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)
(1)NASA/GSFC
(2)USRA
(3)NAS/NRC
(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.