THE HELIUM PARAMETERS OF THE VERY LOCAL INTERSTELLAR-MEDIUM AS DERIVED FROM THE DISTRIBUTION OF HE-WIND( PICKUP IONS IN THE SOLAR)

Until very recently the parameters of the local interstellar hydrogen and helium could only be determined from measurements of solar UV ligh t back-scattered by interstellar neutral gas. In this paper the parame ters (density, temperature and bulk velocity) characterizing the heliu m component in the Very Local Inter-Stellar Medium (VLISM) are derived from pickup helium ion distributions obtained with the time-of-flight spectrometer SULEICA on AMPTE/RM in 1985. Because of the relative sta bility of the Extreme Ultra-Violet (EUV) flux, which is the major cont ributor to the ionization of the interstellar helium, this solar minim um period provides the ideal conditions for inferences based on measur ements in the inner heliosphere. First the time period was investigate d for variations of the pickup ion fluxes with interplanetary paramete rs. For example, radial interplanetary magnetic field conditions showe d extremely low fluxes, while during subsequent turns of the field ext remely high fluxes were observed compared with the average values, Aft er such non-representative cases were excluded from the analysis, the pickup ion fluxes near and in the focusing cone on the downwind side o f the Sun were compared with simulations based on a hot model of the i nterstellar helium distribution in interplanetary space. A total ioniz ation rate for He of 5.5-6.5 10(-8) s(-1) was inferred from the level of the 10.7 cm solar radio flux during the observations. The best agre ement between the data and the model was obtained for a VLISM paramete r set which brackets the range N-He infinity = 0.0067-0.0087 cm(-3) T- He = 9000-15 000 K, and v(b) = 19-25 km s(-1). These values take into account a correction for the spatial diffusion and convection of the i ons after pickup by the solar wind, which leads to a wider cone in the pickup ion distribution compared with that of the neutral gas.