RECENT RESULTS ON THE PARAMETERS OF THE INTERSTELLAR HELIUM FROM THE ULYSSES GAS EXPERIMENT/

Velocity and direction of the flow of the interstellar helium and its temperature and density have been determined from the measurements of the ULYSSES/GAS experiment for two different epochs: during the in-ecl iptic path of ULYSSES, representing solar maximum conditions, and duri ng the south to the north pole transition (11/94-6/95), close to the s olar minimum conditions. Within the improved error bars the values are consistent with results published earlier. The determination of the d ensity n(infinity) of the interstellar helium at the heliospheric boun dary from observations in the inner solar system requires knowledge ab out the loss processes experienced by the particles on their way to th e observer. The simultaneous observation of the helium particles arriv ing on ''direct'' and ''indirect'' orbits at the observer provides a t ool to directly determine the effects of the loss processes assumed to be predominantly photoionization and - for particles travelling close to the Sun - electron impact ionization by high-energy solar wind ele ctrons. Such observations were obtained with the ULYSSES/GAS instrumen t in February 1995, before the spaceprobe passed its perihelion. From these measurements values for the loss rates and the interstellar dens ity could be derived. Assuming photoionization to be the only loss pro cess reasonable fits to the observations were obtained for an ionizati on rate beta = 1.1 . 10-(7) s(-1) and a density n(infinity) approximat e to 1.7 . 10(-2) cm(-3). Including, in addition, electron impact ioni zation, a photoionization beta = 0.6 . 10(-7) s(-1) was sufficient to fit both observations, resulting in a density n(infinity) approximate to 1.4 . 10(-2) cm(-3).