As we have seen in Figure 2, the cusp position is quite variable from orbit to orbit. One important factor of
this variation is the dipole tilt angle which is defined as the angle between the Earth's north dipole axis and
the GSM z-axis. This angle is positive when the dipole tilts toward the Sun, and negative when the dipole tilts
away from the Sun. The IMF also plays a significant role in controlling the cusp location. To study the effect
of tilt angle, we use data only for northward IMF because the cusp location is weakly dependent on the IMF
direction once the IMF is northward. The large variability of the cusp position for southward IMF tends to
obscure the tilt effects. Figure 3a, 3b, 3c show the data sets with tilt angle range of (0^{
°}-10^{°}
), (10^{°}-20^{
°}), and >20^{°}
respectively. The field lines in these figures are from the Tsyganenko 1989 vacuum
model with tilt angle of 5^{°}, 15^{
°}, and 25^{°}
. Again, the bars show the median values. For tilt angle of 0^{
°}-10^{°}
, the medians are located between field lines of 81^{
°}-82^{°}
. For tilts of 10^{°}-20
^{°}, the medians are within field lines of 82
^{°}-83^{°}
invariant latitude. For tilts >25^{°}
, the medians move a little poleward and are around the field line with an invariant latitude
of 83^{°}. Thus, when the dipole tilts more
toward the Sun, the cusp moves poleward, to higher invariant latitudes.

Figure 4 shows the invarant latitude of the cusp crossings versus the dipole tilt angle. Here all the
data regardless of the IMF condition are shown. The bars are the median values. Again, there is an upward
trend in this figure. If we fit these points linearly, we see that the cusp moves 1^{
°} invariant latitude for 14^{
°} increase in the tilt angle.
We have also examined the possibility of the thickness of the cusp depending on the
tilt angle but found no significant dependence.

** Next:**
Discussion and Conclusions
** Up:** The Polar Cusp Location and Dipole Tilt
** Previous:**
The Polar Cusp Location