The Streaming Limit of Solar Energetic-Particle Intensities

TL;DR

This paper investigates how solar energetic particle intensities are limited by self-generated Alfvén waves during their outward streaming, affecting spectral shapes and energy-dependent behaviors.

Contribution

It provides a detailed analysis of the streaming limit mechanism, including the effects of magnetic rigidity, distance, and pitch-angle coupling on SEP spectra.

Findings

Streaming limit varies with particle rigidity and distance from the Sun.

Pitch-angle coupling causes suppression of lower-energy ions by higher-energy protons.

Spectral knees form at high energies where particle flow weakens.

Abstract

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".