Coronal field lines of an evolving bipolar magnetic region

A simple potential field model is presented to illustrate that loops of magnetic flux rise upward through the corona during the relatively short growth phase of a bipolar magnetic region but contract back to the Sun's surface during the much longer decay phase of the photospheric region. To reconcile this behavior with the unidirectional, solar-wind-driven convection of flux outward from the Sun, one must postulate the existence of an x-type neutral line in the middle corona where open field lines can be converted back to closed ones