Effect of guide field on three dimensional electron shear flow instabilities in collisionless magnetic reconnection

Abstract

We examine the effect of an external guide field and current sheet thickness on the growth rates and nature of three dimensional unstable modes of an electron current sheet driven by electron shear flow. The growth rate of the fastest growing mode drops rapidly with current sheet thickness but increases slowly with the strength of the guide field. The fastest growing mode is tearing type only for thin current sheets (half thickness $\approx d_e$, where $d_e=c/\omega_{pe}$ is electron inertial length) and zero guide field. For finite guide field or thicker current sheets, fastest growing mode is non-tearing type. However growth rates of the fastest 2-D tearing mode and 3-D non-tearing mode are comparable for thin current sheets ($d_e < $half thickness $ < 2\,d_e$) and small guide field (of the order of the asymptotic value of the component of magnetic field supporting electron current sheet). It is shown that the general mode resonance conditions for electron-magnetohydrodynamic (EMHD) and magnetohydrodynamic (MHD) tearing modes depend on the effective dissipation mechanism (electron inertia and resistivity in cases of EMHD and MHD, respectively). The usual tearing mode resonance condition ($\mathbf{k}.\mathbf{B}_0=0$, $\mathbf{k}$ is the wave vector and $\mathbf{B}_0$ is equilibrium magnetic field) can be recovered from the general resonance conditions in the limit of weak dissipation. Necessary conditions (relating current sheet thickness, strength of the guide field and wave numbers) for the existence of tearing mode are obtained from the general mode resonance conditions.