The Role of the Drift Kink Mode in Destabilizing Thin Current Sheets

Abstract

The drift kink mode in current sheets of the Harris type is driven by the relative cross-field streaming of the electrons and ions. The properties of this mode are investigated by means of electromagnetic particle simulations and a two-fluid stability analysis, first in a 2-D (y, z) geometry and then in a full 3D. For thin current sheets (ρi0/L ≤ 1, where ρi0 is the ion gyroradius in the lobe field and L is the current sheet half thickness) the fluid theory indicates that the most unstable mode has kyL ∼ (Mi/me)1/2/2 and that the associated real frequency extends up to several times the ion gyrofrequency Ωi0. The simulations indicate, however, that finite gyroradius effects limit the dominant growing modes to longer wavelengths kyL ∼ 1 and to frequencies ∼Ωi0. In 2D the current sheet kinks due to a bulk displacement of the plasma in z, and this displacement appears to be limited only by the simulation boundary. This transport of plasma across flux tubes reduces the electron compressibility associated with the finite Bz field, and, in 3D, allows the collisionless tearing instability to grow at a rate comparable to that for the simple 1-D neutral sheet. Implications of these results for the triggering of substorms are discussed.