Below the cusp the field lines point away from the sun and above it toward the sun. Figure 4a shows an example of measurements below the cusp and Figure 4b shows an example of measurements above the cusp. Unless we are in the cusp itself we do not know where it is exactly since we can determine whether we are above or below the cusp we can determine where the cusp is statistically. Since we expect that the direction of the IMF will affect the cusp location we examine only magnetopause crossings for which there are IMF measurements on the IMP-8 spacecraft. Figure 5a and b show the location of the magnetopause crossings for magnetopause clock angles within 22.5 degrees of the noon-midnight meridian for north and south IMF respectively.
Solid circles show measurements above the cusp and open circles below the cusp.
All locations have been normalized by the sixth root of the dynamic pressure to a common 2nPa pressure. While Figure 5a , Figure 5b show statistically where the cusp is found, it provides little insight into the dependence of the cusp location or dipole tilt angle or even what the tilt angle was during this period. Figure 6a , Figure 6b , Figure 6c , Figure 6d break up Figures 5a and b into IMF north and south and into three tilt angle ranges 10 degrees. The median tilt angle is shown in the Figure as well as being illustrated by a tilted arrow. The polar cusp will be encountered polarward of the below the cusp crossings and equatorward of the above the cusp crossings.