Solar energetic particle (SEP) events exhibit complex variability, making their statistical characterization essential for constraining physical models and accurately modeling SEP dynamics. SEP transport is commonly modeled as a diffusive process. However, recent studies reveal persistent correlations in SEP propagation, suggesting that fractional Brownian motion (fBm) provides a suitable framework for representing nondiffusive transport because it accounts for correlated increments. In this work, we perform the first detailed evaluation of whether SEP flux time series conform to the fBm model, using IS\u2299IS data from the Parker Solar Probe mission. In particular, we test three defining properties of fBm: Gaussianity and stationarity of flux increments, and power-law scaling of the structure function. Our analysis of Low-Energy Telescope measurements across SEP phases and energy channels reveals phase- and energy-dependent behavior inconsistent with fBm, including nonlinear drift, volatility clustering, and heavy-tailed distributions. Consequently, stochastic generators based on fBm cannot reliably reproduce SEP fluxes. While structure functions exhibit power-law scaling with Hurst exponents H < 0.5, indicating anticorrelated increments, H varies with phase and energy without a consistent trend. Overall, SEP fluxes behave as stochastic processes with self-similarity, non-Gaussian statistics, and intermittent volatility—yet a unified statistical model that integrates these properties remains elusive.
", "doi": "10.3847/1538-4357/ae5ba1", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "2026-05-01", "series_number": "1", "volume": "1002", "issue": "1", "pages": "112" }, { "id": "authors:gx2yj-99f03", "collection": "authors", "collection_id": "gx2yj-99f03", "cite_using_url": "https://authors.library.caltech.edu/records/gx2yj-99f03", "type": "article", "title": "The Effect of Large-scale Magnetic Fluctuations on Energetic Particle Profiles across Interplanetary Shocks", "author": [ { "family_name": "Lo", "given_name": "Ming-Yuan", "orcid": "0009-0009-4551-376X" }, { "family_name": "Guo", "given_name": "Fan", "orcid": "0000-0003-4315-3755" }, { "family_name": "Giacalone", "given_name": "Joe", "orcid": "0000-0002-0850-4233" }, { "family_name": "Zhao", "given_name": "Lulu", "orcid": "0000-0003-3936-5288" }, { "family_name": "Lario", "given_name": "David", "orcid": "0000-0002-3176-8704" }, { "family_name": "Koban", "given_name": "Gergely", "orcid": "0009-0006-2646-1501" }, { "family_name": "Ho", "given_name": "George C.", "orcid": "0000-0003-1093-2066" }, { "family_name": "Cohen", "given_name": "Christina M. S.", "orcid": "0000-0002-0978-8127", "clpid": "Cohen-Christina-M" } ], "abstract": "In situ observations of energetic particles associated with interplanetary (IP) shocks, known as energetic storm particle (ESP) events, often show time-intensity profiles quite different from predictions of classical diffusive shock acceleration theory. We use numerical simulations, including test-particle simulations and hybrid simulations (with fluid electrons and kinetic protons) for shocks propagating into a turbulent magnetic field to study the ESP intensity-time profiles across a strong IP shock. We find that several types of energetic particle intensity profiles similar to in situ observations can be produced in our simulations. The peak of energetic particle count is often near, but can be shifted from, the locations of the shock front, with the peak usually being observed downstream of the shock. These findings may help understand particle acceleration at both traveling IP shocks and shocks in other heliospheric and astrophysical environments.
