Magnetic field amplification in turbulent astrophysical plasmas (1610.08132v2)

Abstract: Magnetic fields play an important role in astrophysical accretion discs, and in the interstellar and intergalactic medium. They drive jets, suppress fragmentation in star-forming clouds and can have a significant impact on the accretion rate of stars. However, the exact amplification mechanisms of cosmic magnetic fields remain relatively poorly understood. Here I start by reviewing recent advances in the numerical and theoretical modelling of the 'turbulent dynamo', which may explain the origin of galactic and inter-galactic magnetic fields. While dynamo action was previously investigated in great detail for incompressible plasmas, I here place particular emphasis on highly compressible astrophysical plasmas, which are characterised by strong density fluctuations and shocks, such as the interstellar medium. I find that dynamo action works not only in subsonic plasmas, but also in highly supersonic, compressible plasmas, as well as for low and high magnetic Prandtl numbers. I further present new numerical simulations from which I determine the growth of the turbulent (un-ordered) magnetic field component ($B_\mathrm{turb}$) in the presence of weak and strong guide fields ($B_0$). I vary $B_0$ over 5 orders of magnitude and find that the dependence of $B_\mathrm{turb}$ on $B_0$ is relatively weak, and can be explained with a simple theoretical model in which the turbulence provides the energy to amplify $B_\mathrm{turb}$. Finally, I discuss some important implications of magnetic fields for the structure of accretion discs, the launching of jets, and the star formation rate of interstellar clouds.