Cosmic Rays near the North Geomagnetic Pole in the Summers of 1955 and 1956

Abstract

The experiments herein described are a continuation of similar experiments performed in previous years near the north geomagnetic pole. In 1954 low-energy particles were found at high altitudes which had the characteristics of protons with energies down to 150 Mev. In 1955 flights to higher altitudes (4 g ${\mathrm{cm}}^{\ensuremath{-}2}$) indicated that these energies extended down to at least 100 Mev for protons. The numbers of both the high-energy particles ($E>1$ Bev) and those of low energy, in 1955 were less than in 1954. On one flight in 1955 the very low-energy particles were missing. In 1956, these low-energy particles (300 Mev for protons) were missing completely.The "shape" of the ionization-depth curves at high altitudes appears to be a sensitive index of the presence or absence of these low-energy particles. In 1956 the "shape" of the curves at low pressures (100 g ${\mathrm{cm}}^{\ensuremath{-}2}$) was similar to those of 1951 when the low-energy radiation was missing and when a "knee" was present in the latitude curve at high altidues. We infer that a "knee" was also present in 1956 but not in 1955.The fluctuations were also large in 1955 and 1956 compared with 1954. The average intensity of cosmic rays since 1954 has been decreasing with time as the sun approaches its maximum activity. This is in accordance with Forbush's relationship. However, in 1956 three of six flights showed intensities in the lower part of the atmosphere even less than any of the flights of 1937 at Saskatoon, Canada, in spite of the fact that the sun's activity in 1956 was less than in 1937.