Spatial and Temporal Features of 0.64-35 MeV Protons in the Space Station Environment: EXOS-C Observations

The spatial and temporal features of the peak proton flux detected near the equator in the integral energy channel of 0.64-35MeV by S-1 telescope on board the EXOS-C mission were studied for a period of three years (1984-86). The equatorial zone (±30° geomagnetic latitude) proton data obtained for the angle between the telescope axis and the magnetic field direction in the range of 50°-130°and at the peak efficiency of the instrument, were sorted out from a study of the orientation of the telescope axis and calculation of the response function of the telescope. The data, after freed from the major influence of the South Atlantic Anomaly, and subsequently analyzed, revealed some new results contrary to the earlier observation (Moritz, 1972) in the altitude range of 350 to 850km. The spatial distribution shows that the global profile of the peak proton flux is aligned with the profile of the minimum magnetic field, and, the peak proton population straddles the minimum magnetic field equator with a full-width-at-half-maximum of about 19°. Also, the pitch angle distribution of the equatorial proton population is found to have the anisotropy index in the range 7 to 10. Further, the proton flux shows altitude, L, and longitude dependences. The flux increases as h^1.43 and L^4.25-7.50 (corresponding to different equatorial pitch angles). Besides, in the longitude range 180°<Φ<300°, the flux is significantly larger than that in the range 100°<Φ<180°. The fact of L independence of low energy (<5MeV) protons, and the strong L dependence (L⁸¹) of high energy (>5MeV) protons, has been used to infer the presence of a major low energy component and a minor high energy component. In the study of temporal features, comparison of absolute fluxes during two solar epochs shows that the proton flux during the solar minimum in 1984-86 was, at least, 40 times less than that during the solar maximum condition in 1982, possibly, due to lesser amount of hydrogen escape to the exosphere during the minimum condition, and thereby causing less precipitation of low energy particles in the equatorial atmosphere.