COLLISIONLESS RECONNECTION IN CONFIGURATION WITH A MINIMUM IN THE EQUATORIAL MAGNETIC-FIELD AND WITH MAGNETIC SHEAR

Two-dimensional (x,z) particle simulations are used to investigate the influence of a minimum in the equatorial magnetic field profile and of a B-y field component (sheared magnetic field) on the occurrence of collisionless reconnection in thin current sheets (R(i)/L similar to 1, where R(i) is the ion gyroradius based on the lobe field Bo and L is the current sheet half thickness). It is shown that the pure ion tearing mode (ion to electron mass ratio M(i)/m(e) = 1) first produces an X line in a region of minimum average (over a tearing wavelength) normal field B-z (x,0) and in the region of minimum current sheet thickness; this need not coincide with the actual minimum in B-z. With a mass ratio M(i)/m(e) > 1 the tearing growth rate is strongly reduced as k rho(e) --> 1, where rho(e) is the electron gyroradius in the B-z field. This is an extension of the stabilization associated with electron confinement to a flux tube previously predicted and observed for a constant equatorial field profile. If the slow growth allowed for k rho(e) greater than or similar to 1 eventually produces an X line, then an ''explosive'' stage of reconnection occurs in which the electron dynamics no longer inhibits the tearing mode displacement, and the growth rate approaches that for the one-dimensional neutral sheet. The addition of a uniform and static shear field By greater than or similar to B-z i s found not to remove the stabilizing effect associated with the electron dynamics. The significant differences between the parameter regime of the present simulations and that of analytic treatments of the effect of B-y On thick (R(i)/L < 0.1) current sheets are discussed in detail.