UNDERSTANDING THE ROTATION OF CORONAL HOLES

In an earlier study we found that the rotation of coronal holes could be understood on the basis of a nearly current-free coronal field, with the holes representing open magnetic regions. In this paper we illustrate the model by focusing on the case of CHI, the rigidly rotating, boot-shaped hole observed by Skylab. We show that the interaction between the (axisymmetric) polar fields and the (nonaxisymmetric) flux associated with active regions produces distortions in the coronal field configuration and thus in the polar-hole boundaries; these distortions corotate with the perturbing nonaxisymmetric flux. In the case of CHI, positive-polarity field lines in the northern hemisphere ''collided'' with like-polarity field lines fanning out from a decaying active-region complex located just below the equator, producing a midlatitude corridor of open field lines rotating at the rate of the active region complex. Sheared coronal holes result when nonaxisymmetric flux is present at high latitudes, or equivalently, when the photospheric neutral line extends to high latitudes. We demonstrate how a small active region, rotating at the local photospheric rate, can drift through a rigidly rotating hole like CHI. Finally, we discuss the role of field-line reconnection in maintaining a quasi-potential coronal configuration.