Three‐dimensional stability of thin quasi‐neutral current sheets

In a thin current sheet (ρ i 0 / L ≲ 1, where ρ i 0 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 1 y is an antisymmetric function of z while E 1 z is a symmetric function with E 1 z (0) ≠ 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 ∼ 1 and frequency ω r ∼ Ω i 0 , where Ω i 0 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 (γ/Ω i 0 ≲ 0.05–0.10). The k y L ∼ 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.