Geomagnetic and Solar Modulation Effects of Sea-Level Cosmic Ray Intensity -Summary of Cosmic Ray Latitude Surveys Aboard the Expedition Ship SOYA during 1956-1962-

Five latitude surveys for measurements of the cosmic ray nucleonic and meson components have been carried out aboard M/S SOYA along a definite route between Japan and the Antarctic during 1956-1962. Using the data obtained from these surveys and also some other related data, investigations of the relationship between the cosmic ray intensity and the permanent magnetic field of the earth were performed, and the results are summarized in two parts. In Part 1, latitude variations of cosmic ray intensities in a stationary state are studied with respect to the cosmic ray equator, the cosmic ray latitude knee, and the threshold rigidities. It is shown that the world-wide distribution of the cosmic ray neutron intensities at sea level is, in general, consistent with the spatial distribution of the vertical threshold rigidities determined from the trajectory calculations using a higher order simulation of the geomagnetic field. In this way, a table of the vertical threshold rigidities is prepared at 5°latitude and 10°longitude intervals. From comparisons of the threshold rigidities with experimental data, the overall uncertainty involved in the vertical threshold rigidity is found to be about 0.3 GV. This suggests the limitation of the usefulness of the vertical threshold rigidities which are supposed to account for the actual distribution of cosmic ray intensity over the globe. To minimize the uncertainty, it would be necessary to develop the study of the following three points: representation of the real geomagnetic field, correction for the penumbral effect, and influence of the inclined cosmic ray particles. In Part 2, year-to-year change of the latitude variation of the cosmic ray intensity is examined throughout the last solar cycle 1954-1962. It is found that the position of the latitude knee moves toward a higher rigidity as solar activity increases, from about 1 GV at the solar minimum to about 3 GV at the solar maximum, whereas the geographical position of the cosmic-ray equator remains constant. The change in the slope of the intensity-rigitiy curve, indicating the change in the slope of the primary cosmic ray spectrum, appears to differ during the ascending and descending phases of the solar cycle. The 11-year variation of the response functions for the sea-level nucleonic component is deduced for every year, and is found to be consistent with PARKER\u27S solar wind model proposed for interpretation of the solar cycle modulation. The threshold rigidities and the response functions presented would be available for analyses of other cosmic ray observation results. By actual application to the past special events such as the solar proton increase or the FORBUSH decrease, their general excellency and wide availability are established