début brusque d'orage magnétique

CONT: As a CME [Coronal Mass Ejection] hits the magnetosphere, the compression of the magnetic field produces an increase in the field measured at ground level. This gives a characteristic rise in the field and is known as a Sudden Storm Commencement (SSC). Not all SSCs are followed by auroral activity, but it is a good early indicator. If the solar wind speed and IMF [Interplanetary Magnetic Field] direction are favourable, the energy in the magnetosphere will gradually build up into a large storm, with the magnetic field decreasing and changing rapidly.
CONT: Plasma from a solar eruption impacted the Earth's magnetic field causing a sudden storm commencement (SSC) and the possible beginning of a significant geomagnetic storm.