A substorm, also called a magnetospheric substorm or an auroral substorm, is a short change in Earth's magnetic field that sends energy from the "tail" of the magnetosphere into the high latitude ionosphere. Visually, a substorm appears as a sudden brightening and faster movement of auroral lights. Scientists first described substorms in general terms by Kristian Birkeland, who called them polar elementary storms. Later, Sydney Chapman used the term "substorm" around 1960, which is now the standard name. Syun-Ichi Akasofu first described the shape of auroras during a substorm in 1964 using data from the International Geophysical Year.

Substorms are different from geomagnetic storms. Geomagnetic storms last for several days, can be seen anywhere on Earth, send many ions into 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 solar minimum. Substorms, however, last only a few hours, are mainly seen near Earth's poles, send fewer particles into the radiation belt, and happen often—sometimes within a few hours of each other. During a geomagnetic storm, substorms may start before the previous one finishes, making them more intense and frequent. 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 ionosphere at high latitudes. During a substorm, currents in the magnetotail change direction and flow into the ionosphere through a specific area called the substorm current wedge, moving westward.

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

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