ENERGY-SPECTRA OF IONS ACCELERATED IN IMPULSIVE AND GRADUAL SOLAR EVENTS

We report new high-sensitivity measurements of the energy spectra of i ons from five impulsive solar flares and one gradual event observed du ring solar minimum by the Energetic Particles, Acceleration, Compositi on, and Transport (EPACT) experiment aboard the WIND spacecraft. All o f the impulsive-flare events had intensities too low to be visible on previous spacecraft such as ISEE 3, which observed hundreds of impulsi ve-hare events. Often these events cluster in or behind a coronal mass ejection (CME) where magnetic field lines provide an excellent connec tion to a solar active region where flares are occurring. In most case s we can see velocity dispersion as the ions of 20 keV amu(-1) to 10 M eV amu(-1) streamed out from the impulsive flare at the Sun, arriving in inverse order of their velocity. Ions from a large, magnetically we ll-connected gradual event, associated with a CME-driven shock, also s how velocity dispersion early in the event but show identical time pro files that last for several days late in the event. These time-invaria nt spectra of H, He-4, C, O, and Fe in this gradual event are well rep resented as power laws in energy from 20 keV amu(-1) to similar to 100 MeV amu(-1). In the impulsive-flare events, H, He-3, He-4, C, O, and Fe have more rounded spectra that flatten somewhat at low energies; ye t the intensities continue to increase down to 20 keV amu(-1). Most of the ion energy content appears to lie below 1 MeV in the impulsive ev ents, where it would be invisible to gamma-ray line observations.