Lario, David ; Richardson, Ian G ; Hu, Junxiang ; ManyOtherAuthors, X ; Berghmans, David ; Maksimovic, Milan

Poster presented at AGU Annual Meeting, New Orleans LA on 2025-12-16

Abstract: During the intense period of solar activity in May-June 2024, the fleet of spacecraft distributed throughout the inner heliosphere detected a series of large Solar Energetic Particle (SEP) events. Among them, the SEP event on 2024 May 20 stood out due to its high intensity and broad spatial extent. It was associated with an X16.5 solar flare located 170 degrees from Earth as observed by instruments on Solar Orbiter. A long-lasting, high-energy ($\sim$100 MeV) proton event with a hard-energy spectrum flattening over the $\sim$30-80 MeV energy range was even detected by spacecraft such as SOHO and STEREO-A, which were positioned nearly diametrically opposite to the flare site. This event led to the formation of a long-lasting ($\sim$2 weeks) high-energy particle reservoir extending over at least 170 degrees in longitude as observed by Solar Orbiter, Parker Solar Probe, and near-Earth spacecraft. Notably, this SEP event constituted the most intense Mars ground-level event recorded to date by the Radiation Assessment Detector on the Mars Science Laboratory (MSL/RAD; Posner et al., 2025). We will present a detailed analysis of this event, including its solar origin, the associated coronal mass ejection (CME), the interplanetary environment where the event occurred, and results of modeling the SEP injection at the CME-driven shock and subsequent SEP transport in the inner heliosphere. The arrival of $>$100 MeV protons at Earth, along with the observation of the long-lasting, widespread particle reservoir with flat high-energy spectra, make this event an excellent case study to investigate the key mechanisms behind the spread of SEPs in the inner heliosphere, including particle acceleration and injection from a broad CME-driven shock, efficient cross-field transport, and particle reflection and redistribution by the merging of preceding CMEs located beyond 1 AU.

Keyword(s): Solar Orbiter ; EUI ; SEP
Note: SH23D-2603
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