The Solar Maximum Mission satellite ’ s Gamma Ray Spectrometer spent much of its 1980–1989 mission pointed at Earth , accumulating spectra of atmospheric albedo \gamma -rays .
Its 28 ^ { \circ } orbit ensured that a range of geomagnetic latitudes was sampled .
We measured the variation with time and cutoff rigidity of some key \gamma -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 \sim 3.6 between the lowest ( < 7 GV ) and highest ( > 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 \sim - 1.15 – -1.45 and better constraints on its variability , \leq 5 % over a solar cycle , and \leq 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 .