Cosmic-ray intensity distributions at 640 grams per square centimeter atmospheric depth around Cape Town

Abstract

Worldwide surveys on the nucleon component intensity distributions of cosmic radiation showed consistent results with the calculations of vertical magnetic cutoff rigidities both by Rothwell and by Quenby and Wenk. The calculations of Quenby and Wenk are the least accurate in a region over the North Atlantic, where Pomerantz and Agarwal found the theoretical cutoff rigidities too low by as much as 2 gv, and in a region around Cape Town, for which Pomerantz and Agarwal found consistent results for a crossing at fairly low latitude. In the present investigation the higher‐latitude region around Cape Town was investigated by an airborne standard neutron monitor at 640 g/cm2, and the lines of equal cosmic‐ray intensities were compared with the lines of equal cutoff rigidities calculated according to the geomagnetic theories of Störmer, Rothwell, and Quenby and Wenk. Close agreement was found with the theory of Quenby and Wenk, and compatibility with other theories is excluded statistically within 90 per cent confidence limits. The intensity‐latitude curve along 18°35′E geographic longitude showed no ‘knee’ effect below 40.5°S geomagnetic latitude, whereas Kodama has found a knee at about 35.4°S at sea level. This is discussed in view of the Cape Town magnetic anomaly, but no explanation can be given of this controversial result.