Gamma rays from an ‘‘over-the-limb’’ flare

We present observations taken with the SMM Gamma‐Ray Spectrometer of a giant flare that occurred on 1989 September 29 at a position located approximately 10° beyond the solar limb. The spectral measurements show a rich gamma‐ray spectrum with a remarkably strong neutron capture line at 2.22 MeV. We briefly examine several possible interpretations of the measurements. We conclude that the best interpretation is that the gamma‐ray emission had a spatially extended component which subtended more than 30° on the solar surface in addition to the spatially compact component that normally dominates the total flare fluence. We suggest that the ‘‘back diffusion’’ of solar energetic particles resident on open field lines may have generated the extended component.