LI-7 AND BE-7 DEEXCITATION LINES - PROBES FOR ACCELERATED PARTICLE-TRANSPORT MODELS IN SOLAR-FLARES

The photon energy spectrum of a spectral feature composed of the 429 and 478 keV gamma-ray lines from 7Be and 7Li (produced by interactions of flare-accelerated α particles with ambient He in the solar atmosphere) depends on the angular distribution of the interacting accelerated particles. We calculate this spectrum for limb and disk-centered flares using a loop model for the transport of the ions. In this model, the flux tube magnetic field is constant in the corona and converges in the chromosphere to the photosphere. Magnetic mirroring and MHD pitch-angle scattering are both taken into account. We compare the resulting spectra with data from the 1981 April 27 limb flare obtained with the gamma-ray spectrometer on SMM and provide convincing evidence for the existence of the 7Li-7Be feature in this flare. By comparing the fluence of this feature with that of the 511 keV line, we show that the accelerated α particle abundance or the ambient He abundance, or both, must be enhanced. Furthermore, we show that data of this quality are adequate to distinguish between model spectra calculated for limb flares and spectra calculated for disk-centered flares. But the data are incapable of distinguishing limb-flare spectra derived with pitch-angle scattering from spectra derived without, since both provide good fits to these data. We also show that the data are also consistent with the spectrum derived for an isotropic distribution of interacting accelerated particles, but possibly not with the spectrum derived for a beam normal to the line of sight, as would be a downward beam in a limb flare.