Helium in the Solar Wind as a Tracer of Coronal and Interplanetary Processes

The bulk of the solar wind is fully ionized hydrogen and helium, with helium contributing 5%-45% of the total mass density. Knowledge of the kinetic properties of helium (velocity, density, temperatures) is needed to understand the dynamics of the solar wind, and to this end we have analyzed observations of ion distribution functions by the Solar Wind Experiment Faraday Cup instruments on the Wind spacecraft. From each 92-second spectrum we have extracted velocities, densities, and anisotropic temperatures for hydrogen and helium. In addition to local dynamics, comparison of hydrogen and helium parameters tell us about both kinetic and large-scale processes which occur in the solar corona and in the interplanetary medium during the expansion of the solar wind. Modulation of the abundance ratio He/H traces the evolution of the coronal magnetic field within the solar cycle. Relations between ion temperatures, either equal temperatures or equal thermal speeds, tell us about fluctuations and the prevalence of collective wave-particle interactions. Our separate study of the equilibration of parallel and perpendicular temperatures allows us to distinguish mechanisms for this coupling.