Puzzling Variation of Gamma Rays from the Sun over the Solar Cycle Revealed with Fermi-LAT

Acharyya A, Adelfio A, Ajello M, Baldini L, Bartolini C, Bastieri D, Becerra Gonzalez J, Bellazzini R, Berenji B, Bissaldi E, Blandford RD, Bonino R, Bottacini E, Buson S, Cameron RA, Caraveo PA, Casaburo F, Casini F, Cavazzuti E, Cerasole D, Cibrario N, Ciprini S, Cozzolongo G, Cristarella Orestano P, Cuna F, Cuoco A, Cutini S, D’Ammando F, Depalo D, Digel SW, Di Lalla N, Di Venere L, Domínguez A, Fiori A, Fukazawa Y, Fusco P, Gargano F, Gasbarra C, Gasparrini D, Germani S, Giacchino F, Giglietto N, Giliberti M, Giordano F, Giroletti M, Guiriec S, Gupta R, Hashizume M, Hays E, Hewitt JW, Holzmann Airasca A, Horan D, Hou X, Kayanoki T, Kuss M, Larsson S, Laviron A, Li J, Liguori A, Liodakis I, Loizzo P, Longo F, Loparco F, López Pérez S, Lorusso L, Lovellette MN, Lubrano P, Maldera S, Manfreda A, Martí-Devesa G, Martinelli R, Mazziotta MN, McEnery JE, Mereu I, Michailidis M, Michelson PF, Mirabal N, Mizuno T, Monti-Guarnieri P, Monzani ME, Morselli A, Moskalenko IV, Negro M, Omodei N, Orlando E, Ormes JF, Paneque D, Panzarini G, Persic M, Pesce-Rollins M, Petrosian V, Pillera R, Principe G, Rainò S, Rando R, Rani B, Razzano M, Reimer A, Reimer O, Sánchez-Conde M, Saz Parkinson PM, Serini D, Sgrò C, Siskind EJ, Spinelli P, Tak D, Tibaldo L, Torres DF, Valverde J, Wadiasingh Z, Zhang W (2025)

Publication Type: Journal article

Publication year: 2025

Journal

Book Volume: 989

Article Number: L16

Journal Issue: 1

DOI: 10.3847/2041-8213/adef4d

Abstract

The steady-state gamma-ray emission from the Sun is thought to consist of two emission components due to interactions with Galactic cosmic rays: (1) a hadronic disk component, and (2) a leptonic extended component peaking at the solar edge and extending into the heliosphere. The flux of these components is expected to vary with the 11 yr solar cycle, being highest during solar minimum and lowest during solar maximum, as it varies with the cosmic-ray flux. No study has yet analyzed the flux variation of each component over solar cycles. In this work, we measure the temporal variations of the flux of each component over 15 yr of Fermi Large Area Telescope observations and compare them with the sunspot number and Galactic cosmic-ray flux from AMS-02 near Earth. We find that the flux variation of the disk anticorrelates with the sunspot number and correlates with cosmic-ray protons, as expected, confirming its emission mechanism. In contrast, the extended component exhibits a more complex variation: despite an initial anticorrelation with the sunspot number, we find neither anticorrelation with the sunspot number nor correlation with cosmic-ray electrons over the full 15 yr period. This most likely suggests that cosmic-ray transport and modulation in the inner heliosphere are unexpectedly complex and may differ for electrons and protons or, alternatively, that there is an additional, unknown component of gamma rays or cosmic rays. These findings impact space weather research and emphasize the need for close monitoring of Cycle 25 and the ongoing polarity reversal.

Authors with CRIS profile

Involved external institutions

University of Southern Denmark / Syddansk Universitet Denmark (DK) National Institute for Nuclear Physics / Istituto Nazionale di Fisica Nucleare (INFN) Italy (IT) Clemson University United States (USA) (US) University of Pisa / Università di Pisa (UniPi) Italy (IT) Universidad de La Laguna Spain (ES) California State University, Los Angeles United States (USA) (US) Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) United States (USA) (US) Deutsches Elektronen-Synchrotron DESY Germany (DE) National Institute for Astrophysics / Istituto Nazionale Astrofisica (INAF) Italy (IT) Italian Space Agency / Agenzia Spaziale Italiana (ASI) Italy (IT) Hiroshima University / 広島大学 Japan (JP) National Aeronautics and Space Administration (NASA) United States (USA) (US) University of North Florida (UNF) United States (USA) (US) Yunnan Observatories China (CN) Università degli Studi di Trieste Italy (IT) University of Denver United States (USA) (US) Max-Planck-Institut für Physik (Werner-Heisenberg-Institut, MPP) / Max Planck Institute for Physics Germany (DE) Universidad Autónoma de Madrid (UAM) Spain (ES) Santa Cruz Institute for Particle Physics United States (USA) (US) University of Padua / Universita degli Studi di Padova Italy (IT) Università degli Studi di Perugia Italy (IT) Universidad Complutense de Madrid (UCM) Spain (ES) Columbian College of Arts and Sciences United States (USA) (US) Laboratoire Leprince-Ringuet (LLR) France (FR) University of Science and Technology of China (USTC) China (CN) Foundation for Research and Technology-Hellas (FORTH) / Ίδρυμα Τεχνολογίας και Έρευνας Greece (GR) The Aerospace Corporation United States (USA) (US) Louisiana State University United States (USA) (US) Leopold-Franzens-Universität Innsbruck / University of Innsbruck Austria (AT) NYCB Real-Time Computing, Inc. United States (USA) (US) Seoul National University (SNU) / 서울대학교 Korea, Republic of (KR) Institució Catalana de Recerca i Estudis Avançats (ICREA) / Catalan Institution for Research and Advanced Studies Spain (ES) Institute of Space Sciences (IEEC-CSIC) Spain (ES) Royal Institute of Technology / Kungliga Tekniska Högskolan (KTH) Sweden (SE) Institut de Recherche en Astrophysique et Planétologie (IRAP) France (FR)

How to cite

APA:

Acharyya, A., Adelfio, A., Ajello, M., Baldini, L., Bartolini, C., Bastieri, D.,... Zhang, W. (2025). Puzzling Variation of Gamma Rays from the Sun over the Solar Cycle Revealed with Fermi-LAT. Astrophysical Journal Letters, 989(1). https://doi.org/10.3847/2041-8213/adef4d

MLA:

Acharyya, A., et al. "Puzzling Variation of Gamma Rays from the Sun over the Solar Cycle Revealed with Fermi-LAT." Astrophysical Journal Letters 989.1 (2025).

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