Abstract
We present a study on the abundance of oxygen relative to hydrogen in the solar minimum corona and for the first time we measure this quantity in polar coronal holes. The results are derived from the observations of the extended corona obtained with the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO. The diagnostic method used to obtain the oxygen abundance is based on the resonant components of the O VI 1032 A and HI 1216 A emission lines. This method fully accounts for the effects of the outflow velocity of the solar wind, which can be determined through Doppler dimming, and of the width of the absorbing profiles of the coronal ions or neutral atoms involved in resonant scattering. The oxygen abundance is higher in the polar coronal hole regions, where the fast wind is accelerated, than in the streamer belt. In the polar regions the observed oxygen abundance is consistent with the photospheric value and with the composition results obtained with Ulysses for the fast wind. The oxygen abundance values derived with UVCS suggest that the plasma remains substantially contained in quiescent streamers, that therefore do not contribute significantly to the solar wind.
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