Penetration of solar protons to synchronous altitude

Solar proton fluxes in two energy intervals (5–21 Mev and 21–70 Mev) were measured with detectors aboard the geostationary ATS 1 satellite during the solar proton event that began on January 28, 1967. Comparison with data obtained by satellites outside the magnetosphere shows that (a) protons with energies greater than 21 Mev have essentially free access to synchronous altitude; (b) protons in the 5–21 Mev energy interval show a diurnal variation with the flux approaching the interplanetary flux level near local midnight. The transmission efficiency of the magnetosphere is not well organized by either the local (synchronous) magnetic field or the Kp index.