Magnetospheric mapping with a quantitative geomagnetic field model

The Mead-Fairfield geomagnetic field model has been used to trace field lines between the outer magnetosphere and the earth's surface. The results are presented in terms of ground latitude and local time contours projected to the equatorial plane and into the geomagnetic tail. With these contours, various observations can be mapped along field lines between high and low altitudes. Low-altitude observations of the polar cap boundary, the polar cusp, the energetic electron trapping boundary, and the sunward convection region are projected to the equatorial plane and compared with the results of the model and with each other. The results provide quantitative support to the earlier suggestions that the trapping boundary is associated with the last closed field line in the sunward hemisphere, the polar cusp is associated with the region of the last closed field line, and the polar cap projects to the geomagnetic tail and has a low-latitude boundary corresponding to the last closed field line. The sunward convection region is associated with most of the magnetosphere outside the plasmapause, but whether the region extends all the way outward to the last closed field line or inward to the plasmapause is uncertain. Field lines from 6.6 R/sub E/ in the geomagnetic equatorial plane intersect the earth between 67.40 and 65.10, the exact value depending on the dipole tilt angle, season, local time, and geomagnetic activity. (U.S.)