Trapped electron model for in situ observations of magnetotail reconnection.

Details Trapped electron model for in situ observations of magnetotail reconnection. Trapped electron model for in situ observations of magnetotail reconnection.

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsh42b..02e&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SH42B-02

Physics

Fluid Dynamics

0654 Plasmas, 2723 Magnetic Reconnection (7526, 7835), 2744 Magnetotail, 7526 Magnetic Reconnection (2723, 7835), 7835 Magnetic Reconnection (2723, 7526)

Scientific paper

Numerical simulations of magnetic reconnection [1] show that if the plasma resistivity is uniform then reconnection will be slow and occur in the familiar Sweet-Parker geometry. On the other hand, if some mechanism exists that allows for a locally enhanced resistivity then reconnection can proceed at a much faster Alfvenic rate in geometries similar to those predicted by the Petschek reconnection model. Recently, direct in situ observations by the Wind satellite of the electron distribution function during reconnection in the magnetotail [2] have revealed that electrons in the inner reconnection region follow electrostatically trapped trajectories [3]. Perhaps the most important effect of this trapping is that it implies a nearly symmetric bounce motion of the electrons along the magnetic field; a symmetry which is also reflected in their distributions function f(vallel,v\perp) ~ f (-vallel,v\perp). For reconnection geometries including a guide magnetic field it follows that the current along the reconnection X-line must be limited, jallel=\int vallel f d3v~0. Thus, the kinetic behavior of the trapped electrons causes a locally enhanced resistivity, which in a fluid description would appear as a finite \nabla· P-term in the generalized Ohm's law. In turn this localized effect yields the fast reconnection geometry for which the rate is likely to be controlled by two-fluid dynamics [4] externally to the diffusion region of trapped electrons. Magnetic reconnection including electron trapping is studied experimentally at the Versatile Toroidal Facility (VTF) at MIT. We find that electron trapping is so efficient in limiting the plasma current that in a driven scenario reconnection proceeds at the rate imposed externally. In the talk I will first discuss the evidence in the Wind data for electrostatic electron trapping. Then I will present a theory for why the electrostatic trapping potential develops and finally I will provide direct experimental observations from VTF of how trapped electrons yield a large effective and localized resistivity. [1] D Biskamp and E Schwarz, (2001) Phys. Plasmas 8, 4729. [1ex] [2] M Oieroset, RP Lin, TD Phan, DE Larson, and SD Bale, (2002) Phys. Rev. Lett. 89, 195001. [1ex] [3] J Egedal, M Oieroset, W Fox and RP Lin, (2005) Phys. Rev. Lett. 94, 025006. [1ex] [4] Y. Ren et al., (2005) Phys. Rev. Lett. 95, 055003.[1ex] Work supported by DOE Junior Faculty Award DE-FG02-06ER54878.

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