Interchange Reconnection and Coronal Hole Dynamics

We investigate the effect of magnetic reconnection between open and closed field, (often referred to as "interchange" reconnection), on the dynamics and topology of coronal hole boundaries. The most important and most prevalent 3D topology of the interchange process is that of a small-scale bipolar magnetic field interacting with a large-scale background field. We determine the evolution of such a magnetic topology by numerical solution of the fully 3D MHD equations in spherical coordinates. First, we calculate the evolution of a small-scale bipole that initially is completely inside an open field region and then is driven across a coronal hole boundary by photospheric motions. Next the reverse situation is calculated in which the bipole is initially inside the closed region and driven toward the coronal hole boundary. In both cases we find that the stress imparted by the photospheric motions results in deformation of the separatrix surface between the closed field of the bipole and the background field, leading to rapid current sheet formation and to efficient reconnection. When the bipole is inside the open field region, the reconnection is of the interchange type in that it exchanges open and closed field. We examine, in detail, the topology of the field as the bipole moves across the coronal hole boundary, and find that the field remains well-connected throughout this process. Our results imply that open flux cannot penetrate deeply into the closed field region below a helmet streamer and, hence, support the quasi-steady models in which open and closed flux remain topologically distinct. Our results also support the uniqueness hypothesis for open field regions as postulated by Antiochos et al. We discuss the implications of this work for coronal observations. Subject Headings: Sun: corona Sun: magnetic fields Sun: reconnection Sun: coronal hole