Cosmic-ray changes near the north geomagnetic pole from 1954 to 1956

Abstract

— Since the summer of 1954, when the sun was at a very low state of activity, the intensity of cosmic rays at high altitudes near the poles has been decreasing in accordance with Forbush’s relationship. The decrease in the numbers of primaries from the summer of 1954 to 1955 was only a few percent, but the decrease was approximately a factor of two between 1955 and 1956. At the middle altitudes the intensity was even less in 1956 than in 1937 when the sun was at a maximum of activity. However the ionization at balloon altitudes did not pass through a maximum in 1956 as it did in 1937. The shape of the ionization-depth curve at atmospheric pressures of 10 to 100g cmp-2 is quite sensitive to the relative numbers of low-energy particles in the primaries. If protons, such particles have energies in the range 100 to 500 MeV. By comparing the shapes of such curves taken near the geomagnetic poles at different times, the presence or absence of a « knee » at very high altitudes may be predicted for it is just the absence or presence of such low-energy particles that causes the presence or absence of the « knee ». The relative numbers of low-energy (100 to 500 MeV protons) particles to those of higher energy do not always bear the same relationship to each other. Thus, on some days the numbers of low-energy particles will be higher than on some other days, but the numbers of those of higher energy may be less. The reverse situation may also be true. As to the nature of the primaries that change so radically with time, there is some evidence that they may be almost all protons. Thus, the latitude effect at high altitudes is certainly consistent with this assumption. Also, the experiments of the Minnesota group show an absence of low-energy α-particles even during the low period of solar activity in 1954.