Geomagnetic storms are major disturbances of the magnetosphere that occur when the interplanetary magnetic field turns southward and remains southward for an prolonged period of time. During a geomagnetic storm's main phase, which can last as long as two to two and a half days in the case of a severe storm, charged particles in the near-Earth plasma sheet are energized and injected deeper into the inner magnetosphere, producing the storm-time ring current. This phase is characterized by the occurrence of multiple intense substorms, with the attendant auroral and geomagnetic effects. (The nature of the relationship between magnetic storms and substorms is a matter of some controversy.) When the interplanetary field turns northward again, the rate of plasma energization and inward transport slows and the various loss processes that remove plasma from the ring current can begin to restore it to its pre-storm state. In the case of a great storm, such as the one of 6 February 1986, the ring current can take over a month to fully return to its quiet state. The drop in the surface magnetic field strength during the main phase of a geomagnetic storm is typically preceded by a brief rise in the field strength (see the entry for Dst index). This increase is caused by an intensification of the magnetopause current that occurs as increased solar wind dynamic pressure drives the magnetopause inward by as much as four Earth radii. This phenomenon, which is known as the storm sudden commencement (SSC), marks the beginning of the initial phase of the storm.

Recurrent vs. non-recurrent storms

Geomagnetic storms are classified as recurrent and non-recurrent. Recurrent storms occur every 27 days, corresponding to the Sun's rotation period. They are triggered by the Earth's encounters with the southward- oriented magnetic field of the high-pressure regions formed in the interplanetary medium by the interaction of low- and high-speed solar wind streams co-rotating with the Sun. Recurrent storms occur most frequently in the declining phase of the solar cycle. Non-recurrent geomagnetic storms, on the other hand, occur most frequently near solar maximum. They are caused by interplanetary disturbances driven by fast coronal mass ejections (CMEs) and typically involve an encounter with both the interplanetary shock wave and the CME that drives it.