HIGH LATITUDE FIELD ALIGNED CURRENTS

[摘要] Horizontal ionospheric conduction currents are driven by magnetic field-aligned currents generated in the Earth's magnetosphere. Traditional ideas about field-aligned currents imply that most of the higher latitude set of field-aligned (region-1 Birkeland) current flows on field lines that connect to antisunward-flowing magnetospheric plasma. This conclusion disagrees with recent satellite data, and that disagreement provides a major motivation for this thesis.This research includes a stability analysis that is based on current conservation, which implies that field-aligned current is balanced by the divergence of ionospheric current. This stability analysis demonstrates that there may be a sector, within the plasma sheet in the nightside magnetosphere, where plasma pressures are reduced relative to the surrounding regions.In order to simulate this depleted region, several ;;computer experiments;; have been performed, enforcing gradients in plasma content at the tailward boundary of the calculation. These ;;computer experiments;; use an adapted form of the Rice Convection Model which has previously been used to calculate Birkeland currents (as well as other magnetospheric currents) in the inner magnetosphere by utilizing time dependent data from specific geophysical events. The present work extends the model further out in the magnetosphere.Results of these ;;computer experiments;; applied to the 19 September 1976, substorm event show that it is possible to generate high latitude magnetic field-aligned currents (region-1) connecting to regions of sunward plasma motion in the magnetosphere, indicating a significant departure from classical notions. The new model agrees better with observations than the earlier Rice model, with regard to the latitudinal distribution of the lower latitude (region-2 Birkeland) field-aligned currents, with a general increase in latitudinal extent. The peak magnitudes of the generated currents relative to region-2 current strength are (TURN)50% on the dusk side, and (TURN)100% on the dawn side. However, approximately 50% of the current may exist outside the modeling region. This generating mechanism may very well be the most important region-1 current source in the regions of (TURN)18:00 LT to 21:00 LT and (TURN)3:00 LT to 6:00 LT which causes flow of region-1 current on sunward convecting flux tubes.

[发布机构] Rice University

[效力级别] physics