  Large solar explosions are responsible for space weather that can impact
technological infrastructure on and around Earth. Here, we apply a
retrospective cohort exposure analysis to quantify the impacts of geomagnetic
activity on the U.S. electric power grid for the period from 1992 through 2010.
We find, with more than 3-sigma significance, that approximately 4% of the
disturbances in the U.S. power grid reported to the U.S. Department of Energy
are attributable to strong geomagnetic activity and its associated
geomagnetically induced currents.
