Active regions losing their moorings by subsurface reconnection

Abstract

The properties of emerging active regions suggest that they originate from deep flux bundles with a field strength well above the equipartition value, so that they can resist strong deformation by convection as they surface. Yet upon flux emergence, the field appears in a multitude of bundles with a field strength near to equipartition with the pressure component that is associated with the convective motions. During the subsequent decay of active regions, the flux disperses in a random walk that seems to be caused uniquely by the flows in the near-surface convection. We propose that this apparently untethered random walk is the consequence of subsurface reconnection, which leads to the formation of flexible weak-field connections between the strong, buoyant top segments, cut loose from the deep source region. The frequent reconnection between flux tubes underneath each of the polarities in the active region acts to maintain an approximately vertical organization of the subsurface field. We concur with earlier studies that the distribution of the field can be described by the horizontal dispersal of a scalar at least in these layers near the surface, because in addition to this combing of the field, mixing-length models suggest that the horizontal diffusion coefficient is almost constant down to a depth of some 10 000 km. Our model predicts that the sub-surface reconnection between fields of opposite polarity causes the surface field to be disconnected from the deep source region on a time scale that is in fair agreement with the observed lifetimes of active regions of a range of sizes. We explore whether branching of flux bundles into ever smaller bundles between the bottom of the convective envelope and the photosphere allows the limited bending of flux tubes by convection that is required to induce reconnection.