POLAR magnetic field observations at apogee during the January 1997 magnetic cloud event

Magnetic field measurements from the POLAR satellite are presented for three dusk to dawn apogee passes during the January 1997 magnetic cloud event. The variability in the residual fields from one orbit to the next are investigated in terms of the changing solar wind and magnetospheric current systems. The observed fields are compared to the Tsyganenko 96 (T96_01) model which allows dynamic solar wind and Dst inputs. The model fields are found to generally agree with the observed fields with the greatest discrepancy occurring during periods of large dynamic pressure. An investigation of the contributions to the T96_01 model by the various solar wind and Dst inputs provides insight into the solar wind control of the high altitude magnetosphere above the polar cap. It is found that the ring current is responsible for the largest proportion of the observed magnetic field residuals while the interplanetary magnetic field and solar wind dynamic pressure control a smaller proportion primarily producing the shorter time scale variations in the observed fields.