Extended solar wind helium distribution functions in high-speed streams

The extended velocity distribution function for solar wind doubly charged helium (He+2) has been systematically studied for seven high-speed streams occurring in the first 7 months of 1995, near solar minimum. The two instruments, suprathermal ion composition spectrometer (STICS) and high mass resolution spectrometer (MASS), used in this study are both part of the SMS experiment on board the Wind spacecraft. The helium distributions were fit over the speed range nu/nu(sw) approximate to 0.8-1.6 constituting similar to 7.5 thermal widths and 5 orders of magnitude in phase space density, where nu(sw) is the proton solar wind speed. The distributions are reasonably well fit by kappa functions with values of kappa ranging from 3.4 to 5.8, although there may be systematic deviations from a perfect kappa function over the 5 orders of magnitude in phase space density. This is the first study that characterizes the extended helium distribution functions using kappa functions in the high-speed solar wind.