Substorm

Substorms were first described in qualitative terms by Kristian Birkeland[1] which he called polar elementary storms.

The morphology of aurora during a substorm was first described by Syun-Ichi Akasofu in 1964[2][3] using data collected during the International Geophysical Year.

Substorms[4] are distinct from geomagnetic storms in that the latter take place over a period of several days, are observable from anywhere on Earth, inject a large number of ions into the outer 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.

The source of the magnetic disturbances observed at the Earth's surface during geomagnetic storms is the ring current, whereas the sources of magnetic disturbances observed on the ground during substorms are electric currents in the ionosphere at high latitudes.

The disturbance is much greater in space, as some geosynchronous satellites have registered the magnetic field dropping to half of its normal strength during a substorm.

A series of images made by ultraviolet light imager on the Polar spacecraft showing the aurora and Earth's upper atmosphere. The glowing side is the atmosphere lit up by the Sun's light energy and the oval of light is the aurora. During a substorm the auroral oval brightens in a localized area and then suddenly breaks into many different forms that expand both toward Earth's pole and equator. This is exactly what Shun-ichi Akasofu (1964) drew in his auroral substorm illustration.
Short video featuring commentary by David Sibeck, project scientist for the THEMIS mission, discussing a visualization of reconnection fronts.