The effect of external magnetic field on electron scale Kelvin-Helmholtz instability

We use particle-in-cell, fully electromagnetic, plasma kinetic simulation to study the effect of external magnetic field on electron scale Kelvin-Helmholtz instability (ESKHI). The results are applicable to collisionless plasmas when e.g. solar wind interacts with planetary magnetospheres or magnetic field is generated in AGN jets. We find that as in the case of magnetohydrodynamic KHI, in the kinetic regime, presence of external magnetic field reduces growth rate of the instability. In MHD case there is known threshold magnetic field for KHI stabilization, while for ESKHI this is to be analytically determined. Without a kinetic analytical expression, we use several numerical simulation runs to establish an empirical dependence of ESKHI growth rate, $\Gamma(B_0)\omega_{\rm pe}$, on the strength of applied external magnetic field. We find the best fit is hyperbolic, $\Gamma(B_0)\omega_{\rm pe}=\Gamma_0\omega_{\rm pe}/(A+B\bar B_0)$, where $\Gamma_0$ is the ESKHI growth rate without external magnetic field and $\bar B_0=B_0/B_{\rm MHD}$ is the ratio of external and two-fluid MHD stability threshold magnetic field, derived here. An analytical theory to back up this growth rate dependence on external magnetic field is needed. The results suggest that in astrophysical settings where strong magnetic field pre-exists, the generation of an additional magnetic field by the ESKHI is suppressed, which implies that the Nature provides a "safety valve" -- natural protection not to "over-generate" magnetic field by ESKHI mechanism. Remarkably, we find that our two-fluid MHD threshold magnetic field is the same (up to a factor $\sqrt{\gamma_0}$) as the DC saturation magnetic field, previously predicted by fully kinetic theory.