The energy spectra of low-energy primary cosmic-ray hydrogen and helium nuclei have been determined from nuclear emulsions flown on a high-altitude balloon launched from Forth Churchill on 8 July 1961. The flight was preceded by a period of three months of low solar and geophysical activity. The results indicated that the differential spectrum of hydrogen nuclei in this low-energy interval decreased toward low energies and did not exhibit the sharp rise found with counters in 1960 and 1961 by other experimenters. The low-energy differential spectrum and integral flux value for helium nuclei were found to be intermediate between those typical of solar minimum and solar maximum and are consistent with a modulation mechanism which yields the same rigidity spectrum for a given intensity both before and after solar maximum. Various modulation models of galactic cosmic rays are examined, and it is concluded that none of them seem to be entirely adequate.

• Received 30 August 1963

DOI:https://doi.org/10.1103/PhysRev.133.B818