From March 1996 to December 1997, we have identified a total of 459 polar cusp crossings according to the
criteria discussed above. Figure 2 shows the center of the cusp crossings in solar magnetic (SM) coordinates
in the x-r plane. The three vector components of the positions
are normalized by the sixth root of the solar wind dynamic pressure relative to that of 2 nPa. Also Figure 2a
and 2b show magnetic field lines from Tsyganenko 1989 vacuum model [Tsyganenko, 1989] and the latest Tsyganenko
96 model [Tsyganenko and Stern, 1996] respectively. The short bars are the median locations in radial bins of
the 1 RE width. The points and the medians are identical in the two panels, only the field lines
are different. In the 1989 model, these medians are located in the region between the two fields lines
corresponding to invariant latitudes of 80o and 82o, at roughly 81o, while in
the 1996 model, the medians are located around the field line with an invariant latitude of 80o.
However, when we look at the location of these cusp crossings relative to the open field line region in these
two figures, we can see that the 1989 model places the magnetic "funnel" or bifurcation of the field
closer to the center of the cusp than the 1996 model. Thus the cusp location appears to be better predicted by
the 1989 field model. The scatter in the points about the median is due to several factors one of which is the
tilt angle that we examine below. Another effect is the orientation of the IMF. The IMF effects on the cusp
position are more complex than that of tilt. We defer their discussion to a more detailed examination of the
cusp location.
Next:
The Dipole Tilt Angle Effect on the Cusp Location
Up: The Polar Cusp Location and Dipole Tilt
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Data Analysis