Turbulent reconnection after formation of bipolar magnetic structures in strongly stratified helical dynamos

Nordita Astrophysics seminars

Wednesday 02 December 2015
from 13:30
to 14:30 at
122:026

Speaker :

Sarah Jabbari (Stockholm University and Nordita)

Abstract :

The strongly stratified forced two-layer turbulence with helicity and corresponding large-scale dynamo action in the lower layer, and nonhelical turbulence in the upper is investigated in the plane geometry. In this model the magnetic field concentrates near the surface layer and creates intense bipolar magnetic spots. These spots form elongated band like structures, with a sharp boundary between different polarities. Such model was first studied in plane geometry by Mitra et al. (2014) and then extended to spherical geometry by Jabbari et al. (2015). Here we investigate the effects of Reynolds number, ReM, thickness of the helical forcing layer, the scale separation, and rotation on the formation of the bipolar structures in the plane geometry. Then we concentrate on the lifetime of these structures and verify the formation of a current sheet and occurrence of reconnection when the elongated bipolar connect to each other. We calculate the reconnection rate by both measuring the speed of the inflow in the vicinity of the current sheet and using a more general method suggested by Kowal et al. (2009). Our results show that for small Lundquist number, S < 1000, the reconnection rate follows standard Sweet-Parker model but for larger S, the reconnection rate in nearly independent of Lundquist number. We also study the dependence of the reconnection rate on Alfven Much number, MA, and show that our reconnection rate does not vary by MA, as suggested by Lazarian & Vishniac (1999).