are small. T he present paper explores the response of the X-point to finite amplit ude disturbances under the more realistic conditions of limited compre ssibility and a finite nonplanar magnetic held component. We show that fast reconnection is not inhibited by large amplitudes of the perturb ation-in fact, both the reconnection rate and the ohmic dissipation ra te increase with decreasing plasma resistivity. This ''super fast'' sc aling can be understood by a simple, one-dimensional dynamic collapse model. However, the presence of finite gas pressure or an axial magnet ic held component stalls the collapse by providing backpressure which retards the imploding magnetic wave: the current sheet is prevented fr om thinning and reconnection drops toward the static diffusion rate. T hus we overrule the initial implosion as a means of rapidly liberating magnetic energy when gas pressure or an axial magnetic field are pres ent, as they are in the solar corona. But in this case the initial col lapse does not provide the complete picture. Gas is subsequently squee zed out of the current sheet, allowing a higher current density to be attained. Thus the possibility of fast reconnection remains open. The dynamics of the evolution are complicated; it is not yet clear under e xactly what conditions fast reconnection may be attainable.