", "doi": "10.3847/1538-4357/ae5523", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "2026-05-01", "series_number": "1", "volume": "1002", "issue": "1", "pages": "10" }, { "id": "authors:mr0qs-j7e53", "collection": "authors", "collection_id": "mr0qs-j7e53", "cite_using_url": "https://authors.library.caltech.edu/records/mr0qs-j7e53", "type": "article", "title": "Proton and Heavy Ion Acceleration by Magnetic Reconnection at the Near-Sun Heliospheric Current Sheet", "author": [ { "family_name": "Desai", "given_name": "M. I.", "orcid": "0000-0002-7318-6008" }, { "family_name": "Drake", "given_name": "J. F.", "orcid": "0000-0002-9150-1841" }, { "family_name": "Swisdak", "given_name": "M.", "orcid": "0000-0001-9898-464X" }, { "family_name": "Fitzmaurice", "given_name": "A.", "orcid": "0009-0001-9646-9668" }, { "family_name": "McComas", "given_name": "D. J.", "orcid": "0000-0001-6160-1158" }, { "family_name": "Bale", "given_name": "S. D.", "orcid": "0000-0002-1989-3596" }, { "family_name": "Phan", "given_name": "T.", "orcid": "0000-0002-6924-9408" }, { "family_name": "Berland", "given_name": "G.", "orcid": "0000-0001-6010-6374" }, { "family_name": "Mitchell", "given_name": "D. G.", "orcid": "0000-0003-1960-2119" }, { "family_name": "Cohen", "given_name": "C. M. S.", "orcid": "0000-0002-0978-8127", "clpid": "Cohen-Christina-M" }, { "family_name": "Hill", "given_name": "M. E.", "orcid": "0000-0002-5674-4936" }, { "family_name": "Christian", "given_name": "E. R.", "orcid": "0000-0003-2134-3937" }, { "family_name": "Schwadron", "given_name": "N. A.", "orcid": "0000-0002-3737-9283" }, { "family_name": "McNutt", "given_name": "R. L.", "orcid": "0000-0002-4722-9166" }, { "family_name": "Matthaeus", "given_name": "W. H.", "orcid": "0000-0001-7224-6024" }, { "family_name": "Rahmati", "given_name": "A.", "orcid": "0000-0003-0519-6498" }, { "family_name": "Whittlesey", "given_name": "P.", "orcid": "0000-0002-7287-5098" }, { "family_name": "Livi", "given_name": "R.", "orcid": "0000-0002-0396-0547" }, { "family_name": "Larson", "given_name": "D.", "orcid": "0000-0001-5030-6030" } ], "abstract": "Magnetic reconnection at the near-Sun heliospheric current sheet (HCS) dissipates the Parker spiral and converts magnetic energy into plasma kinetic energy. During Encounter 14 at a radial distance of ~16.25 R\u2299, Parker Solar Probe observed an HCS crossing where reconnection-driven acceleration—likely facilitated by merging large-scale flux tubes—energized protons up to ∼400 keV. This energy gain is ≈1000 times greater than the available magnetic energy per particle. We present here a comprehensive analysis of pitch-angle distributions and differential energy spectra for protons and heavy ions (He, O, and Fe) in conjunction with local wave activity during this crossing. Our results provide the first direct in situ observations of simultaneous proton and heavy-ion energization during HCS reconnection. Crucially, we find that heavy-ion power-law spectral indices differ significantly from those of protons, contradicting previous simulations that predict species-independent slopes. We further demonstrate that ion beams and anisotropies produced during reconnection drive waves in the ion cyclotron range of frequencies. Finally, we show that proton pitch-angle scattering is stronger than that of heavy ions, which may account for the flatter spectra or harder spectral indices observed in the heavy-ion populations. These observations provide definitive evidence for in situ reconnection-driven acceleration at the near-Sun HCS and necessitate the inclusion of species-dependent transport and acceleration efficiencies in contemporary reconnection-based particle energization models.
