Effects of Interplanetary Transport on SEPs with Differing Charge-to-Mass ratios

In large SEP events that are well-connected to the observer it is often observed that particles with different charge-to-mass ratios (e.g, O vs. Fe) have different time-intensity histories, and different event-averaged spectral shapes. Several explanations for this behavior have been suggested, such as a sequence of occurrences as an initial “flare” followed by acceleration in a CME-associated interplanetary shock. Another mechanism may be differing responses of ions to waves generated by the streaming SEP protons. In this paper we investigate the possible role of interplanetary transport in such effects by modeling the ion propagation using an advanced numerical transport model in which the particle pitch angle scattering is determined by the turbulence spectrum of the interplanetary magnetic field. Because the turbulence level is a function of particle gyro-frequency, particles with the same speed but different charge-to-mass ratios will sample different portions of the turbulence spectrum and therefore can have different transport histories. We examine sample calculations for O and Fe ions to explore the transport induced effects on time-intensity profiles, spectral shape, and event-averaged abundances for differing levels of interplanetary turbulence. We will compare calculated time-intensity profiles with SEP observations from ACE instruments.