We report a first systematic morphological study of the boundaries of coronal holes (CHs) as viewed in soft X-ray images from the Yohkoh Soft X-Ray Telescope. The special emphasis is on long-lived (several rotations) CHs that extend from the solar polar regions to midlatitudes. As shown earlier, such equatorward extensions tend to show rigid, rather than differential, rotation. Magnetic reconnection must occur at the "closing" boundary, in such a case, to maintain the CH integrity. We find three kinds of CH boundaries in the soft X-ray observations. The majority are generally ragged and not sharply defined; we also find smooth boundaries to occur near a matching-polarity active region (AR), and loopy boundaries to occur near an opposite-polarity AR. In this latter case the loops clearly do not extend far enough to reach another CH but instead end in normal corona. The CH boundaries evolve slowly, and neither large-scale transient X-ray events nor coronal bright points appeared significant factors in long-term CH boundary development. No direct evidence for magnetic reconnection is seen. We compare these results with those expected from current models, derived largely from considerations of heliospheric conditions rather than the detailed appearance of CHs in the low corona.
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