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The Polar Cusp Location and its Dependence on Dipole Tilt

X. W. Zhou1,2,C. T. Russell1,2,G. Le1, S. A. Fuselier3 and J. D. Scudder4


1Institute of Geophysics, University of California, Los Angeles
2Department of Earth and Space Sciences, University of California, Los Angeles
3Lockheed Martin, Palo Alto
4Physics Department, University of Iowa, Iowa City


Received September 9, 1998, revised November 24, 1998, accepted November 30, 1998


Polar cusp crossings at high altitudes are seen in the POLAR data as decreases in the magnetic field and increases in magnetosheath-like plasma. Close to 500 polar cusp crossings identified from the magnetic field, low energy electron and ion data observed by POLAR, are used to determine the statistical ocation of the polar cusp. When compared with Tsyganenko's 1989 vacuum model with an ellipsoidal magnetopause [Tsyganenko, 1989], the medians of the cusp crossings are located between the magnetic field lines with invariant latitudes of 80 and 82 . Statistically the shape of the polar cusp in this region is consistent with this model although there is much scatter around the median value. The position of the cusp is significantly dependen t on the dipole tilt angle. When dipole tilts more toward the Sun, the cusp moves more poleward to higher invariant latitude from 77.2 at -30 tilt, to 80.0 at 0 tilt, to 81.8 at 30 or roughly 1 for every 14 of tilt.

GRL Space Physics and Aeronomy / Editor - R. M. Winglee /