Cosmic Ray Anisotropy in Interplanetary Space.

The solar diurnal variation of cosmic ray intensity results from an interaction of cosmic rays with the co-rotating spiral interplanetary magnetic field. Changing interplanetary conditions might therefore be reflected in the diurnal variation. In this dissertation, the dependences of the quiet-day solar diurnal variation on the general level of solar activity and on the interplanetary sector structure are investigated. It is found that the amplitude of the solar diurnal variation as recorded by the Embudo and Bolivia multidirectional underground muon telescopes is significantly smaller near solar minimum (1965) as compared to solar maximum (1968). This amplitude dependence is associated with a change in the maximum primary rigidity which can be modulated by the co-rotating interplanetary magnetic field. It is found that this maximum rigidity was as low as 40 GV in 1965, and had risen to approximately 90 GV in 1968. It is shown in addition that the solar diurnal variation recorded by underground telescopes is contributed in part by a Compton­Getting effect which results from the motion of the earth through the interplanetary medium as it orbits the sun. It is found that the solar diurnal variation monitored by underground telescopes is sensitive to the interplanetary sector structure, displaying a dependence of phase on the sense of the interplanetary magnetic field (predominantly into, or out of, the sun). It is shown that this phase dependence can arise from a cosmic ray anisotropy which is normal to the ecliptic plane, and that it is possible that the observed sector structure dependence of the solar diurnal variation and sidereal diurnal variations reported by others can arise from the same source. In order to make a quantitative study of the changes of the diurnal variation with solar cycle, it was necessary to investigate coupling functions applicable to underground telescopes. These coupling functions give the contribution of primary particles of different rigidities to the observed counting rate. It is found that the functions which are in current use are not adequate for describing the observation of a Forbush decrease by the Embudo vertical telescope. A new coupling function model is proposed, and evidence is presented which suggests that underground telescopes respond to lower rigidities than was previously believed. The new functions are shown to satisfactorily describe the observations. In conclusion, the importance of underground measurement is emphasized, because the investigations described in this dissertation would have been impossible through the use of neutron monitors, which respond to much lower rigidities.