A substorm, also called a magnetospheric substorm or auroral substorm, is a short-term change in Earth's magnetosphere that releases energy from the "tail" of the magnetosphere and sends it into the high latitude ionosphere. When a substorm happens, auroras become brighter and move more quickly. Scientists first described substorms in general terms by Kristian Birkeland, who called them polar elementary storms. Later, in about 1960, Sydney Chapman used the term "substorm," which is now the standard name. Syun-Ichi Akasofu first explained how auroras change during a substorm in 1964 using data from the International Geophysical Year.

Substorms are different from geomagnetic storms. Geomagnetic storms last several days, can be seen from anywhere on Earth, add many ions to the outer radiation belt, and happen once or twice a month during the peak of the solar cycle or a few times a year during the solar minimum. Substorms, however, last only a few hours, are mostly visible near the poles, do not add many particles to the radiation belt, and happen often—sometimes only a few hours apart. Substorms can be more intense and occur more frequently during geomagnetic storms, with one substorm starting before the previous one finishes. The cause of magnetic changes during geomagnetic storms is the ring current, while the cause of magnetic changes during substorms is electric currents in the high latitude ionosphere. During a substorm, currents in the magnetotail change direction and flow westward through the ionosphere in a pattern called the substorm current wedge.

Substorms can create magnetic disturbances in the auroral zones as strong as 1000 nT, about 2% of the total magnetic field strength in that area. These disturbances are even stronger in space, with some satellites near Earth recording the magnetic field dropping to half its normal strength during a substorm. The most noticeable sign of a substorm is the sudden increase in brightness and size of polar auroras. Substorms have three stages: the growth phase, the expansion phase, and the recovery phase.

In 2012, the THEMIS satellite mission studied how substorms develop quickly. It found evidence of large magnetic ropes and observed small explosions near the edge of Earth's magnetic field.