Journal of Geophysical Research
The Solar Maximum Mission satellite’s Gamma Ray Spectrometer spent much of its 1980–1989 mission pointed at Earth, accumulating spectra of atmospheric albedo γ-rays. Its 28◦ orbit ensured that a range of geomagnetic latitudes was sampled. We measured the variation with time and cutoﬀ rigidity of some key γ-ray lines which are diagnos-tic of the intensity of the Galactic cosmic radiation penetrating the geomagnetic cutoﬀ and of the secondary neutrons produced in the atmosphere. We found that the inten-sities of nuclear lines at 1.6 MeV, 2.3 MeV and 4.4 MeV varied inversely with solar ac-tivity in cycles 21–22 as expected from the theory of solar modulation of cosmic rays. They were found to be strongly anticorrelated with cutoﬀ rigidity, as expected from the theory of the cutoﬀ, falling by a factor ∼ 3.6 between the lowest (< 7 GV) and high-est (> 13 GV) rigidities sampled. The solar cycle modulation was particularly marked at the lowest rigidities, reaching an amplitude of 16%.
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