Substorms are brief (2-3 hour) magnetospheric disturbances that occur when the interplanetary magnetic field turns southward, permitting interplanetary and terrestrial magnetic field lines to merge at the dayside magnetopause and energy to be transferred from the solar wind to the magnetosphere. The storage of some of this energy in the Earth's magnetotail constitutes the first of the three phases of the substorm, the "growth" phase. During the second phase, the substorm expansion phase, the energy stored in the tail is released when the field lines in the inner magnetosphere relax from their stretched, tail-like configuration and "snap" back into a more dipolar configuration. This process, known as dipolarization, results in the energization of charged particles in the plasma sheet and their injection deeper into the inner magnetosphere. It is not known what triggers substorm expansion, although several theories have been proposed. The role that this process plays in the formation of the storm-time ring current is a matter of controversy. The third phase is the recovery phase, during which the magnetosphere returns to its quiet state. The storage and release of energy in the magnetopshere during a substorm leads to characteristic changes in auroral morphology and emission intensity and to the enhancement of currents flowing in the polar ionosphere and associated disturbances in the strength of the high-latitude surface magnetic field. Substorms occur, on the average, six times a day; they occur more frequently, and are more intense, during geomagnetic storms.

The substorm cartoon is from W. Baumjohan and R.A. Treumann, Basic Space Plasma Physics, 1996. The three auroral images were obtained with the IMAGE WIC instrument.