Coronal mass ejections (CMEs) and their geoeffectiveness

The Sun's activity drives the variability of geospace (i.e., near-Earth environment). Observations show that the ejection of plasma from the Sun, called coronal mass ejections (CMEs), are the major cause of geomagnetic storms. This global-scale solar dynamical feature of coronal mass ejection was discovered almost three decades ago by the use of space-borne coronagraphs (OSO-7, Skylab/ATM and P78-1). Significant progress has been made in understanding the physical nature of the CMEs. Observations show that these global-scale CMEs have size in the order of a solar radius (/spl sim/6.7/spl times/10/sup 5/ km) near the Sun, and each event involves a mass of about 10/sup 15/ g and an energy comparable to that of a large flare on the order of 10/sup 32/ ergs. The radial propagation speeds of CMEs have a wide range from tens to thousands of kilometers per second. Thus, the transit time to near Earth's environment [i.e., 1 AU (astronomical unit)] can be as fast as 40 hours to 100 hours. The typical transit time for geoeffective events is /spl sim/60-80 h. This paper consists of two parts: 1) A summary of the observed CMEs from Skylab to the present SOHO will be presented. Special attention will be made to SOHO/LASCO/EIT observations and their characteristics leading to a geoeffective CME. 2) The chronological development of theory and models to interpret the physical nature of this fascinating phenomenon will be reviewed. Finally, an example will be presented to illustrate the geoeffectiveness of the CMEs by using both observation and model.