Geomagnetic variations

Article By:

Papitashvili, Vladimir O. Space Physics Research Laboratory, University of Michigan, Ann Arbor, Michigan.

Campbell, Wallace H. U.S. Geological Survey, Denver, Colorado.

Last reviewed:November 2019

DOI:https://doi.org/10.1036/1097-8542.286700

Variations in the natural magnetic field measured at the Earth's surface and elsewhere in the Earth's magnetosphere (for example, at the geostationary orbit). These are field changes with periodicities from about 0.3 second to hundreds of years. (These boundaries are set to distinguish geomagnetic variations from the quasipermanent field and higher-frequency waves.) Many of these observed variations—from very short periods (seconds, minutes, hours) to daily, seasonal, semiannual, solar-cycle (11-year), and secular (∼60–80 years) periods—arise from sources that either are external to the Earth (but superposed upon the larger, mainly dipolar field) or internal to the Earth (the magnetic-dipole and higher-harmonic trends and variations on the scales of hundreds and even thousands years). The daily and seasonal motions of the atmosphere at ionospheric altitudes 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 aurorae (northern and southern lights) at high latitudes, very large geomagnetic variations occur that even mask the quiet daily changes. These geomagnetic variations are so spectacular in size and global extent that they have been named geomagnetic storms and substorms, with the latter generally limited to the polar regions.

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