Geomagnetic variations

(geophysics) Temporal changes in the geomagnetic field, both long-term (secular) and short-term (transient).

Variations in the natural magnetic field measured at the Earth's surface. This field changes with periodicities from about 0.3 s to thousands of years. Many of these variations of observed field—the very short-period, daily, seasonal, semiannual, and solar-cycle (11-year) variations—arise from sources that are external to the Earth but are superposed upon the larger main dipolar field of the Earth by the typical measuring instruments. The daily and seasonal atmospheric motions cause field variations that are smooth in form and relatively predictable, given the time and location of the observation. During occasions of high solar-terrestrial disturbance activity that give rise to auroras at high latitudes, very large geomagnetic variations occur that mask the quiet daily changes. These geomagnetic variations are so spectacular in size and global extent that they have been named geomagnetic storms. See also Aurora; Geomagnetism; Ionosphere; Magnetosphere; Solar wind; Sun.