Cosmic rays at balloon altitudes and the solar cycle

Abstract

Measurements of the cosmic-ray intensity have now been made for most of the past solar cycle at balloon altitudes near the north geomagnetic pole. Such experiments have been conducted during the summer months, but these data are thought to represent a relatively true measure of cosmic rays in the vicinity of the earth for that particular phase of the solar cycle, since large solar flare activity was absent when these flights were made. It is found (1) that the energy density of cosmic rays in the vicinity of the earth was 40 per cent less in 1958–1959 than in 1954; (2) that the new particles present in 1954 that were not present in 1951 were almost entirely those absorbed in 200 g cm−2 of the atmosphere; (3) that after the cosmic-ray maximum of 1954 the first particles to disappear were very absorbable, having an absorption length of about 60 g cm−2; (4) that as the solar cycle progressed the particles removed became increasingly energetic, until those that disappeared between 1957 and 1958 were absorbed about like primaries of mean rigidity 9 bv; (5) that the particles now reappearing after the cosmic-ray minimum of 1958–1959 have close to the same rigidity as those removed before the cosmic-ray minimum was reached, when the intensity was approximately the same; (6) that, if the general solar activity can be measured by the Zurich sunspot numbers, there appears to be a 9- to 12-month lag of cosmic–ray change from being anticorrelated with solar activity; and (7) that sunspot numbers averaged over periods like a quarter of a year appear to be a better measure of solar activity as it relates to cosmic-ray intensity than similarly averaged planetary magnetic character figures.