Origin of the high energy proton component below the geomagnetic cutoff in near earth orbit

The high flux proton component observed by AMS below the geomagnetic cutoff can be well accounted for by assuming these particles to be secondaries originating from the interaction of cosmic ray protons with the atmosphere. Simulation results are reported. The existence of a high flux proton component below the earth geomagnetic cutoff (GC), was reported recently by the AMS collaboration [1]. The observed spectrum has an intensity about equal to the cosmic primary proton flux above GC. It has a maximum at low momenta and extends from the momentum threshold of the spectrometer at ≈0.4 GeV/c, up to the GC around 10 GeV/c in the equatorial region, where the high momentum tail merges with the primary proton spectrum above GC. It has been demonstrated long ago that such subGC component cannot be part of the primary proton flux [2, 3]. It has to be a secondary product of the primary cosmic ray (CR) flux on earth. This note reports on an attempt both to identify which part of the incident CR flux on earth generates this spectrum and by which mechanism, and to investigate its dynamical status with respect to the trapping process observed for other circum-terrestrial populations of particles. A similar phenomenon was observed previously with the very high intensity low energy proton flux measured in the Van Allen belts, at kinetic energies typically below 100 MeV and for distances typically beyond 1.2 earth radius, which was interpreted as originating from secondary neutrons produced on atmospheric nuclei by CR protons [4]. Many low energy neutrons are produced by nuclear evaporation, or spallation, or preequilibrium nuclear emission, which can constitute the source term driving the equilibrium of this population. The situation is quite different for the population of higher momentum protons, beyond 0.5 GeV/c, observed here. It is too high in energy to be accounted for by the above quoted mechanism, neither could it be by neutrons from the direct charge exchange reaction pn → np, induced by high energy CR protons of nuclei, because of highly unfavored kinematics and small cross sections of the diffractive production mechanism, and very small probability of neutron decay below the altitude of AMS. The two most significant sources of secondary protons over the observed energy range are the p + A and 4 He+A collisions, A standing for atmospheric nuclei, the CR flux of higher nuclear masses being too small to contribute significantly. …