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Context. The intensity profiles of iron and oxygen in Solar EnergeticParticle (SEP) events often display differences that result in a decreasingFe/O ratio over time. The physical mechanisms behind this behaviour are notfully understood, but these observational signatures provide important tests ofphysical modelling efforts. Aims. In this paper we study the propagation ofiron and oxygen SEP ions using a 3D model of propagation which includes theeffect of guiding centre drift in a Parker spiral magnetic field. We derivetime intensity profiles for a variety of observer locations and study thetemporal evolution of the Fe/O ratio. Methods. We use a 3D full orbit testparticle model which includes scattering. The configuration of theinterplanetary magnetic field is a unipolar Parker spiral. Particles arereleased instantaneously from a compact region at 2 solar radii and allowed topropagate in 3D. Results. Both Fe and O experience significant transport acrossthe magnetic field due to gradient and curvature drifts. We find that Fe ionsdrift more than O ions due to their larger mass-to-charge ratio, so that anobserver that is not magnetically well connected to the source region willobserve Fe arriving before O, for particles within the same range in energy pernucleon. As a result, for the majority of observer locations, the Fe/O ratiodisplays a decrease in time. Conclusions. We conclude that propagation effectsassociated with drifts produce a decay over time of the Fe/O ratio,qualitatively reproducing that observed in SEP event profiles.

Time dependence of Fe/O ratio within a 3D Solar Energetic Particle propagation model including drift