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Geomagnetic storm
A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field. The increase in the solar wind pressure initially compresses the magnetosphere. The solar wind's magnetic field interacts with the Earth’s magnetic field and transfers an increased energy into the magnetosphere. Both interactions cause an increase in plasma movement through the magnetosphere (driven by increased electric fields inside the magnetosphere) and an increase in electric current in the magnetosphere and ionosphere.
During the main phase of a geomagnetic storm, electric current in the magnetosphere creates a magnetic force that pushes out the boundary between the magnetosphere and the solar wind. The disturbance in the interplanetary medium that drives the storm may be due to a solar coronal mass ejection (CME) or a high speed stream (co-rotating interaction region or CIR)〔''Corotating Interaction Regions,'' Corotating Interaction Regions Proceedings of an ISSI Workshop, 6–13 June 1998, Bern, Switzerland, Springer (2000), Hardcover, ISBN 978-0-7923-6080-3, Softcover, ISBN 978-90-481-5367-1〕 of the solar wind originating from a region of weak magnetic field on the Sun’s surface. The frequency of geomagnetic storms increases and decreases with the sunspot cycle. CME driven storms are more common during the maximum of the solar cycle, while CIR driven storms are more common during the minimum of the solar cycle.
Several space weather phenomena tend to be associated with or are caused by a geomagnetic storm. These include: solar energetic Particle (SEP) events, geomagnetically induced currents (GIC), ionospheric disturbances that cause radio and radar scintillation, disruption of navigation by magnetic compass and auroral displays at much lower latitudes than normal. In 1989, a geomagnetic storm energized ground induced currents that disrupted electric power distribution throughout most of the province of Quebec and caused aurorae as far south as Texas.
==History==
In 1931, Sydney Chapman and Vincenzo C. A. Ferraro wrote an article, ''A New Theory of Magnetic Storms'', that sought to explain the phenomenon. They argued that whenever the Sun emits a solar flare it also emits a plasma cloud'','' now known as a coronal mass ejection. This plasma will travel at a velocity such that it reaches Earth within 113 days, though we now know this journey takes 1 to 5 days. The cloud then compresses the Earth's magnetic field and thus increases this field at the Earth's surface.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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