ENERGY-SPECTRA OF IONS FROM IMPULSIVE SOLAR-FLARES

We report on a study of the energy spectra of ions from impulsive solar flares in the 0.1-100 MeV region obtained from the combined observations of three experiments on the ISEE 3 and IMP 8 spacecraft. Most of the events studied are dominated by He and these He spectra show a persistent steepening or break above approximately 10 MeV resulting in an increase in the power-law spectral indices from approximately 2 to approximately 3.5 or more. Spectra of H, He-3, He-3, O and Fe have spectral indices that are consistent with a value of approximately 3.5 above approximately 2 MeV amu-1. One event, dominated by protons, shows a clear maximum in the spectrum near 1 MeV. If the roll-over in the spectrum below 1 MeV is interpreted as a consequence of matter traversal in the solar atmosphere, then the source of the acceleration would lie only approximately 800 km above the photosphere, well below the corona. Alternative interpretations are that trapping in the acceleration region directly causes a peak in the resulting ion spectrum or that low-energy particles encounter significant additional scattering during transport from the flare. In two of the events, the normal velocity dispersion in the arrival of the ions is modulated by the passage of spatial structures in the interplanetary plasma. One of these events was previously interpreted as a " channel " of continuous particle emission from the Sun; we find no evidence for this interpretation.