What Makes Long Duration Gamma Ray Flares Do What They Do?

The phenomenon of Long Duration Gamma Ray Flares (LDGRF) is exciting, but not easy to understand. The four main characteristics are great photon energies; prolonged, but also delayed, emission; the absence of other flare-like signatures and the heavily, if not complete, relativistic hadronic nature of the emission. Three widely varying and mutually exclusive concepts exist to explain these events. They include: protons or ions accelerated by CME shocks, precipitating back to the Sun; large scale, coherent electric fields in current sheets; and quasi-static trapping of protons in the high corona accelerated by the second-order Fermi process. It is difficult, if not impossible, to discriminate among these very different concepts, because of none of this activity takes place near 1 AU and the impracticality of in situ GeV measurements of anything in the inner heliosphere. All our observations and measurements are remote and will be so for the foreseeable future. We attempt here to critically review each of these concepts in terms of physics necessary to make each of them happen with some predictability, some negative examples and telltale subtleties of the data, while seeking out the simplest model, i.e., Occams Razor. We find that none of these concepts is currently bulletproof, but some have near fatal flaws or numerous smaller issues, but again, we lack the measurements to put the last nail in the coffin. It is likely that a preponderance of evidence will ultimately prevail for one model or another, although complementary measurements will help resolve this dilemma.