Gamma-Ray Evidence for Time-Dependent Heavy Ion Enhancement on Flare Particle Acceleration Time Scale
R. Ramaty(1), N. Mandzhavidze(1,2), G. Lenters(1,3) and J. Miller(4)

(4)Univ. of Alabama, Huntsville

Gamma-ray line emission was observed from the very large 1991 June 1 flare. Owing to the fact that this flare was located behind the limb of the Sun, only the coronal part of the gamma-ray source was observed (with Granat/Phebus). The thin-target nature of the coronal source prevents the thick-target suppression of the gamma-ray emission produced by accelerated heavy ions, thus allowing the better observation of the emission produced between about 1 to 2 MeV by accelerated Ne, Mg, Si, Fe than is possible for flares on the solar disk. When this 1-2 MeV emission is compared with that between 4 and 7 MeV produced by C, N, O, detailed analysis shows that the (Ne-Fe)/(C-O) ratio increased with time, while the gamma-ray fluxes themselves declined, and the accelerated particle spectra steepened. We suggest that these results can be understood if the acceleration is due to cascading Alfven turbulence. The decreasing gamma-ray fluxes indicate the decrease in the number of accelerated ions, most likely due to the decreasing turbulent energy density. Because the cascading turbulence is first damped by the heavier ions, the decreasing turbulent energy density implies that less turbulence remains available for accelerating the C, N and O, leading to the inferred anticorrelation between the gamma-ray fluxes and the heavy ion enhancement.