Magnetic Reconnection: Theory and Observations II
Presiding: M Hesse, NASA Goddard Space Flight Center; T G Forbes, University of New Hampshire
SM32A-01 INVITED 10:30h
The "Newton Challenge": Properties of Forced Magnetic Reconnection
Inspired by the observations of thin (ion-scale) current sheets at important magnetospheric boundaries, the study of the properties of thin current sheets has become very popular in recent years. Most of these investigations, however, have ignored the question of how the sheets are formed. Instead, usually a simple Harris-type current sheet is postulated at the outset, and the resulting behavior is then determined. Recently, a collaborative effort, dubbed the "Newton Challenge" and involving J. Birn, K. Galsgaard, M. Hesse, M. Hoshino, J. Huba, G. Lapenta, P.~L. Pritchett, K. Schindler, L. Yin, J. Büchner, T. Neukirch, and E.~R. Priest, was begun to investigate the transition from thicker to thin current sheets that can occur as a result of magnetopause deformations imposed by the solar wind. A standard 2-D model problem in which current sheet thinning was forced by imposing a finite deformation of the field above and below the current sheet was studied by a variety of physical models ranging from resistive MHD to fully kinetic particle models. The aim was to determine whether differences would arise between the fluid and kinetic treatments that might affect the onset of magnetic reconnection. The initial results indicate that full-particle, hybrid, and Hall-MHD models lead to fast reconnection and similar final states despite differences in energy transfer and dissipation. Resistive MHD simulations show reduced reconnection rates that depend on the magnitude of the resistivity. These results will be reviewed, and additional features of forced reconnection, including continuous forcing, open boundaries, the presence of a normal field component, and 3-D effects, will be discussed.
SM32A-02 10:50h
Entropy Conservation in Simulations of Magnetic Reconnection
A crucial element in the physics of reconnection is the local breakdown of the frozen-in field approximation that governs ideal MHD. However, there are other approximations included in ideal MHD, which might also be expected to break down when reconnection is initiated, specifically the conservation of entropy. We use the "Newton challenge" problem of forced current sheet thinning and reconnection to compare entropy conservation in a resistive MHD simulation with a particle-in-cell simulation. When the resistivity is strongly localized in the MHD simulation, the global conservation of entropy in magnetic flux tubes undergoing reconnection is surprisingly similar between fluid and particle simulations. This explains why the final configurations are also very similar, despite the fact that the pressure distributions change drastically and that particle simulations include pressure anisotropy, waves, and dissipation mechanisms different from the resistive fluid model. It further indicates that Joule dissipation is highly localized.
SM32A-03 INVITED 11:05h
Quantifying Magnetic Reconnection in the Solar Corona
Magnetic reconnection is believed to play a role in many aspects of solar activity including flares, CMEs and quiet sun brightenings. The process itself is fundamentally a change field line topology resulting from some non-ideal term in the generalized Ohm's law such as collisional resistivity or electron inertia. Such non-ideal effects may or may not dissipate energy directly but do produce topological field line changes at a rate proportional to the non-ideal electric field. The rate of magnetic reconnection can be measured by quantifying the number of field lines topologically changed over time. Chromospheric flare ribbons are believed to reflect the footpoints of topological boundaries; ribbon motion across photopsheric flux is therefore used to infer reconnection rates. Topology of individual X-ray or EUV coronal loops can be unambiguously defined when the photopsheric field is composed of distinct source regions to which footpoints may be assigned. Reconnection occurs as flux is transfered between these topological regions, and the rate is found by quantifying this change. Several measurements of this type have been made, quantifying reconnection rates in the quiet sun and non-flaring active region evolution. This work was funded by NASA and NSF.
SM32A-04 11:25h
Patchy Reconnection in a Solar Post-CME Current Sheet
We study the dynamics of multiple, highly localized reconnection events in a post coronal mass ejection type current sheet. We impose the reconnection in an MHD simulation by enhancing the resistivity in small regions for a short time, thus allowing a finite amount of flux to reconnect. This forms a pair of 3D reconnected flux tubes piercing the current sheet. These tubes then retract from the reconnection region, pushing their way through the surrounding magnetic field. We will study how these tubes react when they collide with each other, and when they reach the edge of the current sheet and collide with the field there. We will use this model to study the theory that the descending post-eruption voids seen by TRACE and LASCO (see e.g. Sheeley et al 2004) are in fact reconnected flux tubes from patchy reconnection. This work has been supported by NASA and ONR.
SM32A-05 INVITED 11:40h
Flux Pileup Reconnection at the Dayside Magnetopause under Northward IMF Conditions
An unsolved problem in magnetospheric physics concerns the geometry of magnetic reconnection at the dayside magnetopause under generic northward interplanetary magnetic field (IMF) conditions. There are currently two popular and competing models which address this problem. In the "antiparallel" reconnection model, the reconnection line is the locus of points on the magnetopause surface for which the magnetosheath and magnetospheric fields are antiparallel. In contrast, the "component" reconnection model does not constrain the reconnecting fields to be antiparallel, instead identifying the X line direction with that of the local current density. In this presentation, we argue that both of these pictures result from an inappropriate application of ideas from two dimensional reconnection theory to an inherently three dimensional situation. Using global magnetohydrodynamics (MHD) simulations, we demonstrate that, in the context of resistive MHD with constant Lundquist number -- and under generic northward IMF conditions, where the y and z components of the magnetic field (in Solar Magnetospheric coordinates) are comparable -- magnetic reconnection occurs at the subsolar magnetopause via a flux pileup mechanism. Current density is distributed across the dayside magnetopause in a "ribbon" which terminates near two cusp magnetic nulls. Due to the inherently three-dimensional nature of the reconnection geometry, neither the component nor the antiparallel model is relevant; instead, the geometry is more reminiscent of "separator" reconnection, which combines aspects of both component and antiparallel merging. We discuss the possibility of using spacecraft data to test the predictions of the MHD simulations.