Magnetic flux concentrations from dynamogenerated fields 

Axel Brandenburg Submitted: 20150727 14:45
The meanfield theory of magnetized stellar convection gives rise to the two possibility of distinct instabilities: the largescale dynamo instability, operating in the bulk of the convection zone, and a negative effective magnetic pressure instability (NEMPI) operating in the strongly stratified surface layers. The latter might be important in connection with magnetic spot formation, but the growth rate of NEMPI is suppressed with increasing rotation rates, although recent direct numerical simulations (DNS) have shown a subsequent increase in the growth rate. We examine quantitatively whether this increase in the growth rate of NEMPI can be explained by an α squared meanfield dynamo, and whether both NEMPI and the dynamo instability can operate at the same time. We use both DNS and meanfield simulations (MFS) to solve the underlying equations numerically either with or without an imposed horizontal field. We use the testfield method to compute relevant dynamo coefficients. DNS show that magnetic flux concentrations are still possible up to rotation rates above which the largescale dynamo effect produces mean magnetic fields. The resulting DNS growth rates are quantitatively well reproduced with MFS. As expected, for weak or vanishing rotation, the growth rate of NEMPI increases with increasing gravity, but there is a correction term for strong gravity and large turbulent magnetic diffusivity. Magnetic flux concentrations are still possible for rotation rates above which dynamo action takes over. For the solar rotation rate, the corresponding turbulent turnover time is about 5 hours, with dynamo action commencing in the layers beneath.
Authors: Sarah Jabbari, Axel Brandenburg, Illa R. Losada, Nathan Kleeorin, Igor Rogachevskii
Projects: EUNIS

Publication Status: Astron. Astrophys. 568, A112 (2014)
Last Modified: 20150729 13:00


