Evolution of the relationships between helium abundance, minor ion charge state, and solar wind speed over the solar cycle

Abstract

The changing relationships between solar wind speed, helium abundance, and minor ion charge state are examined over solar cycle 23. Observations of the abundance of helium relative to hydrogen (AHe ≡ 100 × nHe/nH) by the Wind spacecraft are used to examine the dependence of AHe on solar wind speed and solar activity between 1994 and 2010. This work updates an earlier study of AHe from 1994 to 2004 to include the recent extreme solar minimum and broadly confirms our previous result that AHe in slow wind is strongly correlated with sunspot number, reaching its lowest values in each solar minima. During the last minimum, as sunspot numbers reached their lowest levels in recent history, AHe continued to decrease, falling to half the levels observed in slow wind during the previous minimum and, for the first time observed, decreasing even in the fastest solar wind. We have also extended our previous analysis by adding measurements of the mean carbon and oxygen charge states observed with the Advanced Composition Explorer spacecraft since 1998. We find that as solar activity decreased, the mean charge states of oxygen and carbon for solar wind of a given speed also fell, implying that the wind was formed in cooler regions in the corona during the recent solar minimum. The physical processes in the coronal responsible for establishing the mean charge state and speed of the solar wind have evolved with solar activity and time. © 2012. The American Astronomical Society. All rights reserved.