Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsm52a..02m&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SM52A-02
Other
2712 Electric Fields (2411), 2740 Magnetospheric Configuration And Dynamics, 2744 Magnetotail, 2764 Plasma Sheet, 2790 Substorms
Scientific paper
THEMIS electric and magnetic field measurements are combined with ion flows determined from the plasma and energetic particle instruments to investigate deviations from the "ion frozen-in" condition (E + VixB = 0) during geomagnetic substorms. It is found that the vast majority of ion flows during inner-magnetospheric dipolarizations can be classified as "frozen-in" within measurement uncertainties. In an unanticipated result, the largest deviations from frozen-in are found at the end of the substorm growth phase as the plasmasheet thins, rather than during the dipolarization process. Plasmasheet thinning is coincident with an increased current density which manifests itself as an increasing ion flow in the duskward direction. This duskward flow is the result of the ion pressure gradient perpendicular to the current sheet and has no associated electric field. Applying these measurements to the generalized Ohms Law, it appear that the non-zero "VixB" term that we measure during growth phase is just that portion of the Hall term, JxB/ne, which results from ion motion in the cross tail current. Deviations from the ion frozen-in condition are also found for brief periods during the dipolarization process, however these deviations are generally associated with ion pressure gradients, and therefore likely due to similar diamagnetic currents. These results suggest that the current disruption model, which requires significant breaking of the frozen-in condition by terms other than those resulting from diamagnetic effects, is inconsistent with the observations.
Angelopoulos Vassilis
Auster Uli
Bonnell Jerry
Carlson Carl W.
Glassmeier K.-
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