Differences Between Solar Wind Alpha Particle and Proton Temperatures at Times of Proton Temperature Depressions

Using ACE SWEPAM measurements from February 5, 1998 through October 30, 2001 we have examined hourly averages of the solar wind alpha particle temperature T_A and compared it to the proton temperature T_P. The ratio T_A/T_P ranges from about 1 to 10, with the most probable value near 4, indicating that protons and alpha particles typically have the same thermal speed. We find that this ratio tends to vary with solar wind speed; ratios less than 4 are more common at lower speeds. In this study we investigate the character of the alpha particle temperature during intervals of depressed proton temperature. In general, the solar wind proton temperature increases with increasing flow speed. The temperature is considered depressed when it is much lower (by a factor of approximately 2) than the typical temperature observed at a given speed. For this work, we developed an expression for the expected proton temperature as a function of speed appropriate for the ACE level 2 data, and used this expression to identify intervals of unusually low proton temperature. When proton temperatures are lower than expected, we find that T_A/T_P tends toward values of 1. We conclude that proton temperature depressions usually have corresponding alpha particle temperature depressions, and the relative difference between typical and depressed temperatures is 4 times greater for alpha particles than for protons. We note that, similar to proton temperature depressions, alpha particle temperature depressions can serve as useful indicators of CME flows.