HEOS Observations of the Configuration of the Magnetosphere

Summary The preliminary results of an analysis of magnetic field measurements made over a five‐year period in the Earth's magnetosphere are presented. These measurements were obtained from Imperial College experiments in the European Space Research Organization's two HEOS satellites which provided data in the near‐Earth region of the tail sheet and in the relatively unexplored polar regions of the magnetosphere. Using a new co‐ordinate system which allows for the variable orientation of the geomagnetic dipole, the data provide a graphic illustration of the field topology in the polar and near‐tail regions: the polar cusp or cleft and the average width of the tail sheet are clearly revealed. The results of a comparison between the measured field and the predictions of magnetospheric models are outlined. The paper concludes by using data taken during two transits through the Earthward edge of the tail sheet to indicate the extreme variability of the field configuration in that region.