||Fe Enhancements in SEP Onsets:
Flare/CME Mixture or Transport Effect?
||484, Outstanding Problems in Heliophysics: From Coronal Heating to the Edge of the Heliosphere
||Mason, G. M.; Li, G.; Cohen, C. M. S.; Desai, M. I.; Haggerty, D. K.; Leske, R. A.; Mewaldt, R. A.; Zank, G. P.
||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).