The Relative Abundances of H, He, and Heavy Ions in Large Solar Energetic Particle Events

Studies have shown that the heavy element abundances in large solar energetic particle (SEP) events can vary tremendously (e.g., by factors of ∼100 for Fe/O) at energies above ∼10 MeV/nucleon. The most iron-rich gradual events often have a heavy element composition more typical of impulsive events,supporting the suggestion that flare seed material plays a significant role in these events. However, most of these studies do not include measurements of protons and alpha particles (H and He),which dominate the particle intensities. Using data from ACE, SAMPEX, Wind, and GOES, we have constructed composite spectra of protons, alpha particles, and heavy ions spanning ∼0.1 to ∼100 MeV/nucleon in energy in many of the large, gradual SEP events of solar cycle 23. We examine the relative abundances and spectral shapes of H and He compared with heavy ions in an effort to better understand the origin of the event-to-event variability. In particular, we test whether the H/Fe ratio in the most iron-rich large events more nearly resembles that in average gradual events or instead approximates the average impulsive event value, which is a factor of ∼10 lower. In addition, the variable proton-to-heavy ratio has important space weather implications, which are briefly discussed. This work was supported by NASA under grants NNG04GB55G, NAG5-12929, and NAG5-8877.