Evidence for the prolonged existence of >100 MeV solar protons in coronal structures

Solar proton events (SPEs) of 1989–1992 accompanied by ground-level enhancements of the cosmic-ray intensity are analyzed. A diffusive-propagation model can well approximate the solar proton intensity at energies of 84–200 MeV measured on board the GOES-7 satellite during the first 15–20 hours of the development of an SPE, assuming several episodes of proton injection into interplanetary space over some hours. The detection of π⁰-decay gamma-ray emission during the flares of June 11 and 15, 1991 on board the CGRO and GAMMA-1 space observatories is the sole experimental evidence for the existence of protons with energies >100 MeV near the Sun over such extended times. The number of protons in the source is estimated for various times based on the propagation model. Moreover, this value is in qualitative and quantitative agreement with the number of protons required to generate the observed gamma-ray emission for the June 15, 1991 event (and in qualitative agreement for the June 11, 1991 event). This provides evidence that processes of prolonged trapping and/or acceleration of protons in coronal structures may be possible in other such events.