High-Resolution Measurements of Atmospheric Gamma Rays from a Satellite

Data are presented on the gamma rays emitted from the atmosphere to satellite altitudes in the absence of bremsstrahlung X rays associated with electron precipitation from the radiation belts. The measurements (40 keV to 2.7 MeV) were performed with a high-resolution germanium spectrometer on board the polar-orbiting satellite 1972-076B. With the collimated spectrometer on the spinning vehicle (5-s period) it was possible to separate sensor and vehicle backgrounds from the atmospheric and diffuse cosmic contributions. The atmospheric component was obtained by unfolding published values for the diffuse cosmic spectrum. Owing to variances in the measured values of the diffuse cosmic spectrum the uncertainties in the atmospheric gamma ray spectrum are greater near the equator and at lower energies than over the polar caps and/or at higher energies, where the atmospheric gamma rays dominate, and the errors in the atmospheric spectrum associated with the diffuse component are small. The variations of the atmospheric gamma rays with invariant latitude are presented. Over the polar caps the energy spectra are similar to those observed near the magnetic equator, and the intensities are higher by about a factor of 4. The spectral indices for least squares power law fits to the continuum are −1.34 ± 0.03 and −1.39 ± 0.08 for the polar cap and equatorial regions, respectively. A search was made for gamma ray lines emitted from the atmosphere in addition to the 511-keV annihilation line. Several peaks were observed in the background spectrum, but no other lines were seen to be emitted from the atmosphere with a significant intensity.