The Relationship between Characteristics of Gradual Solar Energetic Particle Events and their Event-integrated Fe/O Ratios

Abstract Based on multiple-spacecraft observations, we select 157 gradual solar energetic particle (SEP) events spanning from 2007 January to 2017 October. These SEP events are classified into Fe-rich and Fe-poor events in accordance with their event-integrated Fe/O ratio (relative to the coronal reference value of 0.131). A comparative analysis is performed to investigate the correlationship between SEP properties and the associated solar eruptions for these two classes of events, aiming to reveal the impacts of different seed populations on SEP events. The main conclusions are as follows: (1) Fe-rich events are generally associated with coronal mass ejections (CMEs) that have lower speed, mass, kinetic energy, and lower-class flares, along with lower peak fluxes, in comparison to Fe-poor events. Moreover, there is no significant correlation between the peak intensities of Fe-rich events and CME speeds. In contrast, the peak intensities of Fe-poor events exhibit a strong positive correlation with CME speeds, and they encompass nearly all (96%) extremely large SEP events. (2) The pre-CMEs (preceding CMEs occurring within 24 hr prior to the primary CME associated with SEP event) associated with both Fe-rich and Fe-poor events show no significant difference. However, the occurrence rate of Fe-rich events slightly increases with more frequent pre-CME eruptions. The occurrence rates of the two classes of SEP events associated with different type-II radio bursts exhibit distinct differences: the occurrence rate of Fe-rich events decreases progressively from the metric (m), to the decameter-hectometric (DH), and then to the kilometric (km) wavebands. This may imply that the dominant seed particles accelerated by the shock have undergone changes at different altitudes, for instance, from flare materials with a high Fe/O ratio to coronal/solar wind materials with a low Fe/O ratio. (3) The two classes of SEP events display distinct longitudinal distributions and east-west asymmetries. The occurrence rate of Fe-rich events reaches their maxima within the relative longitudinal range of 30°–90° indicating the degrees of the source westward of the magnetic footpoint, which is particularly pronounced in the case of medium- or low-speed CMEs. At various relative longitudes, the peak intensities of the two classes of SEP events also show statistically significant differences, with the Fe-poor event typically exhibiting a notably higher intensity compared to the Fe-rich event.