Simultaneous measurements from the Millstone Hill radar and the Active satellite during the SAID/SAR arc event of the March 1990 CEDAR storm

During a nearby passage of the Active satellite above the Millstone Hill ra dar on 21 March 1990 at local sunset, the satellite and the radar performed simultaneous measurements of upper ionospheric parameters in nearly the sa me spatial volume. For this purpose the radar carried out a special azimuth -elevation scan to track the satellite. Direct comparisons of radar data an d in situ satellite measurements have been carried out quite rarely. In thi s case, the coincidence of co-ordinated measurements and active ionospheric -magnetospheric processes during an extended storm recovery phase presents a unique occasion resulting in a very valuable data set. The measurements s how generally good agreement both during quiet prestorm and storm condition s and the combination of radar and satellite observations gives a more comp rehensive picture of the physical processes involved. We find a close relat ionship between the rapid westward ion drift peak at subauroral latitudes ( SAID event) and the occurrence of a stable auroral red (SAR) are observed a fter sunset by an all-sky imager and reported in an earlier study of this e vent. The SAID electric field is caused by the penetration of energetic ion s with energies between about 1 keV and 100 keV into the outer plasmasphere to a latitude equatorward of the extent of the plasmasheet electrons. Char ge separation results in the observed polarisation field and the SAID. Unus ually high molecular ion densities measured by the satellite at altitudes o f 700-870 km at subauroral and auroral latitudes point on strong upward-dir ected ion acceleration processes and an intense neutral gas upwelling. Thes e structures are collocated with a narrow trough in electron density and an electron temperature peak as observed simultaneously by the radar and the satellite probes.