Observations of the helium focusing cone with pickup ions

The helium gravitational focusing cone has been observed using pickup He, first during the solar minimum in 1984 –1985 with the AMPTE/IRM spacecraft, and again in more detail from 1998 to 2002 with ACE and in 2000 with Nozomi. Five traversals of the cone allow us to obtain an accurate determination of the ecliptic longitude of the interstellar wind flow direction, λ = 74.43◦±0.33◦, while observations of pickup He with Ulysses give us an estimate, relatively free of instrumental systematic uncertainties, of the neutral He density, nHe = 0.0151 ± 0.0015 cm−3, in the Local Interstellar Cloud. From best fits of the measured velocity distributions of pickup He using time-stationary models we deduce the radial dependence and magnitude of electron-impact ionization rates that cannot presently be measured, and find this to be an important ionization process in the inner (<∼ 0.5 AU) heliosphere. We obtain excellent model fits to the 1998 cone profile using measured or deduced rates and known interstellar He parameters, and from this conclude that cross-field diffusion of pickup He is small. Furthermore, we find no evidence for extra sources of He in or near the cone region. Best fits to the velocity distributions of He are obtained assuming isotropic solar-wind-frame distributions, and we conclude from this that the scattering mean free path for pickup He in the turbulent slow solar wind is small, probably less than 0.1 AU. We argue that application of 3-D, time-dependent models for computation of the spatial distribution of interstellar neutral helium in the inner heliosphere may lead to excellent fits of short-term averaged pickup He data without assuming loss rates that are significantly different from production rates.