Kinetic parameters of interstellar neutral helium

The GAS-instrument onboard the space probe Ulysses is designed to measure the local angular distribution of the flow of interstellar neutral He-atoms within ≈3 AU distance from the sun. It allows to infer the kinetic parameters (velocity vector, temperature and density) of these particles outside the heliosphere (“at infinity”). During three observational periods, 1990/1991, shortly after launch and during the two fast latitude scans of Ulysses, from 9/1994 to 8/1996 and from 9/2000 to 8/2002, more than 300 measurements of the distributions were obtained and analyzed in detail. Known issues that relate to the pointing accuracy of the detector and to the efficiency calibration along with their impact on the results are addressed in this paper. The average values, derived from these observations and their refined analysis, are the bulk speed ($v_{\rm He\infty}= 26.3$ ± 0.4 km s-1), the flow direction (ecliptic longitude $\lambda_{\infty} = 74.7^{\circ}$ ± $0.5^{\circ}$, ecliptic latitude $\beta_{\infty} = -5.2^{\circ}$ ± $0.2^{\circ}$) and temperature ($T_{\rm He\infty} = 6300~{\rm K}$ ± 340 K). From 1990 to 2002, covering a complete solar cycle, no significant temporal variations of these parameters were observed, nor variations with solar latitude. In contrast, in the density nHe\infty, derived from the series of local observations along the Ulysses-orbit, substantial apparent variations were seen. After a first attempt to explain these by an inadequate energy calibration of the efficiency they are now interpreted as variations in the loss processes (predominantly photo-ionization), which the neutral gas experiences along its trajectory to the observer. While the temporal variations of the ionization rate were taken into account using the measured solar EUV irradiance (from CELIAS/SEM on SOHO), the residual latitudinal variation has been attributed to a dependence of the solar irradiance on latitude, which thus far has not been accessible to direct observations. As a result of a simple model that includes these effects a density $n_{\rm He\infty}= 0.015$ ± 0.003 cm-3 has been deduced, which is in good agreement with an independent result from pickup ion observations