Sidereal Daily Variations in Cosmic Ray Intensity and Their Relationships to Solar Modulation and Galactic Anisotropies

Abstract

From the earliest days of cosmic ray intensity recording it has been recognised that with a fixed instrument one could use the spin of the earth to scan different parts of the sky and to measure anisotropies by the diurnal periodic variations that should be induced in the data. However, significant daily variations occur in both solar and sidereal times and because of the extreme closeness of the two periodicities (365 solar days and 366 sidereal days in one earth year) one requires unbroken data spanning one complete year or preferably a series of complete years in order to separate the two periodic variations. Even then a direct setting of the data in solar or sidereal time will not result in the separation of the solar or sidereal diurnal variations unless each type of periodic variation retained a constant amplitude and phase throughout the year. This can easily be seen by examining the situation on a harmonic dial. A harmonic dial is a method of representing a sinusoidal variation by means of a vector and is a diagram in the form of a clock-dial which is graduated in 24 hours for displaying diurnal variations. A vector radiating from the centre of the dial has a length equal to the amplitude of the sine wave and points to the time of day at which maximum intensity occurs.