Physical Properties
A typical coronal mass ejection may have any or all of three distinctive features: 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.
Most ejections originate from active regions on the surface, such as groupings of sunspots associated with frequent flares. These regions have closed magnetic field lines, in which the magnetic field strength is large enough to contain the plasma. These field lines must be broken or weakened for the ejection to escape from the sun. However, CMEs may also be initiated in quiet surface regions, although in many cases the quiet region was recently active. During solar minimum, CMEs form primarily in the coronal streamer belt near the solar magnetic equator. During solar maximum, they originate from active regions whose latitudinal distribution is more homogeneous.
Coronal mass ejections reach velocities between 20km/s to 3200km/s with an average speed of 489km/s, based on SOHO/LASCO measurements between 1996 and 2003. The average mass is 1.6×1012kg. The values are only lower limits, because coronagraph measurements provide only two-dimensional data analysis. The frequency of ejections depends on the phase of the solar cycle: from about one every fifth day near the solar minimum to 3.5 per day near the solar maximum. These values are also lower limits because ejections propagating away from Earth (backside CMEs) can usually not be detected by coronagraphs.
Current knowledge of coronal mass ejection kinematics indicates that the ejection starts with an initial pre-acceleration phase characterized by a slow rising motion, followed by a period of rapid acceleration away from the Sun until a near-constant velocity is reached. Some balloon CMEs, usually the slowest ones, lack this three-stage evolution, instead accelerating slowly and continuously throughout their flight. Even for CMEs with a well-defined acceleration stage, the pre-acceleration stage is often absent, or perhaps unobservable.
Read more about this topic: Coronal Mass Ejection
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