Using a field aligned coordinate system Figure 5 plots the observed MFE residuals together with those contributions to the T96_01 model fields from field aligned currents only. Note that the field aligned coordinate system is based upon the T96_01 fields with input parameters Pdyn = 2.0 nPa, Dst = 0, IMF By = IMF Bz = 0, and with IGRF 95 as the internal field. The z-direction is along the local model magnetic field, y is perpendicular to the model field and eastward and x completes the right-handed system. In such a coordinate system field aligned current signatures will be most pronounced in the y-direction. It is found that field aligned currents contribute only a small amount to the total model external fields. However, there are a couple of large features in the observed MFE residuals that appear to be due to field aligned currents. The sharp changes in dBy between 0400 and 0500 UT and between 0900 and 1000 UT on Jan 10 are both consistent with the POLAR satellite passing through the Birkeland field aligned current system. Near these times the T96_01 model predicts some field aligned current effects but they are significantly weaker and are delayed by approximately one hour from the observed signatures.
These field aligned current signatures seen at apogee differ from those seen at perigee on Jan 10 [Le et al., 1998] in that they are almost an order of magnitude smaller and they do not dominate the residual field pattern as they do throughout the perigee pass on this day.
© 1998 AGU