Angular Distribution of Protons Measured by the Energetic Particle Telescope on PROBA-V

Abstract

Angular distribution and contamination of proton spectra measured at LEO are considered as possible sources of discrepancies between fluxes obtained by different instruments. In particular, not accounted for pitch angle distribution and East/West asymmetry of energetic proton fluxes have been suspected of leading to the reported underestimates of these fluxes by the NASA Model AP8. The Energetic Particle Telescope (EPT) was designed as a science-class instrument aimed at providing uncontaminated fluxes of electrons (0.5 – 20 MeV), protons (9.5 – 300 MeV) and α-particles (38 – 1200 MeV) getting into the instrument from within a well-defined Field Of View (FOV). The PROBA-V satellite with EPT was launched on May 7th, 2013 on a LEO, 820 km altitude, 98.7° inclination and a 10:30 – 11:30 Local Time at Descending Node. Based on the data acquired by the EPT on board PROBA-V, we account for flux angular distribution effects to provide a definitive reply to the basic question: “does AP8 underestimate E > 100 MeV proton fluxes around B/B0 = 1.1, L = 1.3”?