Initial Polar magnetic field experiment observations of the low-altitude polar magnetosphere: Monitoring the ring current with polar orbiting spacecraft

The equatorial ring current and the magnetopause current both contribute to the magnetic field at low altitudes over the polar cap. In this region the magnetic field is dominated by the Earth's internal field and is well described by empirical models. The average change in the magnetic field strength due to external sources (the residual of the observed magnetic field strength upon subtracting the International Geomagnetic Reference Field (IGRF) 95 internal field model) is typically a few tens of nanoTesla, or a few tenths of 1% of the total magnetic field over the polar cap at the altitude of the Polar spacecraft. It is easier to measure such small differences in the total field than in the vector components because the accuracy of the residuals in the vector magnetic field depends on the accuracy of the knowledge of spacecraft pointing which is generally less well known than the position of the spacecraft. In order to isolate the ring current effects on the total field we adjust the Dst index for the contributions of the magnetopause current using the solar wind dynamic pressure, and we adjust the observed field values for the same effect using the Tsyganenko [1996] model. After these adjustments the ring current strength as measured by the adjusted Dst index is well correlated with the residual of the field strength observed in the low-altitude polar region over a wide range of local times except when the spacecraft track is near the noon-midnight meridian. These comparisons, using more than a full year of Polar data, demonstrate that the Tsyganenko [1996] model together with the IGRF 1995 internal field model provides a good baseline for the magnetic field at the altitude of the Polar perigee passes (similar to 5000 km above the Earth's surface). Further they demonstrate that total field measurements from low-altitude polar orbiting spacecraft are potentially useful as monitors of the ring current when they cross the polar cap.