The Hydromagnetic Nature of Solar Coronal Mass Ejections

Solar coronal mass ejections (CMEs) are a major form of activity on the Sun. A CME takes 10^15-16 g of plasma from the low corona into the solar wind, to disturb the near-Earth space if the CME direction is favorable. We summarize current observations and ideas of CME physics to provide a hydromagnetic view of the CMEs as the products of continual magnetic flux emergence and an interplay between magnetic reconnection and approximate magnetic-helicity conservation in the corona. Each flux emergence brings helicity to accumulate additively in a coronal structure while excess magnetic energy is flared away by reconnection. Self-confinement eventually fails with a CME shedding the accumulated helicity out of the low corona to enable the field to reach the minimum-energy state. Similar evolutionary processes may occur in other magnetic stars and galaxies.