Presentation #201.06 in the session Suprathermal Particles and their Importance to Understanding Energetic Particles.
The large solar energetic particle (SEP) event on 28 October 2021 was the first ground level enhancement (GLE) event of solar cycle 25, having been detected by a number of ground-based neutron monitors. At the time of the event, Parker Solar Probe (PSP), Solar Orbiter, and STEREO-A (STA) were located within ~60° of near-Earth spacecraft (e.g., ACE) and all observed the SEP event. Despite the radial and longitudinal distribution of the spacecraft, PSP, STA, and ACE measured power law spectra for O and Fe with the same spectral index and identical and constant Fe/O abundance ratios over approximately an order of magnitude in energy. This striking uniformity of the composition and heavy ion spectra might be a result of low-variability in the shock parameters and/or in the seed particle population. The SEP observations will be used to test the results of modeling the shock driven by the coronal mass ejection and the upstream solar wind conditions it propagates into.