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Ann. Geophys., 19, 135-145, 2001
www.ann-geophys.net/19/135/2001/
© European Geosciences Union 2001
Oxygen abundance in coronal streamers during solar minimum
D. Marocchi1, E. Antonucci2, and S. Giordano2
1Dipartimento di Fisica Generale, University of Torino, Torino, Italy
2Osservatorio Astronomico di Torino, Pino Torinese, Italy
Correspondence to: S. Giordano (giordano@to.astro.it)
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)
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