  As solar energetic particles (SEPs) stream outward along the interplanetary
magnetic field after acceleration by shock waves near the Sun, their
intensities are limited by scattering against self-generated Alfv\'en waves,
trapping the particles near their source. This streaming limit varies with the
magnetic rigidity of the particle and with distance from the source.
Pitch-angle coupling can cause higher-energy protons to suppress the
intensities of lower-energy ions causing flattened low-energy spectra on the
early SEP intensity plateau. At sufficiently high energies, particle flow and
wave trapping of particles weakens and the SEP spectra steepen, forming
spectral "knees".
