Fe/O Variations Relative to Source Longitude and Heliospheric Current Sheet in Large Solar Energetic Particle Events

Abstract The Fe/O enhancements exhibit significant variations in gradual solar energetic particle (SEP) events. Several causes have been suggested including transport effects in the interplanetary space and flare contribution. In this study, we investigate the relationship between the integrated Fe/O ratios of 27 gradual SEP events, locations of associated solar flares, and positions along the heliospheric current sheet (HCS) between 2010 and 2014. We employ synchronic potential field source surface (PFSS) extrapolations at 2.5 R ⊙ , derived in near real-time using Artificial Intelligence (AI)-generated far side and Helioseismic and Magnetic Imager (HMI) magnetograms, referred to as AI HMI -PFSS extrapolations. We examine low-energy (∼0.5 MeV/nucleon) Fe and O ion measurements obtained from Suprathermal Ion Telescope on Solar Terrestrial Relations Observatories and Ultra Low Energy Isotope Spectrometer on Advanced Composition Explorer. We found a moderate anticorrelation between the Fe/O ratios and the absolute longitudinal separation angles from the source regions to the spacecraft magnetic footpoints. Furthermore, we investigate the variations in Fe/O ratios with respect to the separation angle, grouped by the same and opposite polarity sectors of the SEP source regions. We found that the mean and median Fe/O values are higher in the same polarity group compared to the opposite polarity group, with the largest contrast at separation angles between 25° and 50°, where the values are approximately 3 times larger. The results imply that the enhanced Fe/O ratios in the examined gradual SEP events are likely associated with direct source regions, while the HCS affects particle transport.