Initial Fe/O Enhancements in Large, Gradual, Solar Energetic Particle Events: Observations from Wind and Ulysses

Shocks driven by fast coronal mass ejections (CMEs) are the dominant particle accelerators in large, gradual solar energetic particle (SEP) events. In these events, the event-integrated value of the iron-to-oxygen ratio (Fe/O) is typically similar to aEuro parts per thousand 0.1, at least at energies of a few MeV/nucleon. However, at the start of some gradual events, when intensities are low and growing, initially Fe/O is similar to aEuro parts per thousand 1. This value is also characteristic of small, impulsive SEP events, in which particle acceleration is due to magnetic reconnection. These observations suggested that SEPs in gradual events also include a direct contribution from the flare that accompanied the CME launch. If correct, this interpretation is of critical importance: it indicates a clear path to interplanetary space for particles from the reconnection region beneath the CME. A key issue for the flare origin is magnetic connectedness, i.e., proximity of the flare site to the solar footpoint of the observer's magnetic field line. We present two large gradual events observed in 2001 by Wind at L1 and by Ulysses, when it was located at > 60(a similar to) heliolatitude and beyond 1.6 AU. In these events, transient Fe/O enhancements at 5 -aEuro parts per thousand 10 MeV/nucleon were seen at both spacecraft, even though one or both is not well-connected to the flare. These observations demonstrate that an initial Fe/O enhancement cannot be cited as evidence for a direct flare component. Instead, initial Fe/O enhancements are better understood as a transport effect, driven by the different mass-to-charge ratios of Fe and O. We further demonstrate that the time-constant of the roughly exponential decay of the Fe/O ratio scales as R (2), where R is the observer's radial distance from the Sun. This behavior is consistent with radial diffusion. These observations thus also provide a potential constraint on models in which SEPs reach high heliolatitudes by cross-field diffusion.