Spectra and solar energetic protons over 20 GeV in Bastille Day event

Solar energetic particles (SEPs) from large solar flares give important information about the physical process in the solar corona and the heliosphere. Several observations have indicated that solar protons could sometimes be accelerated to at least tens of GeV, even hundreds of GeV, in intense solar energetic process. We studied the solar proton differential energy spectra with energy range of 1–500 MeV at several time intervals during Bastille Day event. It was shown that the spectra could be fitted by a power law function before flare and after flare the power law spectra still existed above 30 MeV although spectra became softer with time. There was a spectral “knee” occurring at ∼30 MeV. We constructed a solar proton differential spectrum from 30 MeV to 3 GeV at peak flux time 10:30 UT and fitted it in the same manner. On the basis of a supposition of having the same power law spectrum in higher energy, we calculated the solar proton integrated fluxes in energy range of from 500 MeV to 20 GeV and compared them with other results obtained from experimental, modelling and theoretical calculations in other big historic SEP events. A Monte Carlo simulation was carried out for a primary proton beam at the top of the atmosphere producing secondary muons on the ground. Based on the simulation, possibility of registering the solar energetic proton beams with energies over 20 GeV was discussed.