The Variability of the Solar Coronal Helium Abundance: Polar Coronal Holes Compared to the Quiet Sun
We report on measurements of the abundance ratio He/H in solar polar coronal holes and neighboring quiet coronal regions, using the SUMER spectrometer on board SOHO. In coronal holes we find He abundances smaller than or equal to the value found in the fast solar wind, 5% by number relative to H, suggesting that the flow speed of He up into the wind is equal to or greater than the flow speed for H in the regions we observe. Going to regions above quiet solar corona, we find similar He abundances at altitudes of 50" or more, with some indication that the He abundance is increased toward its photospheric value of 8.5% relative to H lower down. We take this to indicate that the He abundance in open-field regions of the corona is never higher than 5% (as observed in the solar wind), but that in low-lying, closed magnetic field regions higher abundances may result.