Abstract
Abstract. During a nearby passage of the Active satellite above the Millstone Hill radar on 21 March 1990 at local sunset, the satellite and the radar performed simultaneous measurements of upper ionospheric parameters in nearly the same spatial volume. For this purpose the radar carried out a special azimuth-elevation scan to track the satellite. Direct comparisons of radar data and in situ satellite measurements have been carried out quite rarely. In this 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 show generally good agreement both during quiet prestorm and storm conditions and the combination of radar and satellite observations gives a more comprehensive picture of the physical processes involved. We find a close relationship between the rapid westward ion drift peak at subauroral latitudes (SAID event) and the occurrence of a stable auroral red (SAR) arc observed after sunset by an all-sky imager and reported in an earlier study of this event. The SAID electric field is caused by the penetration of energetic ions with energies between about 1 keV and 100 keV into the outer plasmasphere to a latitude equatorward of the extent of the plasmasheet electrons. Charge separation results in the observed polarisation field and the SAID. Unusually high molecular ion densities measured by the satellite at altitudes of 700-870 km at subauroral and auroral latitudes point on strong upward-directed ion acceleration processes and an intense neutral gas upwelling. These 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.Key words. Ionosphere (ionosphere-magnetosphere interactions; plasma temperature and density); Magnetospheric physics (plasmasphere).
Highlights
The incoherent scatter radar facility at Millstone Hill, Westford, Massachusetts (42X6x, 288X5i) used a fully steerable 46-m antenna to track the Active satellite which passed the station about 1000 km eastward during its descending branch of orbit 2174 on March 21, 1990, from about 22:21 UT to 22:33 UT
For a further support of our observational basis we looked for data from the Defense Meteorological Satellite Program (DMSP) satellite F8 which is located in roughly the same local time sector
For the exact time of the DMSP overight it is found at 61X3 magnetic latitude (Fig. 13) which approximately coincides with the estimation from the time moment of our radar observations which gives a value of 61±62 (Fig. 10)
Summary
The incoherent scatter radar facility at Millstone Hill, Westford, Massachusetts (42X6x , 288X5i) used a fully steerable 46-m antenna to track the Active satellite which passed the station about 1000 km eastward during its descending branch of orbit 2174 on March 21, 1990, from about 22:21 UT to 22:33 UT. The simultaneous radar observations during the Active passage are a unique opportunity because of the rare occasions of direct comparisons between satellite and ground-based data but especially due to the fact that it obviously coincided with a subauroral ion drift (SAID) event during the extended recovery phase of an interesting storm interval which became one of the study objects of the Coupling, Energetics and Dynamics of Atmospheric Regions (CEDAR) initiative. As was already the case for the i, the e plot shows some areas in the auroral and subauroral upper ionosphere with data gaps (white bins) due to poor backscattered radar signals which do not allow the temperature estimation It is obvious, that there are two dierent regions of e enhancement: one near the auroral arc extending northward and an other at subauroral latitudes around 57 geomagnetic latitude or 22:28 UT of the satellite passage. The energy transfer to the ions via Joule heating near the subauroral drift maximum seems to be of minor importance, at least at this particular moment during the event under study
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