Atmospheric gamma rays at geomagnetic latitudes of −29° and +43

We present results of atmospheric gamma ray measurements obtained during two balloon flights from Alice Springs, Australia (λ = −29°), and Fort Sumner, New Mexico, United States of America (λ = 43°) at geomagnetic cutoff rigidities of 8.5 GV and 4.3 GV, respectively. The fluxes, in the energy range of 1–15 MeV, are derived as functions of zenith angle, residual depth, and latitudinal rigidity. We find while the downward moving gamma ray flux at the float level (4.8 g cm−2) is not a strong function of rigidity the upward flux at λ = −29° is, on average, by factors of 2 to 4 lower than at λ = 43°. The energy spectra of the downward moving gamma rays at various altitudes are harder than the upward moving gamma rays. The spectral indices for both upward and downward fluxes at λ = −29° are lower than at λ = 43°.