Observational Consequences of Proton-Generated Waves at Shocks

Donald V. Reames
NASA/Goddard Space Flight Center
Greenbelt, Maryland, USA

In the largest solar energetic particle (SEP) events, acceleration takes place at shock waves driven out from the Sun by fast coronal mass ejections. Multi-spacecraft studies show that the particles from the largest SEP events span more than 180 degrees in solar longitude, like the shocks that accelerate them. Protons streaming away from strong shocks generate Alfven waves that trap particles in the acceleration region, limiting outflowing intensities but increasing the efficiency of acceleration to higher energies. Early in the events, with the shock still near the Sun, intensities at 1 AU are bounded and spectra are flattened at low energies. Elements with different charge-to-mass ratios, Q/A, differentially probe the wave spectra near shocks, producing abundance ratios that vary in space and time. An initial rise in He/H, while Fe/O declines, is a typical symptom of the non-Kolmogorov wave spectra in the largest events. Strong wave generation can cause cross-field scattering near the shock and unusually rapid reduction in anisotropies even far from the shock. At the highest energies, shock spectra steepen to form a "knee." For protons, this spectral knee can vary from ~10 MeV to ~10 GeV depending on shock conditions for wave growth. Though rarely measurable, the location of the knee scales approximately as Q/A in the energy/nucleon spectra of other species.