The Multi-Scale Structure of the Electron Diffusion Region: Implications for Observations and Theory

Details The Multi-Scale Structure of the Electron Diffusion Region: Implications for Observations and Theory The Multi-Scale Structure of the Electron Diffusion Region: Implications for Observations and Theory

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsh43a..02d&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SH43A-02

Physics

7526 Magnetic Reconnection (2723, 7835)

Scientific paper

Recent 2D kinetic simulations with open boundary conditions1 along with the largest periodic simulations ever considered have demonstrated that in large-scale systems the electron diffusion region (EDR) expands in time to form a highly elongated current layer with a width on the electron scale but a total length that can exceed tens of ion inertial lengths. This surprising result is nearly two orders of magnitude larger than previous predictions and challenges some of the central assumptions regarding the essential physics of collisionless reconnection. The formation of these layers involves a competition between the outward convection of flux with the non-ideal terms arising from the divergence of the electron pressure tensor. Although it is possible to achieve a balance over limited durations, over longer time scales these electron layers are unstable to secondary-island formation leading to a time dependent reconnection process. The formation of secondary islands is reminiscent of resistive MHD solutions in the presence of an imposed uniform resistivity. However, here the EDR exhibits multiple scales2 in the outflow direction and the resulting structure has no fluid analogue. The elongation of the EDR and secondary island formation appear to be a generic feature of reconnection and remain in the presence of a finite guide field. The implications of these results are profound and bring into question several key expectations based on two-fluid theory including the size of the EDR, temporal behavior of reconnection, importance of the Hall term, role of electrons in the reconnection process, and even the structure of the quadrupole field. Finally, these results offer a wealth of new predictions that should be observationally testable: (1) highly elongated non-gyotropic electron layers extending large distance from the x-line, (2) a continuous but generally time-dependent reconnection rate in the range ~ 0.03-0.14, (3) repeated formation of secondary- islands and (4) strong modifications to the out-of-plane quadrupole field structure. 1Daughton, Scudder and Karimabadi, Phys. Plasmas 13, 072101, 2006 2Karimabadi, Daughton and Scudder, Geophys. Res. Lett. 34, L13104, 2007

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