A substorm is a brief (2- to 3-hour) disturbance in Earth's magnetosphere that happens when the interplanetary magnetic field turns southward, allowing 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 Earth's magnetotail marks the so-called growth phase of a substorm. During the expansion phase, the stored energy is released when the field lines in the inner magnetosphere snap back from their stretched, tail-like configuration. This process results in the energization of charged particles in the plasma sheet and their injection deeper into the inner magnetosphere. The recovery phase is when the magnetosphere returns to its quiet state.

The storage and release of energy in the magnetosphere during a substorm leads to characteristic changes in the appearance of aurorae and of 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, and happen more often, and are more intense, during geomagnetic storms.