On the source and acceleration of energetic He+: A long-term observation with ACE/SEPICA


We report on a systematic study of the He+/He2+ abundance ratio in the energetic population during the years 1998–2000. For the investigation we have used data in the energy range 0.25–0.8 MeV/n from SEPICA on board ACE and from CELIAS STOF on SOHO in the energy range 0.085–0.28 MeV/q. The ratio is quite variable, with values up to 1. Over the entire time period the integral abundance ratio of the energetic population is 0.06, which exceeds the abundance of He+ in the solar wind and corona by several orders of magnitude and even surpasses substantially the average relative contribution of He+ pickup ions. This requires preferential injection and acceleration of He+ over He2+. In a case study of a CME with plasma rich in He+, which drives a shock and is being overtaken by another shock, the largest enhancement is found near the driven shock way ahead of the ejecta. In addition, CMEs which are He+ rich are very rare. This implies that the source of the energetic He+ is primarily interstellar pickup ions and not ejecta material. However, signatures of large-scale structures in the interstellar source, such as the gravitational focusing cone, have not been identified in this survey. Enhanced He+/He2+ ratios are associated with interplanetary structures in the solar wind, such as stream-stream interfaces and interplanetary traveling shocks. In fact, more than 90% of all shocks and stream interfaces show ratios that exceed our lower limit of 0.04. This association points to local acceleration of interplanetary He+, whose injection and acceleration efficiency may vary substantially from event to event, thus probably washing out large-scale structures of the source strength. While the enhancements tend to be very localized at interplanetary traveling shocks, they are much more prolonged in the case of CIRs.

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JGR: Space Physics



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