A coronal mass ejection (CME) is a significant release of magnetized plasma consisting primarily of electrons and protons and accompanying magnetic field from the solar corona. Coronal mass ejections release large quantities of matter and electromagnetic radiation into space above the Sun’s surface, either near the corona, or farther into the planetary system, or beyond (interplanetary CME).
They often follow solar flares and are normally present during a solar prominence eruption. The plasma is released into the solar wind, and can be observed in coronagraph imagery. While the terrestrial effects of solar flares are very fast (limited by the speed of light), CMEs are relatively slow, developing at the Alfvén speed.
Coronal mass ejections are often associated with other forms of solar activity, but a broadly accepted theoretical understanding of these relationships has not been established.
CMEs most often originate from active regions on the Sun’s surface, such as groupings of sunspots associated with frequent flares. Near solar maxima, the Sun produces about three CMEs every day, whereas near solar minima, there is about one CME every five days.
On the Sun, magnetic reconnection may happen on solar arcades—a series of closely occurring loops of magnetic lines of force. These lines of force quickly reconnect into a low arcade of loops, leaving a helix of magnetic field unconnected to the rest of the arcade.
The sudden release of energy during this process causes the solar flare and ejects the CME. The helical magnetic field and the material that it contains may violently expand outwards forming a CME. This also explains why CMEs and solar flares typically erupt from what are known as the active regions on the Sun where magnetic fields are much stronger on average.
When the ejection is directed towards Earth and reaches it as an interplanetary CME (ICME), the shock wave of traveling mass causes a geomagnetic storm that may disrupt Earth’s magnetosphere, compressing it on the day side and extending the night-side magnetic tail. When the magnetosphere reconnects on the nightside, it releases power on the order of terawatt scale, which is directed back toward Earth’s upper atmosphere.
Solar energetic particles can cause particularly strong aurorae in large regions around Earth’s magnetic poles. These are also known as the Northern Lights (aurora borealis) in the northern hemisphere, and the Southern Lights (aurora australis) in the southern hemisphere.
The largest recorded geomagnetic perturbation, resulting presumably from a CME hitting the Earth’s magnetosphere, was the solar storm of 1859 (the Carrington Event), which took down parts of the recently created US telegraph network, starting fires and shocking some telegraph operators. Some telegraphers, on the other hand, were able to continue operating with their batteries disconnected, powered by the aurora-induced currents in the lines, with normal or improved signal quality.
There have been a small number of CMEs observed on other stars, all of which as of 2016 have been found on red dwarfs. These have been detected by spectroscopy, most often by studying Balmer lines: the material ejected toward the observer causes asymmetry in the blue wing of the line profiles due to Doppler shift. This enhancement can be seen in absorption when it occurs on the stellar disc (the material is cooler than its surrounding), and in emission when it is outside the disc.