Cosmic Rays on the Pacific Ocean

Abstract

Records of cosmic-ray intensity obtained on the R. M. S. Aorangi during 12 voyages between Vancouver, Canada and Sydney, Australia, from March 17, 1936, to January 18, 1937, using a Carnegie model C cosmic-ray meter, are described and discussed. Typical records exhibiting the latitude effect are shown. A summary of the data taken at sea is given in the form of graphs, in which each datum point represents the average of 6 hours readings with a probable statistical error of ±0.13 percent. Any variations due to a possible temperature coefficient of the instrument are negligible. The observed minimum of cosmic-ray intensity near the equator averages 10.3 percent less than the intensity at Vancouver, in satisfactory agreement, considering the difference in expermental conditions, with earlier measurements. The critical latitudes above which changes in intensity are less rapid, are found to be somewhat lower, 38.4°N and 34.2°S, than previously reported, and beyond these latitudes the intensity is found to continue to increase with latitude. At the higher latitudes is observed a variation, which appears to be seasonal, with the maximum in the cold months in both hemispheres. This variation is closely correlated with the atmospheric temperature. It is hence ascribed to changes in some atmospheric barrier of unknown nature, such as perhaps an atmospheric potential gradient, of whose strength the temperature of the atmosphere is an approximate but not exact index. Changes in this atmospheric barrier have been approximately allowed for by determining the external temperature coefficient and correcting the observations accordingly (this external temperature coefficient is comparable with that reported by Hess and his collaborators). The latitude effect curves as thus corrected should show the effect of the earth's magnetic field alone. They are now nearly flat beyond the critical latitudes and show a magnetic latitude effect of about 7.2 percent. This implies that a latitude effect of about 3.1 percent owes its origin to the atmospheric barrier. Seasonal variations in the corrected latitude effect curve are almost eliminated. Geomagnetic analysis of the energy distribution of the rays indicates a prominent component with a sharp energy threshold of about 7.5×${10}^9$ ev, and a component so weak as to be questionable, whose energy threshold is not greater than 2.5×${10}^9$ ev. It is not found possible to explain the 7.5×${10}^9$ ev threshold in terms of atmospheric absorption as has previously been supposed. Two alternative interpretations are suggested. The difference in cosmic-ray intensity between the northern and southern hemispheres under comparable conditions, as calculated from these data in various ways, appears to be no larger than the probable error of about ±0.1 percent. This result is in conflict with the prediction by Compton and Getting of an excess in the north of about 0.5 percent, due to the motion of the earth with the rotation of the galaxy, but is not inconsistent with the small diurnal variation that has been found to follow sidereal time, if it is supposed that the cosmic rays acquire a part of the galactic motion.