THE DYNAMIC CUSP AT LOW ALTITUDES - A CASE-STUDY UTILIZING VIKING, DMSP-F7, AND SONDRESTROM INCOHERENT-SCATTER RADAR OBSERVATIONS

Coincident multi-instrument magnetospheric and ionospheric observation s have made it possible to determine the position of the ionospheric f ootprint of the magnetospheric cusp and to monitor its evolution over time. The data used include charged particle and magnetic field measur ements from the Earth-orbiting Viking and DMSP-F7 satellites, electric field measurements from Viking, interplanetary magnetic field and pla sma data from IMP-8, and Sondrestrom incoherent scatter radar observat ions of the ionospheric plasma density, temperature, and convection. V iking detected cusp precipitation poleward of 75.5 degrees invariant l atitude. The ionospheric response to the observed electron precipitati on was simulated using an auroral model. It predicts enhanced plasma d ensity and elevated electron temperature in the upper E- and F-regions . Sondrestrom radar observations are in agreement with the predictions . The radar detected a cusp signature on each of five consecutive ante nna elevation scans covering 1.2 h local time. The cusp appeared to be about 2 degrees invariant latitude wide, and its ionospheric footprin t shifted equatorward by nearly 2 degrees during this time, possible i nfluenced by an overall decrease in the IMF B-z component. The radar p lasma drift data and the Viking magnetic and electric field data sugge st that the cusp was associated with a continuous, rather than a patch y, merging between the IMF and the IMF and the geomagnetic field.