INTERSTELLAR OXYGEN, NITROGEN AND NEON IN THE HELIOSPHERE

Oxygen, nitrogen and neon pick-up ions of interstellar origin were detected 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 abundance as a function of distance from the sun corresponds to the theoretical expectation, and (d) there is no relation to a planetary or cometary source. The interstellar abundance ratios He+/O+, N+/O+, Ne+/O+ were investigated. At approximately 5.35 AU in the outermost part of Ulysses' trajectory He+/O+ = 175(-50)+70, N+/O+ = 0.13(-0.05)+0.05 and Ne+/O+ = 0.18(-0.07)+0.010 were determined. For the interstellar gas passing 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(-0.09)+0.12 were obtained from the pick-up ion measurements. Upper limits for the relative abundances of C+ and C were also determined.