SPATIAL AND TEMPORAL INVARIANCE IN THE SPECTRA OF ENERGETIC PARTICLESIN GRADUAL SOLAR EVENTS

We show evidence of spatial and temporal invariance in the energy spec tra of similar to 1-100 MeV protons from large gradual solar energetic particle (SEP) events. Nearly identical spectra are seen over longitu de intervals of up to 160 degrees, and the intensities at all energies decline with e-folding time constants of 6-18 hr for periods of simil ar to 3 days. The region of the invariant spectra is associated with a cceleration on the eastern flank of the shock wave driven out from the Sun by a coronal-mass ejection (CME). The quasi-parallel shock in thi s region changes only slowly with time, and a quasi equilibrium is est ablished for particles stored on field lines in the expanding volume b etween the shock and the Sun. On the western flank of the shock, the S EP event is more dynamic. Here, the nearly quasi-perpendicular shock p roduces harder spectra that change more rapidly with time as regions o f the shock with varying speed and angle sweep across the observer's f ield line. Gradual SEP events arise from large CMEs with sufficient sp eeds to generate shocks that are fast enough to accelerate particles. In our study, shocks with transit speed >750 km s(-1) always accelerat e particles, while those with speeds of 500-750 km s(-1) sometimes do. However, invariant spectra seem to occur in all gradual SEP events as a consequence of the structure and topology of the CME and shock.