The Darkest Regions Of Solar Polar Coronal Holes Observed By Sumer On Soho

Abstract

The plasma conditions above solar coronal holes are of considerable interest, because it is well established that the high-speed solar wind streams originate in these regions (see, e.g., Krieger et al., 1973; Geiss et al., 1995a; Woch et al., 1997), and the acceleration processes of the fast solar wind thus can be studied there. The most prominent features above polar coronal holes are polar plumes or rays (van de Hülst, 1950a,b; Saito, 1965; Newkirk and Harvey, 1968; Ahmad and Withbroe, 1977; Antonucci et al., 1997; Wang et al., 1997; DeForest et al., 1997; Wilhelm et al., 1998a). The debate whether plumes contribute to the fast solar wind streams is not yet settled. Of crucial importance is the elemental abundance inside and outside plumes. Widing and Feldman (1989, 1992, 1993) deduced a very low neon-to-magnesium ratio relative to the photosphere in a bright plume. As, on the other hand, abundances measured in fast solar wind streams by in situ observations are close to those of the photosphere (e.g., Geiss et al., 1995b), this is an argument against significant contributions to the fast wind by plumes. Wilhelm and Bodmer (1998) found low neon-to-magnesium abundance ratios inside plumes with respect to inter-plume lanes, a distinction that is not seen in the fast solar wind. It thus seems appropriate to assume that the fast wind does indeed emanate from the darkest regions of solar coronal holes. The study of inter-plume lanes is hampered by the fact that the relatively bright plumes dominate the EUV emission inside the northern and southern plume assemblies. East and west of these assemblies, very dark regions of the corona could be seen on EIT/SOHO images during the minimum of solar activity. Their investigation is difficult, because of the faint ultraviolet emission, but the danger of any plume “contamination” is small, and the very fact that these regions are so dark might indicate that the energy delivered by the chromospheric network to the corona is transformed predominantly into the acceleration of the solar wind and not into EUV and UV radiation.