geomagnetic storms

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geomagnetic storms

(jee-oh-mag-net -ik) Sudden alterations in and subsequent recovery of the Earth's magnetic field due to the effects of solar flares. The variations are complex in the auroral zone and polar regions but at middle latitudes the horizontal component of the field shows four distinct phases.

The first is storm sudden commencement (SSC), when a sharp rise in field strength (over 2.5 to 5 minutes) is caused by compression of the magnetosphere by a flare-generated shock wave. The second is the initial phase (IP), when the Earth is surrounded by the high-speed post-shock plasma and field, and is effectively isolated (for between about 30 minutes and several hours) from the interplanetary magnetic field. The surface field strength is higher than the pre-SSC value. In the main phase (MP) an increase in particle population, or in particle acceleration by reconnection of the geomagnetic and interplanetary fields, or in magnetospheric fluctuations produces a ring current at three to five Earth radii. This generates a magnetic field opposed to the Earth's and causes a decrease in the surface field strength of 50–400 nanoteslas, which lasts from a few hours to more than a day. During the fourth phase, recovery phase (RP), which is typically longer than the MP, the ring current decays by diffusion of the trapped particles and plasma instabilities. The surface field strength may return to, or just below, the pre-SSC value.

Geomagnetic storms are usually accompanied by ionospheric and auroral activity, and some may recur after 27 days owing to the persistence of a particular solar active region or coronal hole.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
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