Current Structure and Motion of a Northward IMF X-Line

Details Current Structure and Motion of a Northward IMF X-Line Current Structure and Motion of a Northward IMF X-Line

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsm51c0397w&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #SM51C-0397

Physics

2724 Magnetopause, Cusp, And Boundary Layers

Scientific paper

On March 18, 2002, between 14:50 and 15:03 UT, Cluster passed from the tail lobe northward and sunward into the magnetosheath. The IMAGE FUV instrument observed a proton emission, northward of the auroral zone, that endured for a period of hours, including the time of the Cluster crossing. The location is consistent with the footprint of a northward IMF reconnection site (Frey, et. al.). Phan, et. al., mapped the location of cluster during its magnetopause crossing to the location of the ionsopheric footprint observed by IMAGE. We argue from Cluster PEACE electron and magnetic field data that the Cluster spacecraft passed very closely through an active reconnection site, with the x-line actually passing through the centroid of the Cluster. This proximity allows us to infer the current structure, velocity, orientation, and size of the x-line. The magnetic fields experience minima in the GSE x-z plane that are consistent with an x- line along the y-direction. Meanwhile, near these minima, the y-component of the magnetic field undergoes a large enhancement. The PEACE electron moments, distributions, and pitch angle spectra give evidence of magnetic shear current at the reconnection site, as well as perpendicular currents that bundle an enhanced magnetic field along the reconnection line. We find the spatial derivatives and curlometer shear current at the x-line through a least squares estimate at the centroid of the Cluster spacecraft. The x-line current from the curl of the Cluster measurements is consistent with that found from the PEACE moments on the four spacecraft, emphasizing its quasiuniformity on 40-50 km size scales. We map the position of the x-line at the points of closest approach to each spacecraft through inversion of a low-order expansion for the magnetic field near the x-line. From the estimated locations and the measured times, we develop a least squares fit to the x-line velocity. These estimates of x-line location also fit a functional form Bx(z) and Bz(x) for the x-line that allows us to measure its motion and location at times when Cluster is further away. The functional fit to the data also gives an estimate of the thickness of the shear current sheet. The x-line earth and IMF fields are orientated at a large angle (~65) from the GSE x-direction and enhanced in the By direction, consistent with antiparallel reconnection, but with a twist. We propose that the x-line, though it does waver across the spacecraft several times, is largely stable over the times of Cluster observations.

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