Sidereal daily variation of galactic cosmic rays

Abstract

We have analyzed the yearly averaged sidereal daily variations in the count rates of 44 underground muon telescopes by fitting Gaussian functions to the data. These functions represent the Loss-cone (LC) and Tail-in (TI) anisotropies proposed by Nagashima et al.. The telescopes cover the median rigidity range 143GV–1400GV and the viewing latitude range 73°N–76°S. We find that the TI anisotropy has its reference axis located at declination ( δ) ≈ 14°S and right ascension ( α) ≈ 4.7 sidereal hours. We show that the TI anisotropy is asymmetric about the reference axis and its observed α depends on the viewing latitude of the telescopes. We also show that the LC anisotropy is symmetric and has a referenceaxis located at the celestial equator and α ≈ 13 sidereal hours. From the parameters of the Gaussian fits we devise an empirical model of the sidereal anisotropies which implies that the above characteristics of the anisotropies can explain the north-south asymmetry in the amplitude of the sidereal diurnal variation. Furthermore, we find that the phase of the sidereal semi-diurnal variation of cosmic rays should be recorded at later times when measured from the northern hemisphere compared to observations made from the southern hemisphere.