Magnetogenesis in a collisionless plasma: from Weibel instability to turbulent dynamo

TL;DR

This paper presents a comprehensive kinetic simulation study showing how unmagnetized plasmas can generate and amplify magnetic fields through a sequence of instabilities and turbulence, relevant to cosmic magnetic field origins.

Contribution

It introduces a first-principles kinetic framework demonstrating magnetic seed generation and turbulent dynamo amplification from unmagnetized plasma conditions.

Findings

Weibel instability generates seed magnetic fields on plasma scales.

Turbulent motions amplify magnetic fields to energy equipartition.

Magnetic fields influence plasma turbulence and transport properties.

Abstract

We report on a first-principles numerical and theoretical study of plasma dynamo in a fully kinetic framework. By applying an external mechanical force to an initially unmagnetized plasma, we develop a self-consistent treatment of the generation of ``seed'' magnetic fields, the formation of turbulence, and the inductive amplification of fields by the fluctuation dynamo. Driven large-scale motions in an unmagnetized, weakly collisional plasma are subject to strong phase mixing, which leads to the development of thermal pressure anisotropy. This anisotropy triggers the Weibel instability, which produces filamentary ``seed'' magnetic fields on plasma-kinetic scales. The plasma is thereby magnetized, enabling efficient stretching and folding of the fields by the plasma motions and the development of Larmor-scale kinetic instabilities such as the firehose and mirror. The scattering of particles off the associated microscale magnetic fluctuations provides an effective viscosity, regulating the field morphology and turbulence. During this process, the seed field is further amplified by the fluctuation dynamo until they reach energy equipartition with the turbulent flow. By demonstrating that equipartition magnetic fields can be generated from an initially unmagnetized plasma through large-scale turbulent flows, this work has important implications for the origin and amplification of magnetic fields in the intracluster and intergalactic mediums.