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

Tsiklauri, D.

Abstract

We use particle-in-cell, fully electromagnetic, plasma kinetic simulation to
study the effect of external magnetic field on electron scale Kelvin-Helmholtz instabil-
ity (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 an-
alytically determined. Without a kinetic analytical expression, we use several numerical
simulation runs to establish an empirical dependence of ESKHI growth rate, Γ(B0)ωpe,
on the strength of applied external magnetic field. We find the best fit is hyperbolic,
Γ(B0)ωpe = Γ0ωpe/(A + B ¯B0), where Γ0 is the ESKHI growth rate without external
magnetic field and ¯B0 = B0/BMHD 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 astrophysi-
cal settings where strong magnetic field pre-exists, the generation of an additional mag-
netic 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 √γ0) as the DC saturation magnetic field, previously predicted by fully kinetic
theory.

• Journal Article Type: Article
• Acceptance Date: Aug 24, 2024
• Online Publication Date: Sep 20, 2024
• Publication Date: Sep 1, 2024
• Deposit Date: Oct 15, 2024
• Publicly Available Date: Sep 2, 2025
• Journal: Research in Astronomy and Astrophysics
• Print ISSN: 1674-4527
• Publisher: IOP Publishing
• Peer Reviewed: Peer Reviewed
• Volume: 24
• Issue: 9
• Article Number: 095021
• DOI: https://doi.org/10.1088/1674-4527/ad74de