STABILITY OF RESISTIVE MHD TEARING AND BALLOONING MODES IN THE TAIL CURRENT SHEET

We have analytically investigated the evolution of resistive MHD teari ng and ballooning modes by assuming that the dissipation is anomalous in the current sheet region. A generalized technique is diplayed for o btaining the solutions for both these modes near the singular layer, w here the B-x(z) field reverses sign. When the perturbation is limited to two dimensions, we have found that the stability of tearing modes i s controlled by compressibility and the Lundquist number S, where S is the ratio of anomalous diffusion time to Alfven time. For S much less than 5 x 10(3), the fluid compressibility plays a significant destabi lizing role while the normal component B, contributes to a weak stabil ization of tearing modes. In the three-dimensional case with k(y) not equal 0 (k(y) being the wavenumber), it is demonstrated that a linear coupling of the pressure gradient and the magnetic held curvature caus es the excitation of a new class of unstable tearing and ballooning mo des with their growth rates significantly dependent on anomalous resis tivity and the Alfvenic frequency. The resistive ballooning modes, exc ited in the field reversal layer, are shown to enhance the current and the magnetic field gradients in the center of the plasma sheet and th ereby provide the source for the excitation of tearing modes with a ty pical growth;time of 5 s. Finally, the relevance of the newly excited modes to recent AMPTE/IRM, GEOS 2, and Geotail results is discussed.