Spontaneous magnetization of collisionless plasma through the action of a shear flow

TL;DR

This paper demonstrates how shear flows in collisionless plasmas can spontaneously generate magnetic fields via the Weibel instability, with analytical and simulation results confirming the scaling of seed magnetic fields.

Contribution

It provides a comprehensive kinetic model and simulation validation for seed magnetic field generation driven by shear flows in unmagnetized plasmas.

Findings

Magnetic fields grow exponentially during the linear Weibel stage.

Saturation occurs when magnetic fields induce particle gyromotion.

Scaling laws for saturated magnetic fields depend on system parameters.

Abstract

We study in a fully kinetic framework the generation of seed magnetic fields through the Weibel instability driven in an initially unmagnetized plasma by a large-scale shear force. We develop an analytical model that describes the development of thermal pressure anisotropy via phase mixing, the ensuing exponential growth of magnetic fields in the linear Weibel stage, and its saturation when the seed magnetic fields become strong enough to instigate gyromotion of particles and thereby inhibit their free-streaming. The predicted scaling dependencies of the saturated seed fields on key parameters (e.g., ratio of system scale to electron skin depth, the forcing amplitude) are confirmed by 3D and 2D particle-in-cell simulations using an electron-positron plasma. This work demonstrates the spontaneous magnetization of a collisionless plasma through large-scale motions as simple as a shear flow, and therefore has important implications for magnetogenesis in dilute astrophysical systems.