THE ULYSSES NEUTRAL GAS EXPERIMENT - DETERMINATION OF THE VELOCITY AND TEMPERATURE OF THE INTERSTELLAR NEUTRAL HELIUM

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: nu(infinity) = 26 +/- 1 km s-1, downstream direction: lambda(infinity) = 72 +/- 2.4-degrees, beta(infinity) = -2.5 +/- 2.7-degrees, temperature T(infinity) = 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.