The POLAR spacecraft provides us with a good combination of instruments to study the polar cusp at high altitudes. Polar cusp crossings are seen in the POLAR data as a magnetic field depression and an increase in the density of magnetosheath-like plasma. After surveying the magnetic field data along with the Hydra and TIMAS key parameter data from March 1996 to December 1997, we identified 459 cusp crossings. The characteristics of the polar cusp that we deduce from the POLAR data are consistent with many of the previous results obtained at low and high altitudes. The invariant latitude of the cusp boundary can be as low as 69.4° under strongly southward IMF conditions and as high as 87.5°. The cusp location is close to that of the Tsyganenko 1989 vacuum magnetic field model with an ellipsoidal super-conducting magnetopause. The empirical Tsyganenko 96 model does not order the cusp funnel as well as the vacuum model. The shape of the model magnetopause and the amount of field crossing the magnetopause both differ between the models and this may account for the difference between the two models.
Finally, to the low-altitude observations by DMSP [Newell and Meng, 1989] show that the cusp moves 1° in magnetic latitude for 17° increase in the tilt angle. Similarly at high altitudes we see an increase of 1° in invariant latitude for 14o increase in the tilt angle. In view of the scatter in the data caused by the IMF this difference is insignificant. Thus the behavior of invariant latitude of the cusp as seen by POLAR is entirely consistent with that seen at low altitudes.
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Acknowledgements
Up: The Polar Cusp Location and Dipole Tilt
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The Dipole Tilt Angle Effect on the Cusp Location