SOLAR-WIND HELIUM ISOTOPIC COMPOSITION FROM SWICS ULYSSES

This is the first study of the isotopic composition of solar wind heli um with the SWIGS time-of flight mass spectrometer. Although the desig n 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 mag netic mass spectrometer on ISEE-3 and with the Apollo SWC foil experim ents. The ULYSSES spacecraft follows a trajectory which is ideal for t he 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 recu rrent fast stream from the southern polar coronal hole was observed ev ery solar rotation. Solar wind bulk velocities ranged from 350 km/s to 950 km/s which would, in principle allow us to identify velocity-corr elated compositional variations. Our investigation of solar wind heliu m, however, shows an isotopic ratio which does not depend on the solar wind speed.