  We present the detections of 19 solar flares detected in high-energy gamma
rays (above 100 MeV) with the Fermi Large Area Telescope (LAT) during its first
four years of operation. Interestingly, all flares are associated with fairly
fast Coronal Mass Ejections (CMEs) and are not all powerful X-ray flares. 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 flare followed by slowly varying gamma-ray emission over 13 hours,
and the 2011 June 7 M2.5 flare, which was followed by gamma-ray emission
lasting for 2 hours. We compare the Fermi-LAT data with X-ray and proton data
measurements from GOES and RHESSI. We argue that a hadronic origin of the gamma
rays is more likely than a leptonic origin and find that the energy spectrum of
the proton distribution softens after the 2011 March 7 flare, favoring a
scenario with continuous acceleration at the flare site. This work suggests
that proton acceleration in solar flares is more common than previously
thought, occurring for even modest X-ray flares, and for longer durations.
