Extension of the Polar Coronal Hole Boundary into Interplanetary space

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Abstract
White-light measurements made by the SOHO LASCO C2 and C3 coronagraphs and the Mk III Mauna Loa K-coronameter, ranging from 1.15 to 30 R☉, have been combined with Kitt Peak daily He I 1083 nm coronal hole maps, and full Sun Yohkoh soft X-ray images, to show that the boundaries of polar coronal holes, as determined by measurements of path-integrated density, extend approximately radially into interplanetary space. These results are in contrast to the long-standing view that the boundaries of polar coronal holes diverge significantly beyond radial, evolving around the edges of streamers. The combined observations also show that the corona is dominated by raylike structures as small as a few degrees in angular size with respect to Sun center, originating from both coronal holes and the quiet Sun. This analysis provides further support for results originally derived from radio occultation measurements, namely, that the coronal density projects itself almost radially from the Sun into the outer corona, implying that open field lines abound in the quiet Sun from which the fast wind can alsooriginate.