Upon comparing Figures 4a and 4b and taking note of the smaller vertical scale in Figure 4b, it is apparent that the ring current is responsible for most of the By, Bz and total B field residuals in all three time intervals. During each interval the ring current contribution is greatest near the equatorial plane where it produces the large negative Bz and total B field depression. The most pronounced ring current contributions occur during the Jan 10 interval when the Dst index is most negative (see Figure 1).
Referring back to Figure 3 the largest differences between the observed and model fields occur near the equatorial plane where the ring current fields dominate. One example of this occurs from 0600-0700 UT at the end of the January 9-10 pass as POLAR approaches the dawn equatorial plane. During this time the T96_01 model overestimates the negative Bz residual by as much as 50 nT. This discrepancy is attributed to an asymmetric ring current for the following reason. Just a few hours later on Jan 10 between 0900 and 1000 UT when POLAR is passing through the dusk equatorial plane the T96_01 model slightly underestimates the negative Bz residual even though the solar wind conditions are quite similar to the earlier 0600-0700 UT time interval (see Figure 1). Since the T96_01 model does not incorporate an asymmetric ring current the existence of such an asymmetry with a stronger ring current at dusk and a weaker ring current at dawn would produce the observed discrepancies. A similar asymmetry is also seen at low altitudes [Le et al., 1998].
However, not all periods of large discrepancy can be attributed to ring current effects. Some of the largest differences between model and measured fields occur during periods of enhanced dynamic pressure and variable IMF such as at the end of the Jan 10 interval and the beginning of the Jan 11 interval. In such cases many factors contribute to the overall discrepancy between the model and measured magnetic fields.