The Behavior of the Open Magnetic Field of the Sun

Abstract

The magnetic field of the Sun can be considered as having two distinct components: open magnetic flux, in which the field lines remain attached to the Sun and are dragged outward into the heliosphere with the solar wind, and closed magnetic flux, in which the field remains entirely attached to the Sun and forms loops and active regions in the solar corona. In this paper a model is presented for the behavior of the Sun's open magnetic flux. It is argued that the total open flux should tend to be constant in time since it can be destroyed only if open flux of opposite polarity reconnects, a process that may be unlikely since the open flux is ordered into large-scale regions of uniform polarity. The behavior of open flux is thus governed by its motion on the solar surface. The motion may be primarily due to a diffusive process that results from open field lines reconnecting with randomly oriented closed loops, and also to the usual convective motions on the solar surface, such as differential rotation. The diffusion process needs to be described by a diffusion equation appropriate for transport by an external medium, which is different from the usual diffusion equation used in energetic particle transport. The loops required for the diffusion have been identified in recent observations of the Sun and have properties, of both size and composition, consistent with their use in the model. The diffusive motions can account for the concentrations of open flux at the coronal holes, where the loops are reduced and the diffusion limited. The motions result in the reversal of the polarity of open flux over the solar cycle simply by the rotation of the current sheet that separates regions of single polarity. The diffusive process, in which reconnection occurs between open field lines and loops, is responsible for the input of mass and energy into the solar wind. The motions of the open field lines also influence the configuration of the heliospheric magnetic field.