A Solar Anisotropy in Cosmic Radiation, viewed from a Terrestrial Frame of Reference

Abstract

IT is generally recognized that a solar anisotropy in the primary cosmic radiation must be responsible for at least part of the observed terrestrial daily variation of intensity, although it seems that the exact nature of this contribution is not yet clear. However, Rao, McCracken and Venkatesan¹ have recently presented a method of determining the characteristics of a daily variation of this type. Their method takes into consideration the influence of the Earth's magnetic field and in particular they emphasize the part played by the asymptotic cones of acceptance of cosmic ray detectors. Consequently, using experimental values of the daily variation of neutron intensity averaged over the year 1958, they have been able to postulate a type of anisotropy which appears to account for the complex manner in which the amplitude and phase of the first harmonic of the daily variation depends on the geographical location of the detector. However, it would perhaps be truer to say that they predict that if the Earth's atmosphere and its magnetic field were removed the annual mean diurnal variation of intensity observed at the Earth's surface would have the following characteristics : (1) the diurnal variation would be independent of rigidity in the range 1–200 GV; (2) the amplitude would vary as the cosine of geographic latitude and the time of maximum would be in the direction 85° to the east of the Earth–Sun line; (3) the maximum amplitude, observed with narrow-angle detectors at the equator, would be 0.4 per cent of the average primary flux. It is proposed to describe here the co-ordinate transformations which relate these important characteristics to those of a selar anisotropy specified in the frame of reference in which its orientation is fixed.