LWS Studies of Gamma Rays Produced in the Earth's Atmosphere by Cosmic Rays and Solar Energetic Particle Events

We summarize a three-year study of atmospheric gamma-ray line measurements conducted under the Living With a Star program. The observations were made by instruments on three satellites: SMM, Yohkoh, and RHESSI. They detected the 511-keV positron annihilation line produced in electromagnetic showers and de- excitation lines from 14N at 728, 1635, 2313, 3890, and 5106 keV, and from 16O at 6129 keV, as well as spallation lines near 4440 keV from 12C and 11B produced by neutron and proton interactions on Earth's atmosphere. The SMM studies focused on the variation of the cosmic-ray produced lines as a function of magnetic rigidity and solar cycle modulation. We found that the power-law spectral index of the secondary neutrons was relatively constant for the range of times and rigidity intervals sampled. The Yohkoh and RHESSI studies focused on atmospheric gamma rays produced by solar energetic particles interacting in the Earth's atmosphere following flares on 2000 July 14 and 2002 April 21, respectively. The SEP spectra estimated using gamma-ray line ratios were of comparable hardness with the spectrum measured by SMM for the 1989 October 20 intense event, consistent with measurements in space. Improved cross sections for the production of the nuclear lines offer better estimates of the spectra of SEPs impacting the atmosphere. Since the early observations, RHESSI has observed several more SEP events producing detectable fluxes of atmospheric gamma rays. These include large particle events in 2003 October/November and 2005 January.