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Spectral variations of specific solar flare particle events are considered, and an estimate of the doses in free space for different shielding thicknesses for the events of the highly active period from May, 1959 to November, 1960 is given. The high-energy part of the spectra (E greater than 20 Mev), because of minimum amounts of shielding present and the relatively low intensities behind such shielding, is primarily important for its effects on man. With respect to effects on material and sensitive electronic devices, attention is given to low-energy protons in the Mev range, which have been observed since late 1960 to be present in appreciably higher intensity during proton events associated with large magnetic storms. Under the assumption that particles with E > 1 Mev arriving in such large numbers as in the November 12, 1960, and in July 12, 1961, events, are particles trapped in plasma clouds, and are a usual feature of solar events accompanied by large magnetic storms, fluxes between 4 × 1011 to 1012 protons per sq cm are obtained for the 1 1/2 year period between May, 1959 and November, 1960. These proton numbers and the doses are threshold doses for permanent or transient effects on the surface or in thin sheets of sensitive materials and devices. The intensities are of the same order of magnitude as the intensity of low-energy protons in the maxima of the belts. For long-term excursions during solar activity years these low-energy protons may have to be taken into account with respect to the selection of materials and protection of sensitive devices, especially if the spacecraft approaches the sun more closely.
Staff scientist National Aeronautics and Space Administration, Langley Research Center, Hampton, Va.