Three-dimensional stability of thin quasi-neutral current sheets

In a thin current sheet (rho(i0)/L less than or similar to 1, where rho(i0) is the ion gyroradius in the lobe field and L is the current sheet half thickness) of the generalized Harris type, the relative ion-electron cross-field drift is comparable to the ion thermal velocity. The three-dimensional stability properties of such a thin current sheet are investigated by means of nonlocal two-fluid theory and two-dimensional and three-dimensional full particle simulations. As was suggested originally by Zhu et al. [1992], the drift kink mode is found to be of critical importance. For the simple case of no initial B-z field the fluid theory demonstrates that the drift kink mode is a non-MHD mode with a polarization structure such that E(1y) is an antisymmetric function of z while E(1z) is a symmetric function with E(1z)(0)not equal 0. Two-dimensional (y,z) particle simulations indicate that the nonlinear behavior of this mode is dominated by long-wavelength modes with k(y)L similar to 1 and frequency omega(r) similar to Omega(i0), where Omega(i0) is the ion gyrofrequency in the lobe field. Three-dimensional particle simulations performed on a massively parallel computer show that while the growth rates for the drift kink mode are reduced by the finite B-z, they can still be appreciable (gamma/Omega(i0)less than or similar to 0.05-0.10). The k(y)L similar to 1 drift kink modes are always the first to grow in the simulations; subsequently, tearing-like modes with a dominant k(x) wave vector also become unstable. Implications of these results for the triggering of substorms are discussed.