Turbulent reconnection of magnetic bipoles in stratified turbulence

Abstract

We consider strongly stratified forced turbulence in a plane-parallel layer with helicity and corresponding large-scale dynamo action in the lower part and non-helical turbulence in the upper. The magnetic field is found to develop strongly concentrated bipolar structures near the surface. They form elongated bands with a sharp interface between opposite polarities. Unlike earlier experiments with imposed magnetic field, the inclusion of rotation does not strongly suppress the formation of these structures. We perform a systematic numerical study of this phenomenon by varying magnetic Reynolds number, scale separation ratio, and Coriolis number. We focus on the formation of a 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 > 10 3 , the reconnection rate is nearly independent of S in agreement with results of recent numerical simulations performed by other groups in simpler settings.

On the energization of charged particles by fast magnetic reconnection

“…Kowal et al (2009) have measured it by using a contour encompassing the reconnection region and calculating the flux that flows into the region. This incoming flux can also be estimated by finding out the typical speed of the inflow towards the current sheet (Jabbari et al 2016). In this paper, we suggest a prescription that is similar to the one suggested by Kowal et al (2009).…”

Magnetic bipoles in rotating turbulence with coronal envelope

“…In the upper part, the magnetic field displays the formation of strongly concentrated BRs. This was seen in DNS in both Cartesian domains (Mitra et al 2014;Jabbari et al 2016Jabbari et al , 2017 and spherical shells (Jabbari et al 2015).…”

Magnetic concentrations in stratified turbulence: the negative effective magnetic pressure instability

“…To investigate the role of magnetic reconnection, the flow around the sharp interface was zoomed in, and the dynamics of the current sheet in this region was studied [35]. The reconnection rate was determined independently through the inflow velocity in the vicinity of the current sheet and via the electric field in the reconnection region.…”

“…Some of them involve turbulent convection [16,27,[29][30][31], or a stably stratified polytropic atmosphere [32], so it remains to be clarified, whether gravity plays a direct role, or whether the magnetic field concentrations are mainly the result of converging downdrafts. Other simulations involve forced turbulence in isothermally stratified domains, where no thermally driven convection is possible [10][11][12][13][14][33][34][35], and yet one finds the formation of large-scale magnetic structures. What is remarkable is that these structures extend over the scale of many turbulent eddies.…”

“…dynamo simulations of forced turbulence, where rotation also did not suppress the formation of structures [35]. Thus, the question of the origin of these structures remains unsettled.…”