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In late October and early November 2003 scientists witnessed some of the largest solar flares ever recorded. These massive outpourings of charged particles were obvious on and near Earth--a full 150 million kilometers away from the source. For example, the barrage of particles reaching our neighborhood in space was at times so great that many scientific and communications satellites had to be temporarily shut down. A few suffered permanent damage. Astronauts on the International Space Station were endangered as well and had to take refuge in their facility's relatively well shielded service module. Closer to home, airliners were routed away from high latitudes, where pilots would have encountered problems with radio communications and passengers and crew could have been subjected to worrisome levels of radiation. Also, electrical grids had to be carefully monitored for surges. Despite those efforts, 50,000 residents in southern Sweden briefly lost power.
Fortunately, Earth's magnetic field and atmosphere protect the overwhelming majority of people from the ravages of even the worst solar storms. But society's increasing reliance on technology makes nearly everyone vulnerable to some extent [see "The Fury of Space Storms," by James L. Burch; Scientific American, April 2001]. The greatest potential for damage during a large flare comes from material shot rapidly off the sun's outer atmosphere--coronal mass ejections, in space physicist lingo. Some of these events send huge quantities of ionized gas on a collision course with Earth, as was the case for more than one of the exceptionally large flares that occurred in 2003.