A Coronal Mass Ejection (or CME) is a giant cloud of solar plasma drenched with magnetic field lines that are blown away from the Sun during strong, long-duration solar flares and filament eruptions.
A solar flare on the sun where a CME has been emitted.
The first proof of these dynamic events came from observations made with a coronagraph on the OSO 7 spacecraft between 1971 and 1973. A coronagraph produces a solar eclipse by covering the Sun with a small disk. Coronal Mass Ejections are very faint and can not be observed otherwise. SOHO and the STEREO space crafts currently have coronagrahps on-board to detect Coronal Mass Ejections.
A Coronal Mass Ejection usually happens during eruptions on the Sun like solar flares and filament eruptions. However, not every eruption has a CME accompanied with it. Most of the times only strong flares (M and X-class) emit Coronal Mass Ejections. C-class solar flares can also produce Coronal Mass Ejections but only the long-duration and stronger C-class flares might do this. It also depends on the duration of the solar flare how much solar plasma get's blown into space. For example, when there is a solar flare with a peak strength of X2 and with a total duration of two hours then it will certainly be accompanied with a bright, large and fast Coronal Mass Ejection. Depending on the location of the eruption, the blast could either miss Earth, be partially or fully Earth-directed. An Earth-directed Coronal Mass Ejection will look like a partial or full-halo CME on images coming from SOHO. When this happens the CME will arrive at Earth after 24 hours or more (depending on the speed) and will likely cause a geomagnetic storm.
A full-halo CME on it's way to Earth.
When we are at solar minimum, the Sun is not very active and we only observe a Coronal Mass Ejection about once a week or even less. When we build up to solar maximum the amount of Coronal Mass Ejections can build up to 2 or 3 a day.
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07:45 Dec 07 2013
|Moderately strong M1.27 solar flare|
13:12 Nov 23 2013
|Moderately strong M1.08 solar flare|
02:48 Nov 23 2013
|Moderately strong M1.18 solar flare|
|Last geomagnetic storm:||2013/10/09||Kp5 (G1)|