Cosmic ray intensity variations in the lower atmosphere

Worldwide cosmic ray intensity measurements at aircraft altitudes have been made over a solar cycle. Data obtained with a neutron monitor aboard the U.S. Naval Oceanographic Office Project Magnet aircraft during different epochs have been compared to determine the long-term variation of the nucleonic intensity at latitudes ranging from north to south geomagnetic poles. The measurements were normalized to a constant pressure-altitude, 500 mm of Hg, on the basis of experimentally determined time- and rigidity-dependent attenuation coefficients. Between October 1958 and January 1965, the intensity increased 8.7% at a rigidity of 17 Gv, and 20.9% at 2 Gv. The measured intensities as a function of threshold rigidity are in good agreement with the predictions of trajectory calculations based on a sixth-degree simulation of the geomagnetic field. At solar minimum, measurements of the latitude effect were also obtained for five groups of multiple events and for mesons; the highest group, multiplicity K≥6, varies less with latitude than do the mesons. The latitude effect for K=3 is intermediate between that for the neutron total counting rate and the meson counting rate. The attenuation length for mesons is found to vary with altitude in reasonable agreement with the predictions of Wada [1960].