  Little is known about the origin of the high-energy and sustained emission
from solar Long-Duration Gamma-Ray Flares (LDGRFs), identified with the Compton
Gamma Ray Observatory (CGRO), the Solar Maximum Mission (SMM), and now Fermi.
Though Fermi/Large Area Space Telescope (LAT) has identified dozens of flares
with LDGRF signature, the nature of this phenomenon has been a challenge to
explain both due to the extreme energies and long durations. The highest-energy
emission has generally been attributed to pion production from the interaction
of >300 MeV protons with the ambient matter. The extended duration suggests
that particle acceleration occurs over large volumes extending high in the
corona, either from stochastic acceleration within large coronal loops or from
back precipitation from coronal mass ejection driven shocks. It is possible to
test these models by making direct comparison between the properties of the
accelerated ion population producing the gamma-ray emission derived from the
Fermi/LAT observations, and the characteristics of solar energetic particles
(SEPs) measured by the Payload for Matter-Antimatter Exploration and Light
Nuclei Astrophysics (PAMELA) spacecraft in the energy range corresponding to
the pion-related emission detected with Fermi. For fourteen of these events we
compare the two populations -- SEPs in space and the interacting particles at
the Sun -- and discuss the implications in terms of potential sources. Our
analysis shows that the two proton numbers are poorly correlated, with their
ratio spanning more than five orders of magnitude, suggesting that the back
precipitation of shock-acceleration particles is unlikely the source of the
LDGRF emission.
