Spurious sidereal daily variation of cosmic rays produced from stationary anisotropy of solar origin

Abstract

Cosmic-ray sidereal daily variations can be produced from some kinds of stationary cosmic-ray anisotropy of solar origin, due to the obliquity of the ecliptic. For example, the sector-structure-dependent sidereal diurnal variation produced from the cosmic-ray radial density gradient and the North-South asymmetric sidereal diurnal variation produced from the bidirectional anisotropy responsible for the solar semi-diurnal variation belong to this category. Owing to the obliquity, these diurnal variations show the characteristic eigenphase of 18 h or 6 h in sidereal time, depending on the kind of the sector structure (toward or away) for the former or on the observation site (northern or southern hemisphere) for the latter. It is noted, however, that the above eigenphase is only approximate and, if we consider the effect of the inclination of the solar equatorial plane against the ecliptic plane on these variations, the eigenphase lags by about 0.5 to 0.7 h, depending on the kind of the variation. The present paper provides the exact representation of these variations, for the use of eliminating them from the observed sidereal variation in order to obtain the galactic anisotropy of cosmic rays.