Asubstorm, sometimes referred to as amagnetospheric substorm or anauroral substorm, is a brief disturbance in theEarth's magnetosphere that causes energy to be released from the "tail" of the magnetosphere and injected into the high latitude ionosphere. Visually, a substorm is seen as a sudden brightening and increased movement ofauroral arcs. Substorms were first described in qualitative terms byKristian Birkeland[1] which he called polar elementary storms.Sydney Chapman used the term substorm about 1960 which is now the standard term. The morphology of aurora during a substorm was first described bySyun-Ichi Akasofu in 1964[2][3] using data collected during theInternational Geophysical Year.
Substorms[4] are distinct fromgeomagnetic storms in that the latter take place over a period of several days, are observable from anywhere on Earth, inject a large number ofions into theouter radiation belt, and occur once or twice a month during the maximum of the solar cycle and a few times a year during solar minimum. Substorms, on the other hand, take place over a period of a few hours, are observable primarily at thepolar regions, do not inject many particles into the radiation belt, and are relatively frequent — often occurring only a few hours apart from each other. Substorms can be more intense and occur more frequently during a geomagnetic storm when one substorm may start before the previous one has completed. The source of the magnetic disturbances observed at the Earth's surface during geomagnetic storms is thering current, whereas the sources of magnetic disturbances observed on the ground during substorms are electric currents in theionosphere at high latitudes.[5] During a substorm, the currents flowing across the magnetotail divert into the ionosphere through thesubstorm current wedge, flowing westward.[6]
Substorms can cause magnetic field disturbances in theauroral zones up to a magnitude of 1000 nT, roughly 2% of the total magnetic field strength in that region. The disturbance is much greater in space, as somegeosynchronous satellites have registered the magnetic field dropping to half of its normal strength during a substorm. The most visible indication of a substorm is an increase in the intensity and size ofpolar auroras.[5] Substorms can be divided into three phases: the growth phase, the expansion phase, and the recovery phase.[7]
In 2012, the THEMIS satellite mission observed the dynamics of rapidly developing substorms, confirming the existence of giant magnetic ropes and witnessed small explosions in the outskirts of Earth's magnetic field.[8]