The Ulysses neutral gas experiment: Determination of the velocity and temperature of the interstellar neutral helium

Abstract

A new technique to directly detect low energy neutral helium has been developed and successfully used in space for the first time. It makes possible the in-situ measurement of the local angular distribution of the flow of interstellar neutral helium in the inner heliosphere. Based on the transformation of a velocity distribution, using Liouville's theorem, a straightforward model has been developed to determine, from these measurements, the flow parameters of the interstellar helium at infinity. From twelve measurements at radial distances between 1 and 5 AU, average values of the flow parameters were determined, namely: υ_∞ = 26 +/- 1 km s^-1, downstream direction: λ_∞ = 72 +/- 2.4°, β_∞ = -2.5 +/- 2.7°, temperature T_∞ = 6700 +/- 1500 K. These values are in agreement with results found by completely different methods (such as, backscattered UV-light, pick-up ions), although in detail there are significant differences.