Recent results on the parameters of the interstellar helium from the ULYSSES/GAS experiment

Abstract

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-ecliptic path of ULYSSES, representing solar maximum conditions, and during the south to the north pole transition (11/94-6/95), close to the solar minimum conditions. Within the improved error bars the values are consistent with results published earlier.

The determination of the density n∞ of the interstellar helium at the heliospheric boundary from observations in the inner solar system requires knowledge about the loss processes experienced by the particles on their way to the observer. The simultaneous observation of the helium particles arriving on “direct” and “indirect” orbits at the observer provides a tool 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 electrons.

Such observations were obtained with the ULYSSES/GAS instrument in February 1995, before the spaceprobe passed its perihelion. From these measurements values for the loss rates and the interstellar density could be derived. Assuming photoionization to be the only loss process reasonable fits to the observations were obtained for an ionization rate β = 1.1 · 10⁻⁷ s⁻¹ and a density n∞ ≈ 1.7 · 10⁻² cm⁻³. Including, in addition, electron impact ionization, a photoionization β = 0.6 · 10⁻⁷ s⁻¹ was sufficient to fit both observations, resulting in a density n∞ ≈ 1.4 · 10⁻² cm⁻³.