Survey of cosmic-ray intensity in the lower atmosphere

Abstract

Observations of the intensity of the nucleonic component have been obtained with an airborne neutron monitor aboard the U.S. Naval Oceanographic Office Project Magnet aircraft in a worldwide survey extending from the equator to the north and south geomagnetic poles. The data were reduced to a standard pressure altitude (500 mm of Hg), and the effects of temporal variations were removed by a normalization procedure involving four ground-based neutron monitor stations distributed over a wide range of latitude. Representation of the results in the form of an isocosm map revealed general agreement with the Quenby and Wenk calculations of threshold rigidities. Discrepancies are attributable to the inexactness of the approximation adopted for taking into account the effects of the penumbra. The location of the cosmic-ray equator, determined at seven points, was in accordance with calculations using the sixth-order expansion of the geomagnetic potential. The variation in intensity along the equator was in agreement with earlier results obtained during a different phase of the solar cycle. Measurements in the arctic and antarctic regions reveal that, during a period of extreme solar cycle modulation, the intensities in the northern and southern polar caps were equal within less than 0.5%. The latitude dependence of the atmospheric attenuation length of the nucleonic component, L, was determined from observations of the dependence of intensity upon altitude. In the plateau region, the mean value of L was 136.1±0.3 g/cm2. L increased by about 11% over the range of threshold rigidity from 2.5 to 16 Gv. The absorption mean free path, λa, deduced from L, was 160 g/cm2 in the plateau region. Calculations based on currently accepted models of nuclear collision and of the process of nucleon propagation through the atmosphere yielded a value for the interaction length, λa, of about 80 g/cm2. This increased to 85 g/cm2 when the effects of charge exchange were considered.