HIGH-ENERGY GAMMA-RAY EMISSION FROM SOLAR FLARES: SUMMARY OF FERMI LARGE AREA TELESCOPE DETECTIONS AND ANALYSIS OF TWO M-CLASS FLARES

We present the detections of 18 solar flaresdetected in high-energy gamma-rays(above 100 MeV) with the Fermi Large Area Telescope (LAT) during its first 4 yr of operation. This work suggests that particle accelerationup to very high energies in solar flaresis more common than previously thought, occurring even in modest flares, and for longer durations. Interestingly, all these flaresare associated with fairly fast coronal mass ejections(CMEs). We then describe the detailed temporal, spatial, and spectral characteristics of the first two long-lasting events: the 2011 March 7 flare, a moderate (M3.7) impulsive flarefollowed by slowly varying gamma-rayemission over 13 hr, and the 2011 June 7 M2.5 flare, which was followed by gamma-rayemission lasting for 2 hr. We compare the Fermi LAT data with X-ray and proton data measurements from GOES and RHESSI. We argue that the gamma-raysare more likely produced through pion decay than electron bremsstrahlung, and we find that the energy spectrum of the proton distribution softens during the extended emission of the 2011 March 7 flare. This would disfavor a trapping scenario for particles acceleratedduring the impulsive phase of the flareand point to a continuous acceleration process at play for the duration of the flares. CME shocks are known for accelerating the solar energetic particles(SEPs) observed in situ on similar timescales, but it might be challenging to explain the production of gamma-raysat the surface of the Sun while the CME is halfway to the Earth. A stochastic turbulence acceleration process occurring in the solar corona is another likely scenario. Detailed comparison of characteristics of SEPs and gamma-ray-emitting particles for several flareswill be helpful to distinguish between these two possibilities.