Distinguishing the Rigidity Dependences of Acceleration and Transport in Solar Energetic Particles

Abstract

In solar energetic particle (SEP) events, the power-law dependence of element abundance enhancements on their mass-to-charge ratios A/Q provides a new tool that measures the combined rigidity dependences from both acceleration and transport. Distinguishing these two processes can be more challenging. However, the effects of acceleration dominate when SEP events are small or when the ions even propagate scatter-free, and transport can dominate the time evolution of large events with streaming-limited intensities. Magnetic reconnection in solar jets produces positive powers of A/Q from +2 to +7 and shock acceleration produces mostly negative powers from -2 to +1 in small and moderate SEP events where transport effects are minimal. This variation in the rigidity dependence of shock acceleration may reflect the non-planer structure, complexity, and time variation of coronal shocks themselves. Wave amplification by streaming protons in the largest SEP events suppresses the escape of ions with low A/Q, creating observed powers of A/Q from +1 to +3 upstream of the accelerating shock, decreasing to small negative powers downstream. Of course, the powers of A/Q are correlated with the spectral indices of He, O, and Fe, yet unexplained departures exist.