We present a theoretical magnetohydrodynamic (MHD) model describing the time-dependent expulsion of a three-dimensional coronal mass ejection (CME) out of the solar corona. The model relates the white-light appearance of the CME to its internal magnetic Ðeld, which takes the form of a closed bubble, Ðlled with a partly anchored, twisted magnetic Ñux rope, and embedded in an otherwise open background Ðeld. The model is constructed by solving in closed form the time-dependent ideal MHD equations for a c \ 4/3 polytrope making use of a similarity assumption and the application of a mathematical stretching transformation in order to treat a complex Ðeld geometry with three-dimensional variations. The density distribution frozen into the expanding CME magnetic Ðeld is obtained. The scattered white light integrated along the line of sight shows the conspicuous three features often associated with CMEs as observed with white-light coronagraphs : a surrounding high-density region, an internal low-density cavity, and a high-density core. We also show how the orientation of this three-dimensional structure relative to the line of sight can give rise to a variety of di †erent geometric appearances in white light. These images generated from a CME model in a realistic geometry o †er an opportunity to directly compare theoretical predictions on CME shapes with observations of CMEs in white light. The mathematical methods used in the model construction have general application and are described in the Appendices.

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This review puts together what we have learned about coronal structures and phenomenology to synthesize a physical picture of the corona as a voluminous, thermally and electrically highly-conducting atmosphere responding dynamically to the injection of magnetic flux from below. The synthesis describes complementary roles played by the magnetic heating of the corona, the different types of flares, and the coronal mass ejections as physical processes by which magnetic flux and helicity make their way from below the photosphere into the corona, and, ultimately, into interplanetary space. In these processes, a physically meaningful interplay among dissipative magnetohydrodynamic turbulence, ideal ordered flows, and magnetic helicity determines how and when the rich variety of relatively long-lived coronal structures, spawned by the emerged magnetic flux, will evolve quasi-steadily or erupt with the impressive energies characteristic of flares and coronal mass ejections. Central to this picture is the suggestion, based on recent theoretical and observational works, that the the emerged flux may take the form of a twisted flux rope residing principally in the corona. Such a flux rope is identified with the low-density cavity at the base of a coronal helmet, often but not always encasing a quiescent prominence. The flux rope may either be bodily transported into the corona from below the photosphere, or reform out of a state of flaring turbulence under some suitable constraint of magnetic-helicity conservation. The appeal of this synthesis is its physical simplicity and the manner it relates a large set of diverse phenomena into a self-consistent whole. The implications of this view point are discussed.The topics covered are: the large-scale corona; helmet streamers; quiescent prominences; coronal mass ejections; flares and heating; magnetic reconnection and magnetic helicity; and, the hydromagnetics of magnetic flux emergence.