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

The amplification of seed magnetic fields by the turbulent dynamo is believed to be essential in forming pervasive cosmic magnetic fields. Both the mechanisms of seed-field production and the nature of plasma dynamos have been studied independently. However, in the weakly collisional intergalactic/intracluster medium, the turbulent motions of dynamo may also give rise to seed fields and thus magnetize the plasma non-inductively. The strength and morphology of these seed fields have not been studied self-consistently. We study in a fully-kinetic framework the generation of seed fields through the Weibel instability in an initially unmagnetized plasma driven by 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 phase, and its saturation due to the particle magnetization. The predicted scaling dependencies of the saturated seed fields on key parameters (e.g., scale separation between the forcing scale and electron skin depth, the forcing amplitude) are confirmed by our 3D and 2D particle-in-cell simulations. This work demonstrates the spontaneous magnetization of a collisonless plasma through large-scale motions as simple as a shear flow.