A Search for Mev Gamma-ray Emission from the Quiet-time Sun

Until now, solar γ-ray and neutron emissions have only been detected during times of intense solar activity, specifically, during solar flares. These emissions arise from the interaction of accelerated solar particles (both electrons and ions) with the ambient solar material. However, there are several reasons to expect that γ-ray emission might also be detectable at times when there is no significant solar activity. We report on the first results of an effort to measure quiet-time γ-ray emissions using data from the COMPTEL experiment on the Compton Gamma-Ray Observatory (CGRO). INTRODUCTION Several mechanisms have been proposed which might be capable of producing detectable levels of γ-ray emission during periods of low solar activity. These include: 1) Processes related to the general problem of coronal heating may accelerate particles to nonthermal energies, in which case their interactions with the ambient solar material may lead to γ-ray and neutron emission. 2) The decay of long-lived radioactive isotopes produced in flares (Ramaty Kozlovsky & Lingenfelter 1979), may lead to detectable emission long after the flare itself. Of particular interest in this regard are the decays of 56Co (emission at 0.847 and 1.238 MeV with a half-life of 77 days) and 54Mn (emission at 0.835 MeV with a half-life of 303 days). 3) The precipitation of downstream shock-accelerated ions back to the solar surface from coronal mass ejections (e.g., Vestrand & Forrest 1993). Such coronal-mass ejections may or may not be associated with a visible solar flare. 4) The interaction of high energy cosmic rays with the solar atmosphere is expected to produce γ-ray and neutron emissions much like those generated by similar processes in the Earth’s atmosphere (Seckel, Stanev & Gaisser 1991). 5) Some models involving the radiative decay of massive neutrinos imply that solar neutrino decay might result in the generation of MeV γ-rays in a region just outside the solar surface (Miller