We report a first systematic morphological study of the boundaries of
coronal holes (CH) as viewed in soft X-ray images from the Yohkoh
Soft X-ray Telescope (SXT).
The special emphasis is on long-lived (several rotations)
CHs which 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
In this latter case the loops clearly do not extend far enough to reach
another CH, but intead 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.
Authors: S.W. Kahler and H.S. Hudson
Publication Status: ApJ (accepted)
Last Modified: 2002-04-01 23:16