A Comprehensive Study of Sustained >100 MeV Gamma-Ray Emissions Observed by Fermi and their Association with High-Energy Solar Eruptive Events (Invited)

The Fermi Large Area Telescope (LAT) has detected more than twenty >100 MeV sustained gamma-ray events that appear to begin minutes to tens of minutes after impulsive flares visible on the solar disk and that can last for several hours. Three of these events have been discussed in detail by the Fermi LAT Collaboration in papers submitted for publication. High-energy events with such characteristics have been observed on earlier satellites and were given the name Long Duration Gamma Ray Flares (LDGRFs) and they were all associated with X-class soft X-ray flares. In contrast, at least 50% of the LAT events were from M-Class flares. LDGRFs are likely to originate from interactions deep in the solar atmosphere from precipitating protons stored and further accelerated in large magnetic loops, protons accelerated in shocks associated with fast coronal mass ejections or perhaps in a reconnection current sheet behind a coronal mass ejections (CME). We discuss a comprehensive study of the Fermi events beginning with a list of 101 solar eruptive events (SEEs) from 2008 to 2012 June with broad/fast (>800 km/s) or >100 keV hard X-ray emission or SEPs with >10 MeV proton fluxes above 1 proton flux unit. Our study indicates the following preliminary characteristics of LAT LDGRFs: none were detected from the 33 broad/fast CMEs originating from active regions behind the disk, none were detected in 21 events with broad/fast CMEs when hard X-ray emission did not exceed 100 keV, 12 were detected in 19 events with broad/fast CMEs and hard X-ray emission >100 keV, and of these twelve 10 were clearly associated with SEPs. From these associations one might conclude that 1) the LDGRFs are located near the active region, 2) the presence of a broad/fast CME and >100 keV hard X-ray flare emission is a necessary condition for an LDGRF, but perhaps not a sufficient condition (due perhaps to duty cycle and directionality). It is interesting to speculate that the requirement for >100 keV hard X-ray emission indicates the presence of a seed population of sub-MeV to MeV ions at the flare site. We also discuss timing and spectroscopy of individual events and estimate the numbers of >200 MeV protons at the Sun during the impulsive flare and the sustained emission phase, and the number in the accompanying SEP. This work is funded by the NSF/SHINE program and NASA's Fermi/GI program.