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Coronal Mass Ejections (CMEs)

A CME is a large cloud of magnetised coronal plasma which is injected into the interplanetary medium, sometimes in association with a solar flare. CMEs create disturbances in the background solar wind which can lead to geomagnetic storms when they reach the Earth between 2 and 6 days after leaving the Sun.

CMEs are best observed using a type of telescope called a coronagraph. These make use of an 'artificial moon' called an 'occulting disk' to block out the light emitted by the solar disk. This allows us to see the much fainter structures of the solar corona: the Sun's atmosphere. The Large Angle and Spectrometric Coronagraph  LASCO onboard the ESA-NASA mission SOHO has offered us much new information about these extraordinary events.

The coronal mass ejection seen in this movie took place on June 6th 2000. This type of CME is called a 'halo' because material is seen expanding around the occulting disk in a bright halo. This CME expanded away from the Sun and towards the Earth, eventually causing a huge geomagnetic storm. In this movie, the small, central white circle represents the sun.

Movie courtesy of SOHO/LASCO consortium.