STREAMING-LIMITED INTENSITIES OF SOLAR ENERGETIC PARTICLES ON THE INTENSITY PLATEAU

We examine the energy spectra of H, He, O, and Fe ions on the temporal intensity plateau region in large solar energetic-particle (SEP) events, where intensities may be "streaming limited." Upstream of shock waves near the Sun, equilibrium may occur when outwardly streaming protons amplify resonant Alfven waves that then scatter subsequent protons sufficiently to reduce the streaming. In the largest SEP events, the so-called ground-level events (GLEs), we find proton energy spectra that are peaked near similar to 10 MeV with the energy of similar peaks decreasing for heavier ions and for smaller events. These spectra contrast sharply with spectra near the time of shock passage which rise monotonically above the plateau spectra with decreasing energy. We suggest that strong suppression of upstream ion intensities near similar to 1 MeV amu(-1) on the plateau occurs when those ions resonate with waves amplified earlier by streaming protons of similar to 10 MeV and above. GLEs with much lower intensities of 10-100 MeV protons on the plateau show spectra of ions that rise monotonically toward low energies with no peaking and no suppression of low-energy ions. Wave amplification by streaming protons and the pitch-angle dependence of the resonance condition are essential factors in our understanding of the limiting behavior.