Invited Talk - Splinter Magnetic

Thursday, 24 September 2020, 16:15   (virtual room H)

Dynamos of the Sun and stars

Axel Brandenburg
Nordita, Stocholm

Flux transport dynamos have been successful in reproducing a number of behaviors seen in the Sun. This includes in particular the equatorward migration of the toroidal magnetic field and the rush to the poles of poloidal magnetic field at high solar latitudes. Full numerical simulations have failed to produce these aspects in a satisfactory manner, so something must be wrong with the simulations. The problem starts already when comparing the large-scale flows, which display markedly cylindrical patterns in both angular velocity and meridional circulation, which appear incompatible with helioseismology. Some simulations display equatorward migration, but this is because of a localized dip in angular velocity at mid-latitude, which again is not found in the Sun. In principle, of course, the simulations ought to reproduce the solar behavior eventually, as better resolution can be achieved. The biggest resolution deficit is near the surface, where global simulations are unable to capture the effects of what is known as entropy rain. In my talk, I will review aspects of the solar dynamo and will discuss how the effects of entropy rain may change the picture. I will also touch upon dynamo-related aspects of the physics just above the solar dynamo, i.e., in the lower corona. This is where the magnetic Prandtl number becomes large and the plasma beta drops. In the MHD picture, most of the energy dissipation is now through viscous heating rather than on current sheets. Through the Lorentz force, the magnetic field does work on the flow, rather than the flow doing work on the field against the Lorentz force, as in a dynamo. We therefore have here a reversed dynamo. This is also where the magnetic helicity seems to reverse its sign, but this has so far only been seen in simulations and in the solar wind far away from the Sun. Diagnosing it in the solar corona remains a challenge. I will also discuss corresponding numerical simulations of the dynamo exterior where the solar wind is accelerated. I will finish by discussing two diagnostic techniques that employ linear polarization: Faraday depolarization and the parity-odd contribution to linear polarization. Both are potentially useful to diagnosing not only the Sun's corona, but possibly also laboratory plasmas.