The Structure and Stability of the Diffusion Region in Collisionless Magnetic Reconnection

Details The Structure and Stability of the Diffusion Region in Collisionless Magnetic Reconnection The Structure and Stability of the Diffusion Region in Collisionless Magnetic Reconnection

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm41a1445d&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SM41A-1445

Physics

2723 Magnetic Reconnection (7526, 7835), 7526 Magnetic Reconnection (2723, 7835), 7835 Magnetic Reconnection (2723, 7526)

Scientific paper

One of the central open questions in the physics of collisionless reconnection is the structure and stability of the diffusion region in various parameter regimes. Presently, there is no rigorous theory to describe the complicated multi-scale structure and thus simulations and scaling arguments have played a prominent role. From these efforts, researchers have argued that the electron diffusion region is microscopic in both the inflow and outflow directions with an outflow velocity limited by the electron Alfvén speed. If these assumptions are correct, the inner electron layer does not control the reconnection rate and similar results would be expected from Hall MHD, hybrid and full particle simulations as reported in the GEM challenge and related studies. This would also imply that the chances of encountering the electron diffusion region in spacecraft observations would be very small. In sharp contrast to this conventional thinking, recent results from large-scale fully kinetic particle-in-cell (PIC) simulations indicate the electron diffusion region does not remain microscopic but expands in time to form an extended electron current layer\footnote{Daughton, Scudder, Karimabadi Phys. Plasmas 13, 072101, 2006}. As a result, the electron diffusion region forms a bottleneck limiting the reconnection rate and under certain circumstances the electron layer becomes unstable to secondary island formation leading to a reconnection process that is time-dependent. This is a dramatic shift in the standard thinking, since it implies that the physics of the inner electron layer is essential in understanding both the rate and the time-dependence of reconnection. Scaling these new results to physically realistic parameter regimes remains a significant challenge, but already it seems clear that the length of the electron diffusion region will be much larger ( ≥ 10 c/ωpi) than previously thought. The implications of these new results for understanding and interpreting observational data from Cluster and the upcoming MMS mission will be discussed.

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