Why are astrophysical plasmas always magnetized?

Prof. Axel Brandenburg - Nordita (Svezia)
Why are astrophysical plasmas always magnetized?

Abstract
Long ago, magnetic fields in astrophysics used to be something for specialists. This has drastically changed over the past few decades. Nowadays, many scientists invoke magnetic fields in simulations of astrophysical flows. In fact, the concept of purely hydrodynamic turbulence without magnetic fields seems to be an idealization that hardly exists anywhere. We now understand that this is because of the universality of dynamo action in many types of flows. Dynamos convert kinetic energy into magnetic; they are self-excited and work without wires, but without short-circuiting themselves in spite of their high conductivity everywhere.

In my talk, I will start with a historical perspective, going back to the days when the existence and origin of magnetic fields was still very obscure. We knew about the Earth's magnetic field since the 1600s, and astronomical observations have revealed magnetic fields in sunspots and eventually in other stars and galaxies during the last century. To understand their origin, people had to struggle with Cowling's anti-dynamo theorem that magnetic fields cannot be generated from kinetic energy in a simple axisymmetric geometry. Gradually, it became clear that in three-dimensional settings, self-excited dynamos do actually work.

Meanwhile, with the emergence of three-dimensional simulations, where the plasma motions tend to be turbulent, dynamo action appears to be a natural by-product. Dynamos have also been realized in the lab in various configurations. But some basic questions in astrophysical applications are still troubling us: why exactly is the Sun's magnetic field exhibiting equatorward migration and why do the most realistic simulations not yet reproduce the large-scale magnetic fields observed in spiral galaxies by the present time. I will finish with applications to the early Universe, where decaying magnetic turbulence governs the entire radiation-dominated era and many of the relationships can be understood from dimensional arguments.