OBSERVATIONS OF IRON, SILICON, AND OTHER HEAVY-IONS IN THE GEOSTATIONARY ALTITUDE REGION DURING LATE MARCH 1991

Following the great sudden storm commencement of March 24, 1991, obser vations were made aboard CRRES of Fe, Mg, Si, and other heavy ions in the energy range of several tens to a few hundred keV/n. The charge st ates of these heavy ions were observed to change abruptly. For example , the dominant Fe charge state changed at one point from q = 9(+) to q = 16(+). We suggest that ions from the solar corona, retaining the ch arge state frozen in near the Sun, were convected inward to the locati on of CRRES from the plasma sheet within minutes. The ions were energi zed in the convection process, and the ionic charge state was unaltere d by charge exchange. The very large magnetospheric electric fields as sociated with the highly disturbed geomagnetic conditions were require d to enable the ions to reach CRRES. Guiding center simulations were c arried out which support this hypothesis. At least two ion populations were observed, corresponding to temperatures at two different coronal locations, and the observed changes in heavy-ion charge state were ca used by a sequence of solar wind parcels sweeping over the Earth's mag netosphere. The signature of a change from low to high temperature see ms to fit well the suggestions of Tsurutani and Gonzales [1994], of a fast solar wind stream tamped by a slower one, as a generator of great storms.