", "doi": "10.3847/1538-4357/ae48f2", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "2026-04-01", "series_number": "2", "volume": "1000", "issue": "2", "pages": "300" }, { "id": "authors:pdxtg-n0e58", "collection": "authors", "collection_id": "pdxtg-n0e58", "cite_using_url": "https://authors.library.caltech.edu/records/pdxtg-n0e58", "type": "article", "title": "The High-Energy Ion Telescope (HIT) for the Interstellar Mapping And Acceleration Probe (IMAP) Mission", "author": [ { "family_name": "Christian", "given_name": "E. R.", "orcid": "0000-0003-2134-3937", "clpid": "Christian-E-R" }, { "family_name": "Mitchell", "given_name": "J. G.", "clpid": "Mitchell-J-G" }, { "family_name": "Bruno", "given_name": "A.", "clpid": "Bruno-A" }, { "family_name": "Albaijes", "given_name": "D.", "clpid": "Albaijes-D" }, { "family_name": "Bailey\u00a0Newman", "given_name": "S.", "clpid": "Bailey-Newman-S" }, { "family_name": "Boggs", "given_name": "R. C.", "clpid": "Boggs-R-C" }, { "family_name": "Choi", "given_name": "M. K.", "clpid": "Choi-M-K" }, { "family_name": "Cohen", "given_name": "C. M. S.", "orcid": "0000-0002-0978-8127", "clpid": "Cohen-Christina-M" }, { "family_name": "Cook", "given_name": "W. R.", "clpid": "Cook-Walter-Richardson-III" }, { "family_name": "Darwish", "given_name": "Y. S.", "clpid": "Darwish-Y-S" }, { "family_name": "Davis", "given_name": "A. J.", "orcid": "0000-0002-9922-8915", "clpid": "Davis-Andrew-J" }, { "family_name": "de Nolfo", "given_name": "G. A.", "clpid": "de-Nolfo-G-A" }, { "family_name": "Dumonthier", "given_name": "J. J.", "clpid": "Dumonthier-J-J" }, { "family_name": "Garnett", "given_name": "R. D.", "clpid": "Garnett-R-D" }, { "family_name": "Gkioulidou", "given_name": "M.", "clpid": "Gkioulidou-M" }, { "family_name": "Gregory", "given_name": "K. J.", "clpid": "Gregory-K-J" }, { "family_name": "Guzman\u00a0Garcia", "given_name": "D.", "clpid": "Guzman--Garcia-D" }, { "family_name": "Haas", "given_name": "J. P.", "clpid": "Haas-J-P" }, { "family_name": "Hollenhorst", "given_name": "C.", "clpid": "Hollenhorst-C" }, { "family_name": "Jeunon", "given_name": "M.", "clpid": "Jeunon-M" }, { "family_name": "Kaiser", "given_name": "M. A.", "clpid": "Kaiser-M-A" }, { "family_name": "Kanekal", "given_name": "S. G.", "clpid": "Kanekal-S-G" }, { "family_name": "Kecman", "given_name": "B.", "clpid": "Kecman-Branislav" }, { "family_name": "Leske", "given_name": "Richard A.", "orcid": "0000-0002-0156-2414", "clpid": "Leske-R-A" }, { "family_name": "Letzer", "given_name": "J. D.", "clpid": "Letzer-J-D" }, { "family_name": "Liceaga\u00a0Indart", "given_name": "I.", "clpid": "Liceaga-Indart-I" }, { "family_name": "McComas", "given_name": "D. J.", "clpid": "McComas-D-J" }, { "family_name": "McNeel", "given_name": "B.", "clpid": "McNeel-B" }, { "family_name": "Muro", "given_name": "G. D.", "orcid": "0000-0003-0581-1278", "clpid": "Muro-Gabriel-D" }, { "family_name": "Nolan", "given_name": "J. T.", "clpid": "Nolan-J-T" }, { "family_name": "Ogindo", "given_name": "M.", "clpid": "Ogindo-M" }, { "family_name": "Petrowich", "given_name": "P.", "clpid": "Petrowich-P" }, { "family_name": "Reaves", "given_name": "W. J.", "clpid": "Reaves-W-J" }, { "family_name": "Rosnack", "given_name": "T. P.", "clpid": "Rosnack-T-P" }, { "family_name": "Saghafi", "given_name": "N.", "clpid": "Saghafi-N" }, { "family_name": "Salerno", "given_name": "C. L.", "clpid": "Salerno-C-L" }, { "family_name": "Schwadron", "given_name": "N. A.", "clpid": "Schwadron-N-A" }, { "family_name": "Silber", "given_name": "M. A.", "clpid": "Silber-M-A" }, { "family_name": "Smith", "given_name": "E. S.", "clpid": "Smith-E-S" }, { "family_name": "Smith", "given_name": "M. T.", "clpid": "Smith-M-T" }, { "family_name": "Su\u00e1rez", "given_name": "G.", "clpid": "Su\u00e1rez-G" }, { "family_name": "Tatoli", "given_name": "T.", "clpid": "Tatoli-T" }, { "family_name": "Tiu", "given_name": "C. P.", "clpid": "Tiu-C-P" }, { "family_name": "Wiedenbeck", "given_name": "Mark Edward", "orcid": "0000-0002-2825-3128", "clpid": "Wiedenbeck-M-E" }, { "family_name": "Windhausen", "given_name": "M. H.", "clpid": "Windhausen-M-H" }, { "family_name": "Xu", "given_name": "Z.", "orcid": "0000-0002-9246-996X", "clpid": "Xu-Zigong" } ], "abstract": "The Interstellar Mapping and Acceleration Probe (IMAP; McComas et al. in Space Sci Rev 214:116, 