THEORETICAL-ANALYSIS OF DRIVEN MAGNETIC RECONNECTION EXPERIMENTS

In this paper we present a theoretical framework for the Magnetic Reco nnection Experiment (MRX) [M. Yamada et al., Bull. Am. Phys. Sec. 40, 1877 (1995)] in order to understand the basic physics of the experimen t, including the effect of the external driving force, and the differe nce between co-and counterhelicity cases of the experiment. The proble m is reduced to a one-dimensional (1-D) resistive magnetohydrodynamic (MHD) model. A special class of holonomic boundary conditions is defin ed, under which a unique sequence of global equilibria can be obtained , independent of the rate of reconnection. This enables one to break t he whole problem into two parts: a global problem for the ideal region , and a local problem for the resistive reconnection layer. The calcul ations are then carried out and the global solution for the ideal regi on is obtained in one particular case of holonomic constraints, the so called ''constant force'' regime, for both the co- and counterhelicit y cases. After the sequence of equilibria in the ideal region is found , the problem of the rate of reconnection in the resistive reconnectio n region is considered. This rate tells how fast the plasma proceeds t hrough the sequence of global equilibria but does not affect the seque nce itself. Based on a modified Sweet-Parker model for the reconnectio n layer, the reconnection rate is calculated, and the difference betwe en the co- and counterhelicity cases, as well as the role of the exter nal forces is demonstrated. The results from the present analysis are qualitatively consistent with the experimental data, predicting faster reconnection rate for the counterhelicity merging and yielding a posi tive correlation with external forcing. (C) 1996 American Institute of Physics.