Long Period Variations of Cosmic Rays

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Abstract
With the data from the Carnegie Institution's Model C cosmic-ray meters at four widely separated stations, a study of yearly variations and of a variation of cosmic-ray intensity with a 28-day period, apparently connected with the sun's rotation, has been made. The amplitude of the first annual harmonic with a maximum in colder months varies from 2.15 ±0.06 percent at Cheltenham, 38°.7 N to 0.15±0.03 percent at Huancayo, 12°.05 S. The amplitude of a 27.9-day period is 0.18 percent. Values of the atmospheric temperature coefficient at the different stations are given. The results give some support to Blackett's theory that the annual variation is due to changes in elevation of the barytron producing layer with the thermal expansion of the atmosphere. No other essential relationship between observed annual variation of cosmic-ray intensity and meteorological or astronomical phenomena has shown itself.

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