A typical CME has a three part structure consisting of a cavity of low electron density, a dense core (the prominence, which appears as a bright region on coronagraph images) embedded in this cavity, and a bright leading edge.
CMEs generally originate from groupings of sunspots associated with frequent flares. These regions have closed magnetic field lines, where the magnetic field strength is large enough to allow the containment of the plasma. The CME must open these field lines to escape from the sun. CMEs can also be initiated in quiet sun regions. During solar minimum, CMEs form primarily in the coronal streamer belt near the solar magnetic equator. During solar maximum, CMEs originate from active regions whose latitudinal distribution is more homogeneous.
Coronal Mass Ejections range in speed from about 20 km/s to 2,700 km/s with an average speed (based on SOHO/LASCO measurements between 1996 and 2003) of 489 km/s. The average mass based on coronagraph images is 1.6 x 1015 g. Due to the two-dimensional nature of the coronagraph measurements, these values are lower limits. The frequency of ejections depends on the phase of the solar cycle: from about one every other day near solar minimum to 5-6 per day near solar maximum.