Spectral Characteristics of Gradual Solar Energetic Particle Events
Allan J. Tylka1, Paul R. Boberg2,1, Robert E. McGuire3, 
Donald V. Reames4, Chee K. Ng4,5

1. E O Hulburt Center for Space Research, Code 7654, Naval Research Laboratory
2. Consultant
3. National Space Science Data Center, NASA/Goddard Space Flight Center
4. Lab. for High Energy Astrophysics, NASA/Goddard Space Flight Center
5. Dept. of Astronomy, University of Maryland

We examine the characteristics and temporal evolution of particle spectra in several large 'gradual' particle events, in which acceleration is caused by CME-driven shocks. These events generally last for several days, so we have divided the events into ~8-hour intervals in order to study the evolution of the spectra. By combining data from ACE/SIS, Wind/EPACT, and IMP-8/GME, we follow this spectral evolution for all major species from protons to iron. This evolution reveals significant departures from the generally-expected power laws which are not simply due to velocity dispersion. At low energies, these spectra generally show flattening that can be explained by rigidity-dependent escape from the intense particle-generated wave field near the shock. At high energies, the spectra often exhibit exponential roll-offs, which can reflect both the ionic charge states of the high-energy heavy ions and characteristics of the near-shock diffusion coefficients. In early times in some events, these exponential roll-offs are not observed, suggesting that they initially occur at energies which are beyond the range of current instrumentation. Both the low-energy flattenings and the high-energy roll-offs evolve in an orderly fashion within each event. These characteristics also show significant event-to-event variability in the energies at which they occur. We will try to understand this evolution and variability in terms of the characteristics of the CME-driven shocks.