Coronal mass ejections: origins, evolution, and role in space weather

Coronal mass ejections (CMEs) are an important element of coronal and interplanetary dynamics. They can inject large amounts of mass and magnetic fields into the heliosphere, causing major geomagnetic storms and interplanetary shocks, which are a key source of solar energetic particles (E>1 MeV). Until recently, our understanding of the origins and early development of CMEs at the Sun was very limited. We knew that CMEs were frequently associated with erupting prominences and long-enduring X-ray arcades, but our physical understanding of how and why CMEs are initiated was poor. However, recent studies using the excellent data sets from the Yohkoh, SOHO, Wind, ACE and other spacecraft and ground-based instruments have improved our knowledge of the mass ejection process and how it effects space weather. The author reviews some of the well-determined coronal properties of CMEs, what is known about their source regions, and what their manifestations are in the solar wind. One exciting new type of observation is of halo-like CMEs, which suggest the launch of a geoeffective disturbance toward Earth. Several studies have shown a good correspondence between halo CMEs accompanied by near-sun center surface activity and subsequent magnetic clouds and geomagnetic storms at earth. In addition, halo CMEs are important for understanding the internal structure of CMEs since their source regions are near Sun center and near-earth spacecraft may be likely to sample material along their central axes.