Abstract.A coordinated effort to combine all three methods that are used to determine the physical parameters of interstellar gas in the heliosphere has been undertaken. In order to arrive at a consistent parameter set that agrees with the observations of neutral gas, pickup ions and UV backscattering we have combined data sets from coordinated observation campaigns over three years from 1998 through 2000. The key observations include pickup ions with ACE and Ulysses SWICS, neutral atoms with Ulysses GAS, as well as UV backscattering at the He focusing cone close to the Sun with SOHO UVCS and at 1 AU with EUVE. For the first time also the solar EUV irradiance that is responsible for photo ionization was monitored with SOHO CELIAS SEM, and the He I 58.4 nm line that illuminates He was observed simultaneously with SOHO SUMER. The solar wind conditions were monitored with SOHO, ACE, and WIND. Based on these data the modeling of the interstellar gas and its secondary products in the heliosphere has resulted in a consistent set of interstellar He parameters with much reduced uncertainties, which satisfy all observations, even extended to earlier data sets. It was also established that a substantial ionization in addition to photo ionization, most likely electron impact, is required, with increasing relative importance closer to the Sun. Furthermore, the total combined ionization rate varies significantly with solar latitude, requiring a fully three dimensional and time dependent treatment of the problem.

Abstract. 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, n He = 0.0151 ± 0.0015 cm −3 , in the Local Interstellar Cloud. From best fits to 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 3D, 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.

The rates of the most important ionization processes acting in interplanetary space on interstellar H, He, C, O, Ne and Ar atoms are critically reviewed in the paper. Their long-term modulations in the period 1974 -1994 are reexamined using updated information on relevant cross-sections as well as direct or indirect data on variations of the solar wind/solar EUV fluxes based on IMP 8 measurements and monitoring of the solar 10.7 cm radio emission. It is shown that solar cycle related variations are pronounced (factor of ,-, 3 between maximum and minimum) especially for species such as He, Ne, C for which photoionization is the dominant loss process. Species sensitive primarily to the charge-exchange (as H) show only moderate fluctuations ,,~ 20% aromld average. It is also demonstrated that new techniques that make use of simultaneous observations of neutral He atoms on direct and indirect orbits, or simultaneous measurements of He + and He ++ pickup ions and solar wind particles can be useful tools for narrowing the uncertainties of the He photoionization rate caused by insufficient knowledge of the solar EUV flux and its variations.