Fe Enhancements in SEP Onsets: Flare/CME Mixture or Transport Effect?

During the onset phases of SEP events, the Fe/O ratio is often observed to be initially enhanced (˜1) over typical SEP values, followed by a decline to values close to typical averages over entire events (Fe/O ˜0.1). Two mechanisms have been suggested to explain this behavior, namely (1) a two-step process with an initial injection of "flare" particles with high Fe/O followed by shock-accelerated particles with lower Fe/O, and (2) a transport effect wherein the lower charge-to-mass ratio of Fe vs. O results in faster transport of Fe to the observer, leading to enhanced Fe/O in the early stages of the event. Distinguishing between these two scenarios is important to building a basic picture of processes taking place in large SEP events. We have carried out a detailed study of 17 large SEP events where energetic particle data were fitted by a state-of-the-art model whose computed time-intensity profiles were compared to the observed profiles of H, He, O, and Fe over a very broad energy range. We find that the observed decrease in Fe/O during the rise phase can be reasonably fitted by the transport model where the differences in Fe vs. O transport are due to the slope of the turbulence spectrum of the interplanetary magnetic field (IMF).