Cosmic Rays in Space

The energetic particle populations observed in the MeV and greater range in interplanetary space are of both galactic and solar origin. The galactic cosmic rays are ever present though strongly modulated by solar activity while the solar cosmic rays are highly variable. Of particular interest in attempting to assess the radiation hazards for long missions is a more detailed knowledge of the charge and energy spectrum of the galactic component. Measurements of hydrogen, helium, and heavy nuclei down to 100 MeV/nuc have been obtained by groups at the University of Minnesota, the University of Chicago, and the Goddard Space Flight Center using balloon borne instrumentations at high altitudes. Results obtained on the IMP-1 satellite have furnished proton and helium data in the region 10 to 100 MeV. These combined data show a differential proton spectrum which has a broad maximum around 400 MeV and is steeply falling towards lower energies. The helium nuclei and protons seem to exhibit the same relative differential rigidity spectrum above 1.5 BeV. Below this rigidity the relative helium spectrum appears to be more rapidly falling than the proton spectrum. The IMP-1 proton results indicate that the galactic proton differential energy spectrum decreases by a factor of 5 going from 70 MeV to 20 MeV. Thus less than 1% of the total galactic cosmic rays appears to be contained in the low energy region below 100 MeV.