Observations of iron, silicon, and other heavy ions in the geostationary altitude region during late March 1991

Abstract

Following the great sudden storm commencement of March 24, 1991, observations 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 states 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 charge state frozen in near the Sim, were convected inward to the location of CRRES from the plasma sheet within minutes. The ions were energized in the convection process, and the ionic charge state was unaltered by charge exchange. The very large magnetospheric electric fields associated with the highly disturbed geomagnetic conditions were required to enable the ions to reach CRRES. Guiding center simulations were carried 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 caused by a sequence of solar wind parcels sweeping over the Earth's magnetosphere. The signature of a change from low to high temperature seems 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.