The September 1977 cosmic ray storm observed in a wide rigidity range

Characteristic features of the Forbush decrease observed during September, 1977, were studied in a wide rigidity range, using the data from Japanese cosmic ray observation network. The network consists of four neutron monitor stations and two multi-directional meson telescopes at surface and mountain altitude, and three underground multi-directional telescopes at depths of 34 mwe, 54 mwe and 80 mwe. The hourly values of cosmic ray intensity recorded between September 16 and 30, 1977, were obtained for the representative stations and components. Observations of the sudden commencement of geomagnetic storm, ssc, are also shown to indicate the passes of shock fronts over the earth (1106 UT on September 19 and 204 UT on September 21). The cosmic ray storm started at around the second ssc and reached the maximum depression at about middle of September 22. The magnitude of depression was about 3% for neutron monitors and decreased with the increase of telescope median rigidities. It should be noted that the Forbush decrease was accompanied by the pronounced anisotropic intensity variations. (Yoshimori, M.)