Physics of Coronal Mass Ejections: A New Paradigm of Solar Eruptions

Abstract

The prevailing framework of understanding coronal mass ejections (CMEs) and, indeed, solar eruptions in general is the hypothesis that the quasi-static changes in the photospheric magnetic field increases the magnetic energy in the corona and causes sudden release of the stored energy. However, this hypothesis, which may be called the `storage-release' paradigm, has yet to produce a quantitative model of CMEs and their heliospheric consequences. Recently, a new theory has been proposed to explain the physics of CMEs. This theory posits that the initial structure is a magnetic flux rope that is ultimately connected to the solar dynamo in the convection zone and that magnetic energy propagating from the source along the submerged magnetic structure enters the corona and drives the eruption. Specifically, the theory describes CMEs as the dynamical response of coronal flux ropes to the `injected' poloidal flux and predicts that CMEs evolve into interplanetary magnetic clouds (MCs). In a recent series of studies, the physics-based theory was shown to correctly describe the observed dynamics of a class of CMEs and the properties of MCs, providing the first unified description of the CME-MC dynamics. The apparent success of the theory suggests a new paradigm in which CMEs are viewed as a relaxation process in response to the increased magnetic energy propagating from the dynamo. The motion of a flux rope and its magnetic energy is everywhere given by \(\tilde \upsilon \lesssim 1\), where \(\tilde \upsilon \equiv \upsilon /V_M {\text{ and }}V_M \) is the local characteristic speed. It is predicted that the injected poloidal flux can cause a subtle but distinct signature in the tangential field at the base of the corona. The tangential field, which is difficult to observe, and the poloidal magnetic energy have been neglected in previous theories. This paper reviews the current CME research, compares the physics of the two competing paradigms, and suggests new observable magnetic field signatures.