Understanding CMEs and their source regions

CMEs are an important aspect of coronal and interplanetary dynamics. They c an eject large amounts of mass and magnetic fields into the heliosphere whi ch can drive large geomagnetic storms and interplanetary shocks, a key sour ce of solar energetic particles. However, our knowledge of the origins and early development of CMEs at the Sun is limited. CMEs are most frequently a ssociated with erupting prominences and long-enduring X-ray arcades, but so metimes with weak or no observed surface activity. I review some of the wel l-determined coronal properties of CMEs and what we know about their source regions, including recent studies using Yohkoh,SOHO and radio data. One ex citing, new type of observation is of halo-like CMEs which suggest the laun ch of a geoeffective disturbance toward Earth. Besides their utility for fo recasting the arrival at Earth of magnetic clouds and geomagnetic storms, h alo CMEs are important for understanding the development and internal struc ture of CMEs since we can view their source regions near Sun center and can measure their in-situ characteristics along their central axes. (C) 2000 E lsevier Science Ltd. All rights reserved.