Generation of near-equipartition magnetic fields in turbulent collisionless plasmas

TL;DR

This study uses kinetic simulations to demonstrate how turbulence in collisionless plasmas can generate and amplify magnetic fields to near-equipartition levels, shedding light on cosmic magnetogenesis.

Contribution

It provides the first detailed kinetic simulation analysis of magnetic field generation and growth via the Weibel instability in turbulent collisionless plasmas.

Findings

Magnetic fields grow exponentially with rate ~0.4 u_rms/L.

Magnetic energy reaches about half of turbulent kinetic energy at saturation.

Magnetic spectrum peaks at wavenumber ~12π/L at saturation.

Abstract

The mechanisms that generate "seed" magnetic fields in our Universe and that amplify them throughout cosmic time remain poorly understood. By means of fully-kinetic particle-in-cell simulations of turbulent, initially unmagnetized plasmas, we study the genesis of magnetic fields via the Weibel instability and follow their dynamo growth up to near-equipartition levels. In the kinematic stage of the dynamo, we find that the rms magnetic field strength grows exponentially with rate $\gamma_B \simeq 0.4\,u_{\rm rms}/L$, where $L/2 \pi$ is the driving scale and $u_{\rm rms}$ is the rms turbulent velocity. In the saturated stage, the magnetic field energy reaches about half of the turbulent kinetic energy. Here, magnetic field growth is balanced by dissipation via reconnection, as revealed by the appearance of plasmoid chains. At saturation, the integral-scale wavenumber of the magnetic spectrum approaches $k_{\rm int}\simeq 12\pi/L$. Our results show that turbulence -- induced by, e.g., the gravitational build-up of galaxies and galaxy clusters -- can magnetize collisionless plasmas with large-scale near-equipartition fields.