2018a; McComas et al. in Space Sci Rev 221:100, 2025) is a NASA mission designed to study two of the most important issues in heliophysics: the interaction of the solar wind with the local interstellar medium and the acceleration of energetic particles. These two puzzles are surprisingly intertwined because particle acceleration in the inner heliosphere plays a large role in the interactions at the edge of the heliosphere. The ten instruments on IMAP, situated near the Earth-Sun L1 Lagrange point, include both in situ and remote-sensing observations and are designed to work together to explore these questions. The High-energy Ion Telescope (HIT), described herein, measures in situ ions from hydrogen through nickel at energies ranging from a few MeV/nucleon to some tens of MeV/nucleon, depending upon species. While the primary focus of HIT is Solar Energetic Particles (SEPs), HIT is also sensitive to Anomalous Cosmic Rays (ACRs) and Galactic Cosmic Rays (GCRs) that are also present at these energies. Working with the other IMAP instruments, HIT studies how the variation in SEP intensities, spectra, anisotropies, and composition inform the acceleration processes that generate these particles, and, in turn, how these high-energy particles affect the outer heliosphere. This paper details the design and operation of the HIT instrument.
", "doi": "10.1007/s11214-026-01279-6", "issn": "0038-6308", "publisher": "Springer Science and Business Media LLC", "publication": "Space Science Reviews", "publication_date": "2026-02-27", "volume": "222", "pages": "23" }, { "id": "authors:gh0bd-sme31", "collection": "authors", "collection_id": "gh0bd-sme31", "cite_using_url": "https://authors.library.caltech.edu/records/gh0bd-sme31", "type": "article", "title": "Two Phases of Particle Acceleration of a Solar Flare Associated with In Situ Energetic Particles", "author": [ { "family_name": "Wang \u738b", "given_name": "Meiqi \u7f8e\u797a", "orcid": "0000-0002-2633-3562" }, { "family_name": "Chen \u9648", "given_name": "Bin \u5f6c", "orcid": "0000-0002-0660-3350" }, { "family_name": "Knuth", "given_name": "Trevor", "orcid": "0000-0002-4795-7059" }, { "family_name": "Cohen", "given_name": "Christina", "orcid": "0000-0002-0978-8127", "clpid": "Cohen-Christina-M" }, { "family_name": "Lee", "given_name": "Jeongwoo", "orcid": "0000-0002-5865-7924" }, { "family_name": "Wang", "given_name": "Haimin", "orcid": "0000-0002-5233-565X" }, { "family_name": "Yu", "given_name": "Sijie", "orcid": "0000-0003-2872-2614" } ], "abstract": "How impulsive solar energetic particle (SEP) events are produced by magnetic-reconnection-driven processes during solar flares remains an outstanding question. Here we report a short-duration SEP event associated with an X-class eruptive flare on 2021 July 3, using a combination of remote sensing observations and in situ measurements. The in situ SEPs were recorded by multiple spacecraft including the Parker Solar Probe. The hard X-ray (HXR) light curve exhibits two impulsive periods. The first period is characterized by a single peak with a rapid rise and decay, while the second period features a more gradual HXR light curve with a harder spectrum. Such observation is consistent with in situ measurements: the energetic electrons were first released during the early impulsive phase when the eruption was initiated. The more energetic in situ electrons were released several minutes later during the second period of the impulsive phase when the eruption was well underway. This second period of energetic electron acceleration also coincides with the release of in situ energetic protons and the onset of an interplanetary type III radio burst. We conclude that these multimessenger observations favor a two-phase particle acceleration scenario: the first, less energetic electron population was produced during the initial reconnection that triggers the flare eruption, and the second, more energetic electron population was accelerated in the region above the loop-top below a well-developed, large-scale reconnection current sheet induced by the eruption.
", "doi": "10.3847/1538-4357/adbdd0", "issn": "0004-637X", "publisher": "American Astronomical Society", "publication": "Astrophysical Journal", "publication_date": "2025-04-10", "series_number": "1", "volume": "983", "issue": "1", "pages": "33" } ]