The solar flare which erupted on March 7 was the most powerful eruption ever observed by Fermi's Large Area Telescope (LAT).
The flare, classified as X5.4, made the sun briefly the brightest object in the gamma-ray sky.
"For most of Fermi's four years in orbit, its LAT saw the sun as a faint, steady gamma-ray source thanks to the impacts of high-speed particles called cosmic rays," says Nicola Omodei, an astrophysicist at Stanford University in California. "Now we're beginning to see what the sun itself can do."
At the flare's peak, it was emitting gamma rays with two billion times the energy of visible light, or about four billion electron volts - easily setting a record for the highest-energy light ever detected during or immediately after a solar flare.
The flux of high-energy gamma rays, defined as those with energies beyond 100 million electron volts, was 1,000 times greater than the sun's usual output. And it lasted a long time, with high-energy gamma rays being recorded for about 20 hours, two and a half times longer than ever before.
Another strong, but less powerful solar flare, was observed on June 12, 2010.
"Seeing the rise and fall of this brief flare in both instruments allowed us to determine that some of these particles were accelerated to two-thirds of the speed of light in as little as three seconds," says Michael Briggs, a member of GBM team at the University of Alabama in Huntsville.
And we can expect to see more major flares over the next year or so as the sun progresses toward the peak of its 11-year cycle, now expected in mid-2013.
"Merged with available theoretical models, Fermi observations will give us the ability to reconstruct the energies and types of particles that interact with the sun during flares, an understanding that will open up whole new avenues in solar research," says Gerald Share, an astrophysicist at the University of Maryland in College Park.