An intensification of activity along the auroral oval at high latitudes, resulting from a relatively minor geomagnetic disturbance. Substorm activity initially takes the form of a bright arc on the night-side of the oval, then spreads polewards and westwards before subsiding. The associated ground-level magnetic effects can be detected with sensitive magnetometers. There may be as many as five substorms per day, each lasting 1–3 hours.
Subjects: Astronomy and Astrophysics.