A Coronal Mass Ejection (or CME) is a big bubble of gas drenched with magnetic field lines that are blown away from the Sun with a duration of a couple of minutes till a couple of hours.
A solar flare on the sun where CME has been emitted
The first proof of these dynamic events came from observations of a coronograph on the OSO 7 spacecraft between 1971 and 1973. A coronograph produces a solar eclips by covering the sun with a small disk.
A coronal mass ejection usualy happens with eruptions on the sun (solar flares, filament eruptions) but not every eruption has a CME accompanied with it. Only with the major flares (C, M and X class) the chance of CME emission is much higher. Depending on the duration of the eruption less or more CME is blown into space. For example, when there is an X2 class flare with a duration of 2 hours, this will certainly be accompanied with a very bright CME. Depending of the location of the eruption on the Sun, the blast could be partialy or fully Earth directed. On the satellite images, like the one below, we observe this as a partial or fully halo CME. When this happens the CME will arrive at Earth after 12 tot 48 hours (depending on the speed) and could cause aurora.
A full halo CME on its way to Earth
When we are at solar minimum, the sun is less active and we only observe a CME once a week or less. When we build up to solar maximum the ammount of CMEs build up to 2 or 3 a day.
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06:24 May 18 2013
|G2 - Moderate geomagnetic storm (Kp 6.00) - Observers at high latitudes may see some nice periods of visual aurora. The chances for the Middle latitude is still relatively low.|
09:15 May 17 2013
|Strong M3.21 solar flare|
22:09 May 16 2013
|Moderately strong M1.31 solar flare|