SOLAR-WIND HELIUM ISOTOPIC COMPOSITION FROM SWICS ULYSSES

This is the first study of the isotopic composition of solar wind helium with the SWIGS time-of flight mass spectrometer. Although the design of SWIGS is not optimized to measure He-3 abundances precisely, He-4/He-3 flux ratios can be deduced from the data. The long term ratio is 2290 +/- 200, which agrees with the results obtained with the ICI magnetic mass spectrometer on ISEE-3 and with the Apollo SWC foil experiments. The ULYSSES spacecraft follows a trajectory which is ideal for the study of different solar wind types. During one year, from mid-1992 to mid-1993, it was in a range of heliographic latitudes where a recurrent fast stream from the southern polar coronal hole was observed every solar rotation. Solar wind bulk velocities ranged from 350 km/s to 950 km/s which would, in principle allow us to identify velocity-correlated compositional variations. Our investigation of solar wind helium, however, shows an isotopic ratio which does not depend on the solar wind speed.