COUNTERSTREAMING SOLAR-WIND HALO ELECTRON EVENTS - SOLAR-CYCLE VARIATIONS

Previous work has suggested that counterstreaming solar wind halo electron events are a reliable signature of coronal mass ejections, CMEs, in the solar wind at 1 AU. Using observations of counterstreaming events made by the Los Alamos plasma experiment on ISEE 3 / ICE, we have determined the percentage of time that counterstreaming was observed each year during an approximately 12.4-year interval extending from August 1978 through December 1990. These annual percentages vary roughly in phase and amplitude both with the 11-year sunspot cycle and with annual CME rates derived from coronagraph observations. The total variation in annual percentages over the solar cycle was approximately a factor of 21. Near solar activity maximum, counterstreaming events accounted for approximately 14.7% of all ISEE 3 / ICE solar wind measurements, while near solar minimum they accounted for approximately 0.7% of all the measurements. Inferred rates of counterstreaming events at a single point in the ecliptic plane ranged from approximately 72/yr near solar maximum to less than 12/yr near solar minimum. These inferred rates axe in reasonable quantitative accord with predicted rates for CME events in the ecliptic plane derived from SMM coronagraph observations, further supporting the thesis that most CMEs in the solar wind at 1 AU can be reliably identified by the counterstreaming electron signature.