THE OXYGEN CHARGE-STATE RATIO AS AN INDICATOR OF FOOTPOINT FIELD STRENGTH IN THE SOURCE REGIONS OF THE SOLAR WIND

Abstract

Because of its distinctive compositional properties and high variability, the slow solar wind is widely believed to originate from coronal streamers, unlike high-speed wind, which emanates from coronal holes. Based on measurements from the Advanced Composition Explorer , it has been proposed that an oxygen charge-state ratio O 7+ /O 6+ of 0.145 is the threshold that separates streamer from coronal hole wind. During the 2007–2009 sunspot minimum, however, the median value of O 7+ /O 6+ fell to only 0.06, implying that almost all of the near-Earth wind came from coronal holes, despite the fact that the streamer belt lay much closer to the ecliptic plane at that time than at solar maximum. Employing extrapolations of photospheric field maps to derive the footpoint field strengths B 0 of the near-Earth wind, we find that the median value of B 0 decreased to only 2.6 G during 2007–2009, from a value of 21 G during 1998–2004. The factor of ∼2 decrease in the median value of O 7+ /O 6+ thus reflects the factor of ∼8 decrease in the footpoint field strength. Variations in O 7+ /O 6+ are strongly anticorrelated with the wind speed on timescales of days, but not on long timescales, which are dominated by changes in B 0 . We suggest that the charge-state ratio is determined by the amount of energy deposited near the coronal base, which in turn depends on B 0 and the local flux-tube expansion rate. High values of O 7+ /O 6+ are associated with rapidly diverging flux tubes rooted just inside the boundaries of coronal holes with strong footpoint fields.