Formation of dynamo-driven bipolar magnetic spots in stratified turbulence
Astronomy and astrophysics
Friday 08 April 2016
to 11:30 at
Sarah Jabbari (Nordita / SU Astronomy)
We consider strongly stratified forced turbulence in a two-layer model with helicity and corresponding large-scale dynamo action in the lower part and nonhelical turbulence in the upper. We perform direct numerical simulations in both spherical and Cartesian geometries using the Pencil Code. Our results show that, when the stratification is high enough, the magnetic field develops strongly concentrated super-equipartition bipolar structures near the surface in both geometries. In the Cartesian case, they form elongated bands with a sharp interface between opposite polarities. We carry out a systematic numerical study of this phenomenon by varying magnetic Reynolds number, scale separation ratio, and Coriolis number. Unlike earlier experiments with imposed magnetic field, the inclusion of rotation does not strongly suppress the formation of these structures. We also focus on the formation of the current sheet between bipolar regions where reconnection of oppositely oriented field lines occurs. We determine the reconnection rate by measuring either the inflow velocity in the vicinity of the current sheet or by measuring the electric field in the reconnection region. We demonstrate that for large Lundquist numbers, S > 1000, the reconnection rate is nearly independent of S, in agreement with results of recent numerical simulations performed by other groups in a simpler setups.