Changes in the Low-Energy Particle Cutoff and Primary Spectrum of Cosmic Radiation

Abstract

The low-rigidity cutoff for particles in the primary cosmic-ray spectrum has decreased within the period 1948 through 1951. This decrease corresponds to a 3\ifmmode^\circ\else\textdegree\fi{} change (northward) in the "knee" position of the geomagnetic latitude curve for the nucleonic component. The phenomenon is accompanied by both a change in the primary spectrum for particle rigidities less than approximately 4 Bv and by an increase in total primary intensity. The spectral change is such that if the differential primary intensity, $j$, at low rigidities in 1948 was $j=C{(\frac{p}{z})}^{\ensuremath{-}2}$; then the new spectrum for 1951 through 1954 is approximately $j\ensuremath{\approx}{C}^{\ensuremath{'}}{(\frac{p}{z})}^{\ensuremath{-}2.7}$. The total change of intensity arising from the changes in spectrum and low-rigidity cutoff is more than 13%.The measurements were obtained in 1948 and 1951, and have been confirmed and extended in November, 1954 using nucleonic component detectors. The vertical charged particle intensity was also measured and displays changes equivalent to those reported for the nucleonic component.Three regions of space are considered for the location of the mechanism producing the low energy cutoff; namely, the vicinity of the earth, the region of the solar system, and regions of the galaxy outside our solar system. Although the general solar dipole moment is at least an order of magnitude too small to account for the observations, it is concluded that the mechanism is operative within the solar system and is not a terrestrial phenomenon.