Dissipation in astrophysical jets with velocity and magnetic shear: Interaction of Kelvin-Helmholtz and Drift-Kink Instabilities*

We present 2D particle-in-cell simulations of a magnetized, collisionless, relativistic pair plasma subjected to a strong combined velocity and magnetic-field shear at a thin vortex- and current-sheet interface, a scenario typical for astrophysical black-hole jet-wind boundaries. By considering a 2D simulation plane formed by the velocity flow and the gradient direction, with magnetic field perpendicular to the plane, we focus on the case where only the Kelvin-Helmholtz (KH) and Drift-Kink (DK) instabilities can develop, while tearing (and hence magnetic reconnection) is forbidden. In addition to a control case where only the velocity shear is present, we analyze a sequence of simulations in which the (out-of-plane) magnetic field is reversed across the interface and the velocity shear is progressively increased. This strategy allows us to explore the general dynamics and dissipation driven synergistically by the interplay of the two shears and compare them to the effects of either of the shears taken in isolation. We thus investigate, for the first time, the nonlinear interaction of the KH and DK instabilities, generating qualitatively new structures with very different dissipative behaviors. We find that DKI can effectively disrupt the cat-eye vortices generated by KHI, creating a turbulent shear layer on the joint DK-KHI timescale. This interplay leads to a significant enhancement of dissipation over the velocity-shear-only case. In addition, we find a special, relatively narrow range in velocity shear where the joint DK-KHI is particularly active, resulting in even stronger dissipation. Finally, we observe efficient nonthermal particle acceleration caused by the alignment of the instability-driven electric fields with Speiser-like motion of particles close to the shear interface. This study highlights the sensitivity of flow structures, dissipation, and particle acceleration to multiple simultaneously operating instabilities, thus providing a strong motivation for further studies of their nonlinear interaction at the kinetic level. Such studies will help elucidate the nature of dissipation and particle acceleration, and in particular explain the observed emission limb-brightening, in relativistic jets from astrophysical supermassive black holes.