Longitudinal Dependence of Heavy Ion Composition in the 2021 October 28 Ground Level Enhancement Event

The 2021 October 28 solar energetic particle (SEP) event was a rare ground level enhancement (GLE) event, where secondary particles from the interactions of SEPs with the Earth’s atmosphere were detected by neutron monitors on the ground. A number of papers have examined the solar signatures, neutron monitor observations, and the characteristics of the SEP protons and electrons for this event. Here we describe the heavy ion signatures, specifically O and Fe, observed by multiple spacecraft. Parker Solar Probe, Solar Terrestrial Relations Observatory-Ahead, and Advanced Composition Explorer were distributed over nearly 60° in solar longitude and 0.4 au in heliocentric distance. Despite their separations, all three spacecraft measured event-integrated O and Fe spectra, well represented by power laws, with nearly the same power-law index of approximately −1.7, which is significantly harder than most large SEP events and many GLE events. Moreover, the Fe/O abundance ratio determined from these spectra was also found to be spatially invariant over the 60° in longitude and 0.4 au in heliocentric distance. Such near uniformity is highly unusual, and only one similar occurrence was found in a previous multispacecraft. The observed Fe/O ratio of 0.39 is higher than typical for large SEP events but not unusual for GLE events.