A precision world survey of sea-level cosmic-ray intensities

Abstract

With sensitive, vibration-free, self-recording electroscopes sent to many parts of the globe on twelve different voyages a precision survey has now been completed of the variation of cosmic-ray intensities both with latitude and longitude, so that the earth as a whole can now be covered with sea-level, equal intensity, cosmic-ray lines. In going along the longitude line 75° W., which runs from the far north through Washington, D. C., and along the west coast of South America, there is no appreciable change until the magnetic latitude of about 41° is reached. The equatorial dip then begins to set in and shows a maximum decline of 8 percent off Peru and returns again to its normal value off Cape Horn. In going along longitude line 80° E. through southern India the maximum dip is 12 percent. In going south from Alaska in longitude 165° W. to New Zealand the maximum dip is 10 percent. In going south from Liverpool through the Atlantic Ocean—longitude 30° W.—and around Cape Horn the maximum dip is 8.5 percent. In the region most accurately studied—the west coast of the United States—the intensity remains exceedingly constant until the latitude of Pasadena—41° magnetic—is reached, and then drops remarkably suddenly. In the Atlantic Ocean the drop sets in at about the same magnetic latitude with equal suddenness. It appears also to take place quite suddently at about magnetic latitude 41° in the southern hemisphere. Nevertheless, the existence of a longitude effect shows that in strictness there is no such thing as magnetic latitude. In other words, the earth's magnetic field, even at the remote distances of thousands of miles at which these deflections occur, is strikingly dissymmetrical with respect to any line passing through the earth's center. This method of study opens up the possibility of determining these dissymmetries at large distances from the earth. The observed magnetic effects are to be expected quite independently of whether cosmic rays are in their origin photonic or corpuscular.