How strong was the Carrington Event?

by Jay R. Thompson
Wednesday, January 2, 2013

NASA has described the magnetic field of the coronal mass ejection (CME) associated with the 1859 Carrington Event, the largest solar storm ever detected, as “extremely intense” relative to other CMEs. The amount of plasma it ejected is difficult to estimate, but more important in determining its strength is how fast it arrived.

CMEs generally take a few days to travel from sun to Earth. Some, called fast-transit CMEs, make the trip in 20 hours or less, which can increase the intensity of the resulting geomagnetic storm. Several studies suggest that the CME associated with the Carrington Event reached Earth just 17.6 hours after it was ejected from the sun, second in speed to only one other known event, a 1972 CME that reached Earth in only 14.6 hours.

Perhaps more telling, however, is how a geomagnetic storm rates on the “disturbance storm time index.” Expressed in nano¬≠teslas, the index describes magnetic fluctuations along the equatorial plane of Earth’s magnetosphere, a region called the “ring current.” Those magnetic fluctuations are caused by changes in the flow and volume of charged particles trapped in Earth’s magnetic field lines. During geomagnetic storms, fluctuations are higher than normal, and as magnetic field lines move, they induce electric currents at Earth’s surface, including in power grids.

Various studies have estimated the disturbance index of the Carrington Event at approximately minus 1,600 to minus 1,760 nanoteslas, around three times lower (and therefore more intense) than the March 1989 geomagnetic storm that knocked out Quebec’s power grid and left 6 million people without electricity for 12 hours.

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