O5+ in high speed solar wind streams: SWICS/Ulysses results

Recent observations with UVCS On SOHO of high outflow velocities of O5+ at low coronal heights have spurred much discussion about the dynamics of sola r wind acceleration. On the other hand, O6+ is the most abundant oxygen cha rge state in the solar wind, but is not observed by UVCS or by SUMER becaus e this helium-like ion has no emission lines falling in the wave lengths ob servable by these instruments. Therefore, there is considerable interest in observing O5+ in situ in order to understand the relative importance of O5 + With respect to the much more abundant O6+. High speed streams are the pr ime candidates for the search for O5+ because all elements exhibit lower fr eezing-in temperatures in high speed streams than in the slow solar wind. T he Ulysses spacecraft was exposed to long time periods of high speed stream s during its passage over the polar regions of the Sun. The Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses is capable of resolving this ra re oxygen charge state. We present the first measurement of O5+ in the sola r wind and compare these data with those of the more abundant oxygen specie s O6+ and O7+. We find that our observations of the oxygen charge states ca n be fitted with a single coronal electron temperature in the range of 1.0 to 1.2 MK assuming collisional ionization/recombination equilibrium with an ambient Maxwellian electron gas.