Abstract
Abstract. 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 ultra-violet 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 1.3 × 10-4 at 1.5 R⊙, then it drops to 0.8 × 10-4 at 1.7 R⊙, value which remains almost constant out to 2.2 R⊙. In the lateral bright structures that are ob-served to surround the core of streamers in the oxygen emission, the oxygen abundance drops monotonically with heliodistance, from 3.5 × 10-4 at 1.5 R⊙ to 2.2 × 10-4 at 2.2 R⊙. 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⊙ 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.Key words. Interplanetary physics (solar wind plasma) – Solar physics, astrophysics and astronomy (corona and transition region; ultraviolet emissions)
Highlights
The first systematic spectroscopic observations of the streamer belt in the extended corona, above 1.5 R, were obtained during solar minimum with the Ultraviolet Corona-The UVCS observations showed at once a striking difference in the morphology of the quiescent equatorial streamers as observed in the O VI 1032 Aand H I Lyα 1216 Aemission lines
The aim of the present study is to extend the analysis of the oxygen abundance structure within quiescent streamers to several cases in order to: i) assess whether the difference in the hydrogen–oxygen morphology can be ascribed to an abundance variation as a rule, ii) investigate whether the abundance value is fairly constant in quiescent solar minimum streamers, iii) establish the average abundance structure in the streamer belt in the altitude range from 1.5 R to 2.2 R
The results obtained in the analysis of a few quiescent streamers observed during solar minimum shows that, in an altitude range from 1.5 R to 2.2 R, the oxygen abundance structure within the streamer belt follows a regular pattern
Summary
The first systematic spectroscopic observations of the streamer belt in the extended corona, above 1.5 R , were obtained during solar minimum with the Ultraviolet Corona-. Because of a certain similarity in the drop of the oxygen abundance in the slow solar wind (Geiss et al, 1995) and in the branches of quiescent streamers, Raymond et al (1997a,b) have proposed the streamer branches as the sources of the slow wind In this case the severe depletion of oxygen in the center of streamers is explained as an effect of gravitational settling in a closed magnetic field structure. In this hypothesis the streamer branches might be channeling some plasma outflow, contributing to the slow wind (Raymond et al, 1997b, 1998) These initial studies have underlined the importance of the oxygen abundance structure within streamers in relation to the understanding of the formation of the slow solar wind. The analysis is performed considering 5 quiescent streamers observed with the UVCS in the period April–June 1996
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