A1 Refereed original research article in a scientific journal
The Major Solar Energetic Particle Event on 2024 May 20: Multispacecraft Observations of a Long-lasting Energetic Particle Reservoir
Authors: Lario, D.; Hu, J.; Balmaceda, L. A.; Kwon, R. Y.; Wallace, S.; da Silva, D. E.; Mays, M. L.; Krupar, V.; Carcaboso, F.; Warmuth, A.; Stiefel, M. Z.; Khoo, L. Y.; Richardson, I. G.; Kuhl, P.; Wimmer-Schweingruber, R. F.; Vainio, R.; Riihonen, E.; Liu, W.; Aran, A.; Paouris, E.; Kane, M. R.; Lee, C. O.; Sasso, C.; Arge, C. N.; Jones, S. I.; Verniero, J.; Niemela, A. E.; Horbury, T. S.; Maksimovic, M.
Publisher: Institute of Physics Publishing
Publication year: 2026
Journal: Astrophysical Journal
Article number: 260
Volume: 998
Issue: 2
ISSN: 0004-637X
eISSN: 1538-4357
DOI: https://doi.org/10.3847/1538-4357/ae2ea4
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://iopscience.iop.org/article/10.3847/1538-4357/ae2ea4
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/515847096
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
The fleet of spacecraft distributed across the inner heliosphere during 2024 May–June provided a unique opportunity to analyze, over a broad range of heliolongitudes, the effects produced by the complex sunspot group associated with Active Region 13664. The intense solar activity from 2024 May 8 to 2024 June 21 produced unusually elevated solar energetic particle (SEP) intensities observed at heliocentric distances ≲1 au by Solar Orbiter, Parker Solar Probe, STEREO-A, and near-Earth spacecraft. This activity resulted in a >40 day period of enhanced ≲10 MeV proton intensities spanning at least ∼170° in longitude. Among these events, the 2024 May 20 SEP event was remarkable for its high intensity and wide spatial extent. It was associated with a fast (≳1500 km s−1) halo coronal mass ejection (CME) and an X16.5 solar flare located ∼171° in longitude from Earth. Near Earth, the event produced a long-lasting ∼100 MeV proton intensity enhancement with energy spectra flattening between 30 and 80 MeV. It also led to the formation of an energetic particle reservoir that persisted for ∼2 weeks and extended across at least ∼170° in longitude. Several mechanisms contributed to the broad longitudinal spread of SEPs and formation of the energetic particle reservoir, including acceleration at an extended CME-driven shock, particle drifts along the heliospheric current sheet, particle reflection and redistribution caused by the merging of preceding CMEs beyond 1 au, and predominantly cross-field particle transport processes.
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We thank NASA and ESA for their support of the space missions whose data were used in this paper. Solar Orbiter is a mission of international cooperation between ESA and NASA, operated by ESA. SOHO is a mission of international cooperation between ESA and NASA, operated by NASA. STEREO is the third mission in NASA Solar Terrestrial Probes program. SDO is a mission of the NAS. Living With a Star Program. ACE is a mission of the NASA Explorer Program. Wind is a mission of the NASA Heliophysics System Observatory. PSP was designed, built, and is now operated by the Johns Hopkins Applied Physics Laboratory as part of NAS. Living with a Star (LWS) program (contract NNN06AA01C). Data used in this paper can be downloaded from https://soar.esac.esa.int/soar/, https://cdaweb.gsfc.nasa.gov/, https://stereo-ssc.nascom.nasa.gov/, and https://sdo.gsfc.nasa.gov/. The IS⊙IS data and visualization tools are available at https://spacephysics.princeton.edu/missions-instruments. We acknowledge use of the SOHO/LASCO CME catalog generated and maintained at the CDAW Data Center by NASA and The Catholic University of America in cooperation with the Naval Research Laboratory. We acknowledge use of the HELIO4CAST ICME catalog (ICMECAT) generated and maintained at the Austrian Space Weather Office (https://helioforecast.space/). Simulation results have been provided by the Community Coordinated Modeling Center (CCMC) at Goddard Space Flight Center through their publicly available simulation services (https://ccmc.gsfc.nasa.gov; ENLIL - David_Lario_051925_SH_2). The WSA model was developed by N. Arge now at NASA GSFC, and the ENLIL model was developed by D. Odstrcil now at GMU. D.L. and I.G.R. acknowledge support from NAS. Living With a Star (LWS) program NNH19ZDA001NLWS, the Guest Investigator Program NNH23ZDA001NHGIO (80NSSC24K0555), and NASA Space Weather Center of Excellence program under award 80NSSC23M0191 (CLEAR). D.L. acknowledges support from the Strategic Capability project under NASA grant 80NSSC22K0892 (SCEPTER). I.G.R. also acknowledges support from the ACE mission. Work at the University of Kiel was supported by the German Federal Ministry of Economic Affairs and Energy and the German Space Agency (Deutches Zentrum für Luft- und Raumfahrt, e.V., (DLR)), grant number 50OT2002. R.Y.K. acknowledges support by basic research funding from the Korea Astronomy and Space Science Institute (KASI; KASI2025185005). A.A. acknowledges the project PID2022-136828NB-C41 that received financial support from MICIU/AEI/10.13039/501100011033 and FEDER, UE; the support from grant CEX2024-001451-M funded by MICIU/AEI/10.13039/501100011033; and the Departament de Recerca i Universitats of Generalitat de Catalunya through grant 2021SGR 00679.
