Recent developments in collisionless reconnection theory: Applications to laboratory and space plasmas

Recent developments in the theory and simulation of nonlinear collisionless reconnection hold the promise for providing solutions to some outstanding problems in laboratory and space plasma physics. Examples of such problems are sawtooth oscillations in tokamaks, magnetotail substorms, and impulsive solar flares. In each of these problems, a key issue is the identification of fast reconnection rates that are insensitive to the mechanism that brea ks field lines (resistivity and/or electron inertia). The classical models of Sweet-Parker and Petschek sought to resolve this issue in the realm of r esistive magnetohydrodynamics (MHD). However, the plasmas mentioned above a re weakly collisional, and hence obey a generalized Ohm's law in which the Hall current and electron pressure gradient terms play a crucial role. Rece nt theoretical models and simulations on impulsive (or triggered) as well a s quasisteady reconnection governed by a generalized Ohm's law are reviewed . In the impulsive reconnection problem, not only is the growth rate fast b ut the time derivative of the growth rate changes rapidly. In the steady-st ate reconnection problem, explicit analytical expressions are obtained for the geometric characteristics (that is, length and width) of the reconnecti on layer and the reconnection rate. Analytical results are tested by Hall M HD simulations. While some of the geometric features of the reconnection la yer and the weak dependence of the reconnection rate on resistivity are rem iniscent of Petschek's classical model, the underlying wave and particle dy namics mediating the reconnection dynamics in the presence of the Hall curr ent and electron pressure gradient are qualitatively different. Quantitativ e comparisons are made between theory and observations from laboratory as w ell as space plasmas. (C) 2001 American Institute of Physics.