Oxygen abundance in coronal streamers during solar minimum

We present a study of the oxygen abundance relative to hydrogen in the equatorial streamer belt of the solar corona during the recent period of activity minimum. The oxygen abundance is derived from the spectroscopic observations of the outer corona performed during 1996 with the Ultraviolet Coronagraph Spectrometer (SOHO) in the ultraviolet region. This study shows that the depletion of oxygen, by almost one order of magnitude with respect to the photospheric values, found in the inner part of streamers by Raymond et al. (1997a) is a common feature of the solar minimum streamer belt, which exhibits an abundance structure with the following characteristics. In the core of streamers the oxygen abundance is 1.3x10(-4) at 1.5 R-circle dot, then it drops to 0.8 x 10(-4) at 1.7 R-circle dot, value which remains almost constant out to 2.2 R-circle dot. In the lateral bright structures that are observed to surround the core of streamers in the oxygen emission, the oxygen abundance drops monotonically with heliodistance, from 3.5 x 10(-4) at 1.5 R-circle dot to 2.2 x 10(-4) at 2.2 R-circle dot. The oxygen abundance structure found in the streamer belt is consistent with the model of magnetic topology of streamers proposed by Noci et al. (1997). The composition of the plasma contained in streamers is not the same as observed in the slow solar wind. Even in the lateral branches, richer in oxygen, at 2.2 R-circle dot the abundance drops by a factor 2 with respect to the slow wind plasma observed with Ulysses during the declining phase of the solar cycle. Hence the slow wind does not appear to originate primarily from streamers, with the exception perhaps of the plasma flowing along the heliospheric current sheet.