The role of heliospheric current sheet effect on solar energetic particles at the injection stage

We investigate Fe/O enhancements in the injection stage for 37 large gradual solar energetic particles (SEPs) events from 2010 to 2014 and examine the associations of the Fe/O enhancements with the structures of the heliospheric current sheet (HCS). For this study we use STEREO SIT Fe and O data in 0.32-0.45 MeV/nucleon channel as well as ACE ULEIS Fe and O data in 0.32-0.64 MeV/nucleon channel. We determine the magnetic footpoints of spacecraft using the potential field source surface model of the coronal field with Parker spiral approximation of interplanetary magnetic field. We also use the magnetic polarity data from in-situ measurements of STEREO and ACE. In our data set the mean of the Fe/O values is 0.59 and the highest ones are seen between 2012 and 2013. There is a weak anti-correlation between the Fe/O values and the absolute values of the longitudinal separation angles from source regions to spacecraft magnetic footpoints. In particular, the mean and the median values of the Fe/O enhancements are 0.84 and 0.61 where the absolute longitudinal separation angles are within 50°, which are larger than a typical value of gradual events. It is noted that the high Fe/O enhancements are mostly observed where their footpoints lie in the same polarity regions of the SEP sources rather than the opposite polarity regions as well as they are associated with strong flare peak fluxes. Although the Fe/O enhancements caused by a transport effect could not be ruled out, our result implies that the HCS likely affect the flare contribution in the injection stage of the gradual SEP event.