The Standard Bearer LDGRF on 1991 June 11

Although not the first of its kind observed, the X12+ flare at 0200UT on 1991 June 11 was the event that spawned wide interest in Long Duration Gamma Ray Flares (LDGRF). This LDGRF class of events is unique because of its wide range of gamma-ray emission from 50 keV to above 1 GeV over protracted periods of time. This event is one of three, observed by all four instruments on CGRO, where the time profile and the spectrum of high-energy gamma rays and neutrons were both measured over a wide energy range. Neutron measurements should be unusually revealing in that the threshold for fast neutron production is 10 MeV rather than 300 MeV for pions—the source of the gamma rays. The 9-100 MeV neutron emission (when corrected for velocity dispersion) significantly contrasts with that of the >100 MeV gamma rays. The neutron emission onset shows no delay with respect to the impulsive phase, whereas the ten-minute delay in the gamma emission has been widely interpreted to be the effect of setting up a coronal shock that accelerates protons that then precipitate back to the Sun to radiate. The broad band, uninterrupted emission profiles we present belie the concept that the high-energy phase of the flare derives solely from the CME shock. Rather, the continuous energetic neutron emissions conforms to a picture of continuous ion acceleration, transport and precipitation within a large coronal loop (Ryan and Lee 1991), with the gamma onset delay being due to high threshold for pion production. Given these neutron measurement and the difficulties inherent in robustly explaining proton precipitation from a CME shock great distances from the Sun (Hutchinson et al. 2020), we rightly turn our attention to quantitatively modeling how the phenomenon occurs in this static or quasi-static magnetic environment. These measurements point to the importance of simultaneous broad band measurements of gamma rays and neutrons from 1 MeV to above 1 GeV, an overlooked hallmark of the Compton Observatory mission.