The application of the ring current correction model to the Magsat data over the southern African region is considered. We found that by using dip latitude computed at 3 R e above the surface of the earth (representing the ring current altitude) to derive the E, I coefficients and apply the correction, the effectiveness of the generally used ring current model is improved. These results are of particular importance to work in the southern African region. However, the physical reasoning should be generally applicable in the use of the ring current correction.
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.
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