We report measurements of isotope abundance ratios for 5-50 MeV/nuc nuclei from a large solar flare that occurred on September 23, 1978. The measurements were made by the Heavy Isotope Spectrometer Telescope (HIST) on the ISEE-3 satellite orbiting the Sun near an Earth-Sun libration point approximately one million miles sunward of the Earth. We report finite values for the isotope abundance ratios 13C/12C, 15N/14N, 18O/16O, 22Ne/ 20Ne, 25Mg/24Mg, and 26Mg/24Mg, and upper limits for the isotope abundance ratios 3He/4He, 14C/12C, 17O/16O, and 21Ne/20Ne.
We measured element abundances and spectra to compare the September 23, 1978 flare with other flares reported in the literature. The flare is a typical large flare with "low" Fe/O abundance (≤ 0.1).
For 13C/12C, 15N/14N, 18O/16O, 25Mg/ 24Mg, and 26Mg/24Mg, our measured isotope abundance ratios agree with the solar system abundance ratios of Cameron (1981). For neon we measure 22Ne/20Ne = 0.109 + 0.026 - 0.019, a value that is different with confidence 97.5% from the abundance measured in the solar wind by Geiss at al. (1972) of 22Ne/20Ne = 0.073 ± 0.001. Our measurement for 22Ne/20Ne agrees with the isotopic composition of the meteoritic component neon-A.
Separate arguments appear to rule out simple mass fractionation in the solar wind and in our solar energetic particle measurements as the cause of the discrepancy in the comparison of the apparent compositions of these two sources of solar material.
