Authors: Antonio E. Niemela (UMBC - NASA/GSFC), David Lario (NASA/GSFC) and Nicolas Wijsen (KU Leuven)

Extreme Solar Energetic Particle (SEP) events are rare, but their radiation hazard to astronauts and space missions is unmatched. They tend to occur during intense period of solar activity when one or more active regions produce multiple coronal mass ejections (CMEs). Several fundamental questions lie at the heart of understanding these events: how do preceding magnetized structures such as CMEs modify SEP transport and acceleration? What role do preceding CMEs play in enhancing particle acceleration? Can a preceding SEP event supply the pre-accelerated seed population that allows particles to reach their highest energies? Are these sequential eruptions a necessary condition for producing truly extreme SEP events? The SEP event of 9 October 2024 offers a compelling case study. Reaching a peak flux of approximately 1800 pfu in protons above 10 MeV as measured by GOES, it stands as one of the most intense radiation storms of Solar Cycle 25, second only to the January 2026 MLK Day event. Crucially, the heliospheric conditions leading up to this event were far from pristine: a series of preceding CMEs and a weaker SEP event observed at the Sun-Earth Lagrangian L1 point on 7 October had already preconditioned the interplanetary medium, raising the possibility that these structures played a decisive role in the event’s exceptional intensity.

To address these questions, we employ the EUHFORIA-PARADISE modeling chain, using the advanced flux rope CME model FRi3D to reconstruct both the preceding and the main CME  on 7 and 9 October 2024. The modeled solar wind shows good agreement with multispacecraft in situ observations, which also reveal strikingly different energetic particle profiles for each of the two SEP events, motivating a systematic exploration of different combinations of injection spectra at each CME-driven shock. This allows us to disentangle the respective contributions of the preceding and the main eruptions to the final observed SEP event. We further investigate the effects of CME-CME interaction on particle acceleration, exploring whether this interaction is a key contributor to the exceptional intensities that characterize this event and, more broadly, whether it represents a necessary ingredient for producing truly extreme SEP events.