Numerical studies of diffusion and amplification of magnetic fields in turbulent astrophysical plasmas

Abstract

In this thesis we investigated two major issues in astrophysical flows: the transport of magnetic fields in highly conducting fluids in the presence of turbulence, and the turbulence evolution and turbulent dynamo amplification of magnetic fields in collisionless plasmas. The first topic was explored in the context of star-formation, where two intriguing problems are highly debated: the requirement of magnetic flux diffusion during the gravitational collapse of molecular clouds in order to explain the observed magnetic field intensities in protostars (the so called “magnetic flux problem”) and the formation of rotationally sustained protostellar discs in the presence of the magnetic fields which tend to remove all the angular momentum (the so called “magnetic braking catastrophe”). Both problems challenge the ideal MHD description, usually expected to be a good approximation in these environments. The ambipolar diffusion, which is the mechanism commonly invoked to solve these problems, has been lately questioned both by observations and numerical simulation results. We have here investigated a new paradigm, an alternative diffusive mechanism based on fast magnetic reconnection induced by turbulence, termed turbulent reconnection diffusion (TRD). We tested the TRD through fully 3D MHD numerical simulations, injecting turbulence into molecular clouds with initial cylindrical geometry, uniform longitudinal magnetic field and periodic boundary conditions. We have demonstrated the efficiency of the TRD in decorrelating the magnetic flux from the gas, allowing the infall of gas into the gravitational well while the field lines migrate to the outer regions of the cloud. This mechanism works for clouds starting either in magnetohydrostatic equilibrium or initially out-of-equilibrium in free-fall. We