POLAR MAGNETOPAUSE CROSSINGS OF MAY 29, 1996 - IMPLICATIONS FOR MAGNETIC-FIELD MODELING

During 1996, the magnetometer and particle instruments on NASA's Polar spacecraft detected several apparent magnetopause crossings. In const ast, with a geocentric apogee of only 9 R-E over the northern pole, th is spacecraft was not expected to leave the magnetosphere. We have inv estigated the modifications that are required in existing magnetospher ic magnetic field models in order to account for these observations. I n order to match the observed particle distributions (reported elsewhe re,) this model includes northward-IMF reconnection. For the event of May 29, 1996, unusual magnetic and particle observations began at an a ltitude of 5 R-E while the spacecraft was sunward of the expected nort hern cusp position. Using it cylindrically symmetric magnetopause, we are able to reproduce the measured held reasonably well but with probl ems in the timing of the boundary crossings. The crossing times are mu ch improved by making an adjustment in the spacecraft position which c ould arise either from motion of the boundary or an indentation in the magnetopause at the cusp. The results from this model are compared ag ainst results from the analytic open-magnetosphere Toffoletto and Hill [1993] model and the empirical Tsyganenko [1996] model; significant i mprovement over both prior models is shown. This improvement is shown not only with respect to data from Polar but also; with respect to dat a from the Interball-Tail spacecraft. The model indicates that the Pol ar spacecraft encountered regions of ''overdraped lobe'' field lines t hat connect to the southern polar cap but exit the magnetosphere throu gh the northern cusp.