On the Formation of Extended Current Sheets in Collisionless Hall-MHD Systems
Brian Sullivan (UNH)
Monday 1st March 2010, 12:00pm
Pratt conference room, 60 Garden Street
The geometry of the electron dissipation region plays a key role in determining the rate of magnetic reconnection. Hall MHD and two-fluid simulations of magnetic reconnection have generally produced fairly cusp-like open outflow geometries. In contrast, particle-in-cell simulations of reconnection tend to produce more extended electron dissipation regions. These differences most likely stem from the effects of the agyrotropic electron pressure tensor, included in fully kinetic models, but not in Hall MHD. Some analytic results have indicated that hyper-resistivity, or electron viscosity may act as a fluid approximation to some of the significant pressure tensor effects, allowing the dissipation region to become highly elongated. This seminar will present Sweet-Parker type scaling arguments in the context of hyper-resistive Hall MHD. The scaling arguments will be bench marked by the results of numerical experiments. Results suggest that very modestly extended reconnection geometries can be realized in the two fluid model. However, the length of the electron dissipation region (taken as a parameter by some studies), is found to depend explicitly on the level of hyper-resistivity. Furthermore, although hyper-resistivity can produce modestly extended electron dissipation regions, the length of the region remains smaller than one ion skin depth for the largest values of hyper-resistivity considered--significantly shorter than current sheets observed in kinetic simulations.