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Current sheet formation in quasi-separatrix layers and hyperbolic flux tubes View all abstracts by submitter

Guillaume Aulanier   Submitted: 2005-07-22 07:49

In 3D magnetic field configurations, quasi-separatrix layers (QSLs) are defined as volumes in which field lines locally display strong gradients of connectivity. Considering QSLs as the preferential locations for current sheet development and magnetic reconnection in general, and as a natural model for solar flares and coronal heating in particular, have been strongly debated issues over the last decade. In this paper, we perform zero-eta resistive MHD simulations of the development of electric currents in smooth magnetic configurations, which are strictly speaking bipolar though they are formed by four flux concentrations, and whose potential fields contain QSLs. The configurations are driven by smooth and large-scale sub-Alfvénic footpoint motions. Extended electric currents naturally form in the configurations, which evolve through a sequence of quasi non-linear force-free equilibria. Narrow current layers also develop. They spontaneously form at small scales, all around the QSLs, whatever the footpoint motions are. For long enough motions, the strongest currents develop where the QSLs are the thinnest, namely at the Hyperbolic Flux Tube (HFT) which generalizes the concept of separator. These currents progressively take the shape of an elongated sheet, whose formation is associated with a gradual steepening of the magnetic field gradients over tens of Alfvén times, due to the different motions applied to the field lines which pass on each side of the HFT. Our model then self-consistently accounts for the long-duration energy storage prior to a flare, followed by a switch-on of reconnection when the currents reach the dissipative scale at the HFT. In configurations whose potential fields contain broader QSLs, when the magnetic field gradients reach the dissipative scale, the currents at the HFT %quasi-separator reach higher magnitudes. This implies that major solar flares, that are not related with an early large-scale ideal instabilities, must occur in regions whose corresponding potential fields have broader QSLs. Our results lead us to conjecture that physically, current layers must always form on the scale of the QSLs. This implies that electric currents around QSLs may be gradually amplified in time only if the QSLs are broader than the dissipative length-scale. We also discuss the potential role of QSLs in coronal heating in bipolar configurations made of a continuous distribution of flux concentrations.

Authors: G. Aulanier, E. Pariat and P. Demoulin
Projects: None

Publication Status: A&A (accepted)
Last Modified: 2005-07-22 07:49
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