| Summary: | The experiments herein described support and extend the relationship found by Forbush, namely, that there is an inverse relationship between solar activity and cosmic-ray intensity. During the summer of 1954 the sun was at a minimum of activity. Ionization chambers set up in the Arctic as well as at intermediate latitudes showed a considerable increase in intensity, during the period, compared with 1951. Atmospheric absorption combined with magnetic rigidity requirements indicate that protons with energies down to at least 150 Mev were coming to the earth in the summer of 1954. We conclude that there was no cutoff of primary particles, i.e., no "knee," at least for protons down to this energy. Using the increased areas under the ionization-depth curves, combined with geomagnetic calculations, an estimate may be made of the numbers of primary particles coming in near the poles in 1937 (solar maximum), 1951 and 1954 (solar minimum). These values are 0.10, 0.14, and 0.24 cm-2 sec-1 sterad-1, respectively. The fluctuations in 1954 were also small compared with those measured in 1937, 1938, and 1951. The evidence points to low intensity and large fluctuations of cosmic rays when the sun is active and vice versa. These changes may be understood qualitatively by assuming a modulating mechanism in the form of clouds ejected from the sun and varying with the solar cycle. ©1956 The American Physical Society. Received 2 March 1956. I would like to express my appreciation to Dr. Edward Stern, Mr. Alan Johnston, and Mr. Robert Morris for assistance in making the flights and in reducing the data. The cooperation of the U.S. Weather Bureau, both at Bismarck and Washington, D.C. is gratefully acknowledged. The very fine assistance of the Office of Naval Research in making arrangements for the expeditions and in helping coordinate the flights is also appreciated. I wish also to thank Commander Jacobsen of the U.S.S. Atka, Captain G.H. Bowerman of the Coast Guard Cutter Eastwind and their officers and men for their cooperation ...
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