Accretion discs are a common phenomenon in astrophysics. They can be found in objects spanning a huge range in magnitude. I will start my talk presenting results from simulations of interstellar turbulence driven by SNe. The occurring galaxy dynamo can successfully be described by means of kinematic mean-field theory.
With magnetic fields becoming dynamically important, we leave the realms of kinematic theory. Going to smaller scales, as relevant for protoplanetary discs, I will show that dynamically quenched mean-field models can reasonably well reproduce the “butterfly” diagram seen in stratified MRI simulations.
Finally, in compact binary systems, we find strongly magnetised discs. Here the MRI will not be the only player but will be intertwined with the Parker instability. I confirm recent results by Johansen and Levin, and show that differentially rotating discs can in fact retain their azimuthal flux, counteracting magnetic buoyancy.
