Variability of spectra in large solar energetic particle events
Using data from the Solar Isotope Spectrometer on the Advanced Composition Explorer obtained during 36large solar energetic particle events (SEPs) that occurred during 1997–2002 we have examined the spectral characteristics of oxygen and iron. Based on the shape of the oxygen spectrum during the decay phase following the peak in particle intensity, each SEP event was categorized as either exponential (7 events) or power law (29 events). We find that the exponential events were typically the larger events (in terms of peak oxygen intensity) and had Fe/O ratios that strongly decreased with increasing energy. Event-averaged Fe/O ratios (integrated over 12 to 60 MeV/nucleon) were at or below coronal abundances for nearly all these events, while the ratios obtained in the power law events were typically enhanced over coronal values. The majority of the power law events had the same spectral index for both O and Fe resulting in an Fe/O ratio independent of energy. However 6 of the 29 power law events had Fe/O ratios that increased with increasing energy due to an Fe spectral index less negative than that of O. We consider simple diffusion theory in an effort to understand the nature of these events and obtain preliminary but promising results.