Spatial and temporal variability of the gamma radiation from Earth's atmosphere during a solar cycle

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 diagnostic 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 intensities of nuclear lines at 1.6 MeV, 2.3 MeV and 4.4 MeV varied inversely with solar activity 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 (13 GV) rigidities sampled. The solar cycle modulation was particularly marked at the lowest rigidities, reaching an amplitude of 16%. The ratios of the intensities of the lines produced by nuclear de‐excitation (1.6 MeV, 2.3 MeV) and those from nuclear spallation (4.4 MeV) did not vary with either solar activity or cutoff rigidity, indicating that the shape of the secondary neutron spectrum in the atmosphere above 5 MeV was approximately constant over the times and regions sampled. If it is approximated by a power law in energy, we derive constraints on the absolute value of the power law index ∼−1.15–−1.45 and better constraints on its variability, ≤5% over a solar cycle, and ≤6% over SMM's range of sampled cutoff rigidities. We also measured the intensity of the electron‐positron annihilation line at 0.511 MeV. This line also varies with the solar cycle, but its variation with cutoff rigidity is weaker than that of the nuclear lines, falling by a factor 2 (rather than 3.6) over SMM's range of sampled cutoff rigidities. This can be understood in terms of the energy dependences of the cross sections for positron production and for the hadronic interactions which produce secondary neutrons.