Geomagnetic storms caused by solar winds pose a risk of major wide-area power blackouts. We just got past the 3-year maximum (2000-2002) of the current 11-year solar cycle, but this is always a risk. In March of 1989, such a storm caused a blackout for 6 million people in Quebec, for example.
When the Earth's magnetic field captures ionized particles carried by the solar wind, geomagnetically induced currents (GIC) can flow through the power system, entering and exiting the many grounding points on a transmission network. GICs are produced when shocks resulting from sudden and severe magnetic storms subject portions of the Earth's surface to fluctuations in the planet's normally stable magnetic field. These fluctuations induce electric fields in the Earth that create potential differences in voltage between grounding points—which causes GICs to flow through transformers, power system lines, and grounding points. Only a few amps are needed to disrupt transformer operation, but over 100 amps have been measured in the grounding connections of transformers in affected areas. [from American Geophysical Union]
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