Further Study of Cosmic Rays on the Pacific Ocean

Abstract

Records of cosmic-ray intensity obtained on the R. M. S. Aorangi during 15 new voyages between Vancouver, Canada, and Sydney, Australia, from July 28, 1937, to September 23, 1938, with a Carnegie model C cosmic-ray meter, are discussed and compared with records taken during 11 voyages on the same route previously reported by Compton and Turner. The observed minimum of cosmic-ray intensity near the equator averages 10.3 percent less than the intensity at Vancouver, in good agreement with the value given by Compton and Turner. The correlation between the cosmic-ray intensity and the atmospheric temperature is confirmed. An atmospheric temperature coefficient is found to be a function of latitude, with its highest numerical value of -0.25 percent per \ifmmode^\circ\else\textdegree\fi{}C for latitudes higher than 40\ifmmode^\circ\else\textdegree\fi{} (N and S). With this variable temperature coefficient a latitude effect (about 8.5 percent) of magnetic origin alone is found. The mean latitude effect curve for 25 trips, corrected for external temperature, is flat beyond the critical latitudes (about 40\ifmmode^\circ\else\textdegree\fi{}N and 38\ifmmode^\circ\else\textdegree\fi{}S). The difference in cosmic-ray intensity between the Northern and Southern Hemispheres beyond the critical latitudes after this temperature correction is found to be 0\ifmmode\pm\else\textpm\fi{}0.1 percent (probable error). This is inconsistent with a galactic rotation effect as great as the 0.5 percent predicted by Compton and Getting, but does not definitely rule out a more recent modification of their calculation. The origin of the latitude effect knee is ascribed to the minimum energy required for a primary electron to produce mesotrons capable of traversing the atmosphere. The small magnitude of the latitude effect is shown to supply strong evidence of the secondary nature of mesotrons.