Primary Cosmic-Ray Proton and Alpha-Particle Intensities and Their Variation with Time

Abstract

A series of high-altitude balloon flights was carried out in 1957 and 1958 to study the flux of primary cosmic-ray protons and $\alpha$ particles during variations in the total cosmic-ray intensity. The following results are obtained for $\alpha$ particles with energies exceeding 530 Mev/nucleon under 13.5 g/${\mathrm{cm}}^2$ of air: (a) During a large Forbush-type decrease the $\alpha$-particle and proton intensities were closely correlated. This demonstrates that a modulation mechanism is operating on both components. (b) At certain times variations in the $\alpha$-particle intensity were observed within a few hours which were not accompanied by corresponding changes in the proton flux. This is tentatively ascribed to an anisotropy in the $\alpha$-particle flux that reaches the earth. (c) While there existed an intensity decrease in the proton flux between 1957 and 1958 which is also observed in the neutron monitor station data, no such variation occurred in the $\alpha$-particle flux. A division of the $\alpha$ particles into two energy groups ($450 \mathrm{Mev}/\mathrm{nucleon}<~E_1<~960 \mathrm{Mev}/\mathrm{nucleon}$ and $E_2>~960$ Mev/nucleon) shows (a) that the Forbush decrease is of the same magnitude in both energy groups, (b) that the hourly flux increase observed in some flights is about the same in both energy groups, and (c) that from 1957 to 1958 the flux in the low-energy group increased, while it decreased in the high-energy interval, contrary to the well-known behavior of the proton flux. These independent $\alpha$-particle flux variations cannot be explained by any of the modulation mechanisms so far proposed. We suggest that occasional solar production of $\alpha$ particles may be responsible for our results.