Annual modulation of solar diurnal variation of the cosmic ray nucleonic component in three-dimensional space.

Abstract

A study was made of the annual modulation of the cosmic ray solar diurnal variation to search one of the characteristics of the three-dimensional cosmic ray anisotropy in space. The present analysis, using the neutron monitor data from the worldwide network for the period of [1964][1965][1966][1967][1968], is the extension work to confirm the results already derived from the meson data by Nagashima et al. In order to show the annual change of solar diurnal variations clearly, differences between the diurnal vectors at conjugate pairs of the observational stations in northern and southern hemispheres were exclusively utilized. A significant and systematic annual variation is found in the harmonic dial of monthly difference vectors moving in an ellipse having its major axis with the range of about 0.05% and the direction in 9 hr of FebruaryAugust in a year. This annual movement of solar diurnal variations can be interpreted in the framework of Nagashima's theory, due to the P2-type diurnal term arising from the second harmonic component of the cosmic ray solar anisotropy in space. The space distribution responsible for the annual variation of the nucleonic component is re-examined in terms of both solar and sidereal time variations.