The structure and evolution of coronal holes

When observed at soft X-ray wavelengths coronal holes are seen as open features, devoid of X-ray emission and bounded by apparently divergent coronal loop structures. Inspection of the topology of the photospheric magnetic fields associated with these features suggests that holes are formed when the remnants of active region fields, emerging in both hemispheres over a period of several solar rotations, combine to form a large area of essentially unipolar field. Remnants of opposite polarity fields surround these features resulting in a divergent magnetic configuration at the hole boundaries. Holes are seen to form and evolve while the large scale divergent field pattern is reinforced and to close when large scale remnants occur which disrupt the general field pattern. Two types of holes are observed in the early Skylab observations. The first are elongated features which are aligned approximately north-south extending from one solar pole to a polar filament channel in the opposite hemisphere. The polar holes and somewhat lower latitude holes appear to lie in unipolar areas which are completely confined by opposite polarity fields. Studies of the rotation properties of an elongated hole, which extended from the north pole to a latitude of approximately 20° S, showed it to rotate with a synodic rate of (13.25±0.03)?(0.4±0.1 sin²φdeg day^?1. Possible explanations for the almost rigid rotational characteristics of this feature are discussed.