Large-Scale Magnetic Field Generation via the Kinetic Kelvin-Helmholtz Instability in Unmagnetized Scenarios

Abstract

Collisionless plasma instabilities are fundamental in magnetic field generation in astrophysical scenarios, but their role has been addressed in scenarios where velocity shear is absent. In this work we show that velocity shears must be considered when studying realistic astrophysical scenarios, since these trigger the collisionless Kelvin-Helmholtz instability (KHI). We present the first selfconsistent three-dimensional (3D) particle-in-cell (PIC) simulations of the KHI in conditions relevant for unmagnetized relativistic outflows with velocity shear, such as active galactic nuclei (AGN) and gamma-ray bursts (GRBs). We show the generation of a strong large-scale DC magnetic field, which extends over the entire shear-surface, reaching thicknesses of a few tens of electron skin depths, and persisting on time-scales much longer than the electron time scale. This DC magnetic field is not captured by MHD models since it arises from intrinsically kinetic effects. Our results indicate that the KHI can generate intense magnetic fields yielding equipartition values up to ǫ B /ǫ p ≃ 10 −3 − 10 −2 in the electron time-scale. The KHI-induced magnetic fields have a characteristic structure that will lead to a distinct radiation signature, and can seed the turbulent dynamo amplification process. The dynamics of the KHI are relevant for non-thermal radiation modeling and can also have a strong impact on the formation of relativistic shocks in presence of velocity shears.

Transverse electron-scale instability in relativistic shear flows

“…In conclusion, we have described a fast-growing electronscale instability (MI) that develops in unmagnetized sheared flows, and that explains the structures observed in [17,19,20,22]. We have shown via analytic theory that the MI grows faster than the closely related ESKHI (that operates in the perpendicular plane) in relativistic shears.…”

“…These new unstable modes explain the transverse dynamics and structures observed in PIC simulations in [17,19,20,22]. We label this effect the mushroom instability (MI) due to the mushroomlike structures that emerge in the electron density.…”

“…Sheared flow settings have been traditionally studied using the MHD framework [12][13][14], where the Kelvin-Helmholtz instability (KHI) is the only instability known to develop [15]. Only very recently have collisionless unmagnetized sheared plasma flows been addressed experimentally [16] and using particle-in-cell (PIC) simulations, revealing a rich variety of electron-scale processes, such as the electron-scale KHI (ESKHI), dc magnetic field generation, and unstable transverse dynamics [17][18][19][20][21][22]. The generated fields and modified particle distributions due to these microscopic processes can strongly impact succeeding macroscopic dynamics of the sheared flow.…”

“…In the limit, n − n + /γ 3 + , γ + 1, and v − = 0, relevant for astrophysical jetinterstellar medium shear interaction, the maximum growth rate yields max /ω pe+ 1/ √ 2γ + . The MI and ESKHI [17] growth rates are compared for different shear Lorentz factors and velocities in Fig. 1.…”

The Role of Macroscopic and Microscopic Jet Instabilities

“…Counter-streaming simulations show that the KKHI leads to magnetic field generation and particle acceleration [139,140]. As KKHI grows the electrons from the counterstreaming flows cross the shear-surface.…”