Theoretical Interpretation of Observations by Fermi-LAT and Other Instruments of Solar Flares

The Fermi Large Area Telescope (LAT) is the most sensitive instrument ever deployed in space for observing high-energy gamma-rays. During the past active period of the Sun the LAT has detected more than 20 flares up to GeV energies, not all of which are GOES X-class events. A few of these show impulsive phase emission (e.g. 2010 June 12), but most have a sustained emission lasting from a few to 20 hours (e.g. March 7-8. 2011 and March 8, 2012). The long duration and relatively hard spectrum of the Fermi-LAT gamma-rays pose a challenge to theoretical modeling. We will review the observations of these flares by Fermi and other instruments, notably RHESSI, GOES and SDO. Interestingly all LAT flares are associated with fairly fast coronal mass ejections and solar energetic particles. The SEP spectra combined with the electromagnetic signature can shed considerable light on the acceleration, transport and radiative processes. We will explore the possibility of electron bremsstrahlung vs pion decay emission, stochastic acceleration in corona vs acceleration at the CME shock and issues related to the transport from the acceleration site to radiation region deep in chromosphere. This paper is presented on behalf of Fermi Collaboration