We characterize and catalog 30 solar eruptive events observed by the Fermi Large Area Telescope (LAT) having late-phase >100 MeV γ-ray emission (LPGRE), identified 30 yr ago in what were called long-duration gamma-ray flares. We show that LPGRE is temporally and spectrally distinct from impulsive phase emission in these events. The spectra are consistent with the decay of pions produced by >300 MeV protons and are not consistent with primary electron bremsstrahlung. Impulsive >100 keV X-ray emission was observed in all 27 LPGRE events where observations were made. All but two of the LPGRE events were accompanied by a fast and broad coronal mass ejection (CME). The LPGRE start times range from CME onset to 2 hr later. Their durations range from ∼0.1 to 20 hr and appear to be correlated with durations of >100 MeV solar energetic particle (SEP) proton events. The power-law spectral indices of the >300 MeV protons producing LPGRE range from ∼2.5 to 6.5 and vary during some events. Combined γ-ray line and LAT measurements indicate that LPGRE proton spectra are steeper above 300 MeV than they are below 300 MeV. The number of LPGRE protons >500 MeV is typically about 10× the number in the impulsive phase of the solar eruptive event and ranges in nine events from ∼0.01× to 0.5× the number in the accompanying SEP event, with large systematic uncertainty. What appears to be late-phase electron bremsstrahlung with energies up to ∼10 MeV was observed in one LPGRE event. We discuss how current models of LPGRE may explain these characteristics.

An NM-64 neutron monitor latitude survey made by road transport in the summer of 1965 in Canada, the United States, and Mexico was extended, in the summer of 1966, to the western seaboard of the United States and Hawaii. In 1966 in the vicinity of Mt. Hood (2.43 GV), Palomar Mt. (5.71 GV), and Mt. Haleakela (13.3 GV) advantage was taken of the possibility of changing altitude without significant change of geomagnetic cutoff. At each of these places also a smaller lead polyethylene neutron monitor was flown at seven different pressure altitudes between 530 mm Hg and 140 mm Hg. The airborne monitor was calibrated in terms of the NM-64 when the transport van was at 10 000 ft on the summit of Haleakela. It was found that graphical smoothing of vertical trajectory cutoffs in latitude and longitude eliminated an unnatural kink in the latitude curve near Mexico City. The 1965 and 1966 rates were adjusted to May 12–13, 1965, the date of the IQSY maximum, using data from a fixed monitor and allowing for the dependence of the secular fluctuations on cutoff and altitude. The attenuation coefficient throughout the atmosphere was determined from the data and also a new specific attenuation coefficient which is a function only of rigidity of the primary flux and depth in the atmosphere. The specific attenuation coefficient has a comparatively high value consistent with that observed for solar proton events.

A 3-NM-64 neutron monitor and a 2-MT-64 muon monitor were operated at 29 sites near sea level and on mountains on the western seaboard of the USA and in Hawaii in May, June, and July, 1966, in continuation of the latitude survey begun in 1965 and reported in papers I and II of this set of five papers. The original results and also the corrections for temperature structure of the atmosphere and for secular variations of the cosmic radiation are given in detail. While the overland equipment was at its highest altitude on Mt. Hood (2.4 GV) and on the summits of Mt. Palomar (5.7 GV) and Mt. Haleakela (13.3 GV), an airborne neutron monitor was operated at seven different levels between 3000 m and 12 000 m. The pressure-measuring equipment and also the neutron monitor in the aircraft were calibrated in terms of the overland instruments while the aircraft was at the same altitude as the overland equipment on the summit of Mt. Haleakela.

No abstract