PLASMA SHEET ION COMPOSITION AT VARIOUS LEVELS OF GEOMAGNETIC AND SOLAR-ACTIVITY

Statistical data obtained by the Plasma Composition Experiment on the ISEE-1 spacecraft, covering energies in the range 0.1-16 keV/e, show that plasma sheet ions fall into two groups which behave differently in response to variations in the geomagnetic and solar activity. One group, most prominently represented by the O+, is to a large extent associated with substorm activity (large AE-index) and also varies in density with the solar cycle. Ions in this group are clearly of terrestrial origin and may increase in numbers as a result of increased electric currents during substorms and of increased EUV radiation during high solar activity. The other group consists mainly of H+and He2+ions. These are most numerous during geomagnetically quiet conditions and show no significant response to variations in the solar EUV. Ions in this group are the only ones that vary in energy in a systematic fashion with varying substorm activity, being the least energetic during quiet conditions. When such activity goes to zero the mean (thermal) energy-per-nucleon of these ions approaches the bulk energy-per-nucleon in the quiet solar wind. Ions in this group appear to be largely of solar origin. As a corollary it follows that the capture of solar ions in the plasma sheet is not inherently associated with substorm activity. © IOP Publishing Ltd.