Voyager 2 observations of helium abundance enhancements from 1–60 AU

The Plasma Science Experiment on the Voyager 2 spacecraft recently detected helium abundance enhancements (HAEs) near 60 AU in which the He++ ions comprise more than 10% of the solar wind density. These large values may result from solar events related to the rising phase of solar cycle 23. Large He++/H+ density ratios are probably the best signature for tracking coronal mass ejections (CMEs) through the outer heliosphere for the subset of CMEs with this property. We analyze the high‐resolution spectra for the Voyager 2 mission (from 1977 to 1999, 1–60 AU) and find that HAE events are observed ∼1.1% of the time with He++/H+ abundance ratios as high as 0.35. The long‐term variation of HAEs is related to the solar cycle. HAEs occur in all solar wind regimes but are associated with higher speeds, lower proton temperatures, and higher magnetic fields than the ambient solar wind. HAEs are generally not associated with interplanetary shocks. The He++/H+ temperature ratio increases with distance from the Sun, is larger when He++/H+ density ratios are large, and is smaller when proton densities are high.