The large-scale ejection of matter from the Sun’s corona at speeds from a few tens to a few thousand km/s. The mass thrown off in such an event is about 10¹¹–10¹³ kg. Spacecraft coronagraphs show that a typical CME consists of a bright leading edge of gas forming a loop or, more probably, a bubble in the corona, ahead of a dark cavity with an embedded bright core. In Ηα‎ light, this core is seen to be an erupting prominence that moves outwards within the cavity. A pair of legs, where the bright rim originally connected with the Sun, may persist for a day or more. Sometimes a flare seems to be triggered shortly after a CME, but the connection between CMEs and flares is unclear. A CME produces a disturbance in the solar wind preceded by a shock wave. Interplanetary space probes encountering such disturbances have recorded increased wind speeds and densities, and a rapidly varying magnetic field. When these interplanetary disturbances reach the Earth they give rise to geomagnetic storms. Their frequency varies with the sunspot cycle. At solar minimum about one CME occurs a week, rising to an average of two or three per day at solar maximum.

http://earthobservatory.nasa.gov/IOTD/view.php?id=44362