PROF. AXEL BRANDENBURG

Magnetic fields are prominent players in virtually all areas of astrophysics, Theory and simulations demonstrate their exponential growth starting from weak seed magnetic fields that, in turn, are generated by some battery term. This is the self-excited dynamo, a tremendously important concept for the utilization of electricity since the works of Wheatstone, von Siemens, and Varley of 1866. Homogeneous self-excited dynamos, which do not have wires, are susceptible to short-circuiting themselves. Although anticipated already 99 years by Larmor, it was only since the 1970s that astrophysical dynamos became theoretical and numerical reality and not just ideas. Since 2000, experimental realizations have begun to expand our understanding into corners of parameter space not yet accessible to simulations. In my talk, I will highlight the concepts of small-scale and large-scale dynamos, the dependence on the ratio of viscous to resistive dissipation, the inverse turbulent cascade of magnetic helicity, and new classes of large-scale dynamos beyond just helical ones. In density-stratified systems, cross helicity, characterizing the linkage between magnetic and vortex tubes, plays a rote in producing spots, perhaps like those seen on the Sun. In spite of much progress, though, we are still not sure how exactly the solar dynamo works, whether primordial magnetic fields play a role in galactic magnetism, and whether such primordial fields are perhaps even fully helical.