  High-energy solar flares have shown to have at least two distinct phases:
prompt-impulsive and delayed-gradual. Identifying the mechanism responsible for
accelerating the electrons and ions and the site at which it occurs during
these two phases is one of the outstanding questions in solar physics. Many
advances have been made over the past decade thanks to new observational data
and refined simulations that together help to shed light on this topic. For
example, the detection by Fermi Large Area Telescope (LAT) of GeV emission from
solar flares originating from behind the visible solar limb and >100 MeV
emission lasting for more than 20 hours have suggested the need for a spatially
extended source of acceleration during the delayed emission phase. In this work
we will review some of the major results from Fermi-LAT observations of the
24th solar cycle and how this new observational channel combined with
observations from across the electromagnetic spectrum can provide a unique
opportunity to diagnose the mechanisms of high-energy emission and particle
acceleration in solar flares.

Solar and Stellar Astrophysics;High Energy Astrophysical Phenomena