Gamma-Ray Measurements of Flare-to-Flare Variations in Ambient Solar Abundances

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Abstract

We have measured fluxes of 10 narrow γ-ray lines in 19 X-class solar flares observed by the Solar Maximum Mission spectrometer from 1980 to 1989. These lines originate from interaction of energetic protons and α-particles with ambient solar material. Flare-to-flare variations in line fluxes reveal that the abundances of elements in the flare plasma are grouped with respect to their first ionization potentials (FIPs). Line fluxes from elements with similar FIPs correlate well with one another; in contrast, the low-FIP (< 10 eV; e.g., Mg, Si, and Fe) to high-FIP (>11 eV; e.g., C, N, and O) line ratios vary by as much as about a factor of 4 from flare to flare. This factor of 4 is consistent with the enhancement of low-Fl P elements found in the solar corona relative to photospheric abundances. We also find that the Ne/O line ratio increases as the accelerated particle spectrum becomes softer. This can be explained by excitation cross sections less than 10 MeV, but the magnitude of the effect is dependent on the accelerated α-particle/proton ratio. After correcting for this spectral dependence, we find that the Ne/(C + N + O) line ratio is constant from flare-to-flare, indicating that Ne behaves like these other high-Fl P elements in the ambient flare plasma. We discuss a possible explanation, which is based on these observations, for the factor of ∼3 enhancement in the abundance of Ne relative to other high-FIP elements in the 1981 April 27 flare found by Murphy et al. (1991).

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