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The mechanisms that generate "seed" magnetic fields in our Universe and thatamplify them throughout cosmic time remain poorly understood. By means offully-kinetic particle-in-cell simulations of turbulent, initially unmagnetizedplasmas, we study the genesis of magnetic fields via the Weibel instability andfollow their dynamo growth up to near-equipartition levels. In the kinematicstage of the dynamo, we find that the rms magnetic field strength growsexponentially 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 thesaturated stage, the magnetic field energy reaches about half of the turbulentkinetic energy. Here, magnetic field growth is balanced by dissipation viareconnection, as revealed by the appearance of plasmoid chains. At saturation,the integral-scale wavenumber of the magnetic spectrum approaches $k_{\rmint}\simeq 12\pi/L$. Our results show that turbulence -- induced by, e.g., thegravitational build-up of galaxies and galaxy clusters -- can magnetizecollisionless plasmas with large-scale near-equipartition fields.

Generation of near-equipartition magnetic fields in turbulent collisionless plasmas