Magnetic flux emergence into the solar corona. I. Its role for the reversal of global coronal magnetic fields

Some physical insights into how the corona reverses its global magnetic fie ld are described in this paper based on a set of elementary hydromagnetic c alculations. We assume that a fresh magnetic field of opposite polarity has emerged into a corona containing a preexisting magnetic field. The inevita ble magnetic reconnection that takes place between the two magnetic flux sy stems may result in an expulsion of magnetic flux to infinity. Our calculat ions suggest the following physical story of the coronal reversal process : When the emerged flux exceeds the preexisting flux by a critical amount, t he corona will reverse its polarity. Before this critical ratio is attained , the field with the emerged flux may have enough energy to let only one or two bipolar parts of the multipolar field open up. This opening-up process , taking place as a coronal mass ejection (CME), may take some of the preex isting flux out of the corona and thus increase the emerged-to-preexisting flux ratio and bring the corona closer to the critical value for its global magnetic reversal. Our calculations also indicate that it is possible that the position where the field opens up may be different from that where the new flux emerges. This may help explain the difference in the latitude dis tribution of active regions and CMEs during a solar cycle as observed by Hu ndhausen.