Long-term Variation in the Magnitude of the Diurnal Anisotropy of Cosmic Rays

Abstract

OF the several effects of modulation of cosmic rays which are solar in origin the diurnal variation is the most difficult to investigate because of its small amplitude. The precise determination of both the long term and day to day characteristics of the diurnal anisotropy is of prime relevance in the development of theoretical models of the processes whereby the Sun exercises control over electromagnetic conditions in the inner solar system. Different workers, however, have reached widely divergent conclusions concerning even the gross features of the yearly average solar diurnal variation vector, ranging from essential invariance¹ to a change with the level of solar activity dependent on latitude². The objective of the present investigation was to ascertain, by the critical application of rigorous statistical techniques to the considerable body of data that has now become available, what can be concluded quantitatively concerning the long term behaviour of the solar diurnal variation.