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The Fermi Large Area Telescope (LAT) observed two bright X-class solar flareson 2012 March 7, and detected gamma-rays up to 4 GeV. We detected gamma-raysboth during the impulsive and temporally-extended emission phases, withemission above 100 MeV lasting for approximately 20 hours. Accuratelocalization of the gamma-ray production site(s) coincide with the solar activeregion from which X-ray emissions associated with these flares originated. Ouranalysis of the >100 MeV gamma-ray emission shows a relatively rapid monotonicdecrease in flux during the first hour of the impulsive phase, and a muchslower, almost monotonic decrease in flux for the next 20 hours. The spectracan be adequately described by a power law with a high energy exponentialcutoff, or as resulting from the decay of neutral pions produced by acceleratedprotons and ions with an isotropic power-law energy distribution. The requiredproton spectrum has a number index ~3, with minor variations during theimpulsive phase, while during the temporally extended phase the spectrumsoftens monotonically, starting with index ~4. The >30 MeV proton flux andspectra observed near the Earth by the GOES satellites also show a monotonicflux decrease and spectral softening during the extended phase, but with aharder spectrum, with index ~3. Based on the Fermi-LAT and GOES observations ofthe flux and spectral evolution of these bright flares, we explore the relativemerits of prompt and continuous acceleration scenarios, hadronic and leptonicemission processes, and acceleration at the solar corona by the fast CoronalMass Ejections (CME) as explanations for the observations. We conclude that themost likely scenario is continuous acceleration of protons in the solar coronawhich penetrate the lower solar atmosphere and produce pions that decay intogamma-rays.

Impulsive and Long Duration High-Energy Gamma-ray Emission From the Very Bright 2012 March 7 Solar Flares