BOUNDARY-LAYER ION COMPOSITION AT JUPITER DURING THE INBOUND PASS OF THE ULYSSES FLYBY

During the inbound segment of the Ulysses flyby of Jupiter, there were multiple incursions into the dawnside low-latitude boundary layer, as identified by Bame et al. (Science 257, 1539-1542, 1992) using plasma electron data. In the present study, ion composition and spectral mea surements provide independent collaborative evidence for the existence of distinct boundary layer regions. Measurements are taken in the ene rgy-per-charge range of 0.6-60 keV/e and involve mass as well as mass- per-charge identification by the Ulysses/SWICS experiment. Ion species of Jovian magnetospheric origin (including O+, O2+, S2+, S3+) and she ath origin (including He-2+ and high charge state CNO) have been direc tly identified for the first time in the Jovian magnetospheric boundar y layer. Protons of probably mixed origin and He+ of possibly sheath ( ultimately interstellar pickup) origin were also observed in the bound ary layer. Sheath-like ions are observed throughout the boundary layer ; however, the Jovian ions are depleted or absent for portions of two boundary layer cases studied. Ions of solar wind origin are observed w ithin the outer magnetosphere, and ions of magnetospheric origin are f ound within the sheath, indicating that transport across the magnetopa use boundary can work both ways, at least under some conditions. Altho ugh their source cannot be uniquely identified, the proton energy spec trum in the boundary layer suggests a sheath origin for the lower ener gy protons.