Reducing heliospheric magnetic flux from coronal mass ejections without disconnection

Abstract

Until 1995, disconnection at the Sun was thought to be the only solution to the problem of balancing the magnetic flux coronal mass ejections (CMEs) added to the heliosphere, in spite of the fact that the expected solar wind signature of disconnection was rare. Disconnection was pictured as merging between open field lines to create completely disconnected U‐shaped structures or merging between closed field lines to create, in two dimensions, completely disconnected plasmoids. Since 1995, both theoretical and observational studies have made important contributions toward a revision of this solution. On the basis of a synthesis of these studies, we suggest that the primary flux balancing mechanism is not complete disconnection but rather merging between closed and open fields, which we call “interchange reconnection.” Disconnection on closed fields plays a role but, in three dimensions, only as a partial process that prevents expansion of arcade loops into the heliosphere by creating helical fields still connected to the Sun. We suggest that interchange reconnection occurs in the leg of a CME until it is completely open, long after its leading edge has departed from the vicinity of the Sun, and we discuss preliminary findings from Ulysses testing this view. Further, we suggest that interchange reconnection in CMEs may be driven by the large‐scale circulation of open field lines required by the Fisk model.