The polar cusps are regions of very weak magnetic field in which the magnetosheath plasma has easy access to the ionosphere. Thus polar cusps are very important in the interaction of the solar wind with the magnetosphere. Spreiter and Briggs [1962] developed a three-dimensional magnetospheric model, and demonstrated the existence of the neutral points, which are the equivalent of the polar cusps in this idealized model. They also calculated the dipole tilt angle effect on the magnetopause shape and the location of the neutral points. When the dipole tilts more toward the Sun, the neutral point in the northern hemisphere moves poleward. Later Spreiter et al. [1967], using their gasdynamic numerical simulation, addressed the seasonal and diurnal variation of the location of the neutral points caused by the tilt angle effect mentioned above. Consistent with the earlier model the northern cusp occurs at lower invariant latitudes in the winter hemisphere. The polar cusp can now be studied by direct observations at both low and high altitudes. The low-altitude cusp has been studied with satellites such as DMSP and Viking. At high altitudes the main results to date have been provided by HEOS 2 and Hawkeye. With about 12,000 polar cusp crossings observed by DMSP Newell and Meng [1988; 1989] and Newell et al., [1989] studied the local time distribution, the dipole tilt angle and the orientation of the interplanetary magnetic field (IMF) effects on the location of the polar cusp. They calculated the correlation between the magnetic latitude of the cusp with the dipole tilt angle, showing an equtorward shift of the cusp with increasing tilt angles. Using Hawkeye data, Farrell and Van Allen [1990] and Fung et al. [1997], examined the solar wind dynamic pressure and IMF effects on the position and extent of the cusp. Using the magnetopause crossings observed by Hawkeye near the cusp region, Zhou and Russell [1997] showed that at high altitudes the cusp position is also affected by the dipole tilt angle. In this paper, we use POLAR magnetic field observations together with key parameter data from both Hydra [Scudder et al., 1995] and TIMAS [Shelley et al., 1995] to identify the polar cusp along the POLAR trajectory. A total of 459 cusp crossings were identified from March 1996 to December 1997. The cusp entries occur from altitudes of 4.8 to 8.8 Re. We examine herein the dependence of the location of the cusp on the dipole tilt angle. These observations were all obtained at northerly latitudes in contrast to the low altitude observations that were obtained at both northerly and southerly latitudes.
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