INTERSTELLAR OXYGEN, NITROGEN AND NEON IN THE HELIOSPHERE

Oxygen, nitrogen and neon pick-up ions of interstellar origin were det ected for the first time with the Solar Wind Ion Spectrometer (SWICS) on board Ulysses. The interstellar origin of these ions is established by the following criteria: (a) they are singly charged, (b) they have the broad velocity distributions characteristic of pick-up ions, with an upper limit of twice the solar wind speed, (c) their relative abun dance as a function of distance from the sun corresponds to the theore tical expectation, and (d) there is no relation to a planetary or come tary source. The interstellar abundance ratios He+/O+, N+/O+, Ne+/O+ w ere investigated. At approximately 5.35 AU in the outermost part of Ul ysses' trajectory He+/O+ = 175(-50)+70, N+/O+ = 0.13(-0.05)+0.05 and N e+/O+ = 0.18(-0.07)+0.010 were determined. For the interstellar gas pa ssing through the termination region and entering the heliosphere [He/ O]0 = 290(-100)+190, [N/O]0 = 0.13(-0.06)+0.06 and [Ne/O]0 = 0.20(-0.0 9)+0.12 were obtained from the pick-up ion measurements. Upper limits for the relative abundances of C+ and C were also determined.