Helium Abundance and Minor Ion Charge State Variations in the Solar Wind over the Solar Cycle

Prior studies have shown that the abundance of helium relative to hydrogen in the solar wind is a highly variable quantity. Kasper, et al. (2007) used observations from the Wind spacecraft to show that that the helium abundance in slow solar wind is strongly correlated with sunspot number. During the penultimate solar minimum, helium abundance was a linear function of solar wind speed. However, this dependence vanished as solar activity increased. We extended this study through the recent deep solar minimum and found that the strong correlation between helium abundance and sunspot number for slow solar wind continued. Moreover, as the deep minimum persisted, the helium abundance reached even lower values than observed previously. The strong linear dependence of helium abundance on winds speed also returned during this solar minimum. Motivated by these results, we used data from the ACE spacecraft to determine if the charge states of minor ions, which are strong indicators of coronal temperature, show similar trends. Indeed, the charge states of carbon and oxygen appear to be related to both solar activity and wind speed. Higher charge state ratios were observed for periods of greater activity, but lower ratios were seen as activity decreased. However, while helium abundance increased with solar wind speed, charge state (and thus coronal temperature) decreased with speed. This suggests that during solar minimum slow solar wind originates in cooler plasma. More broadly, these results challenge solar wind acceleration models that link the final speed of the solar wind to a single coronal temperature.