University of Oulu

Usoskin, I. G., Koldobskiy, S. A., Poluianov, S. V., Raukunen, O., Vainio, R., & Kovaltsov, G. A. (2023). Consistency of the average flux of solar energetic particles over timescales of years to megayears. Astronomy & Astrophysics, 670, L22.

Consistency of the average flux of solar energetic particles over timescales of years to megayears

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Author: Usoskin, I. G.1; Koldobskiy, S. A.1; Poluianov, S. V.1;
Organizations: 1Space Physics and Astronomy Research Unit and Sodankylä Geophysical Observatory, University of Oulu, 90014 Oulu, Finland
2Space Research Laboratory, University of Turku, Turku, Finland
3Aboa Space Research Oy, Turku, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 0.7 MB)
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Language: English
Published: EDP Sciences, 2023
Publish Date: 2023-03-20


Aims: Solar energetic particles (SEPs) have been measured directly in space over the past decades. Rare extreme SEP events are studied based on terrestrial cosmogenic proxy data for the past ten millennia. Lunar rocks record the average SEP fluxes on the megayear timescale. The question of whether the SEP fluxes averaged over different timescales are mutually consistent is still open. Here we analyze these different datasets for mutual consistency.

Methods: Using the data from directly measured SEPs over the past decades and reconstructions of extreme SEP events in the past, we built a distribution function of the occurrence of annual SEP fluences for SEPs with energies above 30, 60, 100, and 200 MeV. The distribution function was fit with the Weibull and other types of distributions, and the long-term average SEP flux was computed and compared with the megayear SEP flux estimated from lunar data.

Results: In contrast to the current paradigm, the direct space-era data are not representative of the long-term averaged SEP flux because they are only 20–55% of it, while the major fraction was formed by rare extreme SEP events in the past. The combined statistics of direct and proxy data are fully consistent with megayear lunar data, implying that our knowledge of the whole range of the SEP fluxes, from frequent weak to rare extreme events, is now consistent.

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Series: Astronomy and astrophysics
ISSN: 0004-6361
ISSN-E: 1432-0746
ISSN-L: 0004-6361
Volume: 670
Article number: L22
DOI: 10.1051/0004-6361/202245810
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: This work was partly supported by the Academy of Finland (Projects ESPERA no. 321882, QUASARE no. 330064 and FORESAIL no. 336809), University of Oulu (Project SARPEDON). The ISSI Team #510 (Solar Extreme Events: Setting up a Paradigm, led by F. Miyake and I. Usoskin) is acknowledged for stimulating discussions.
Academy of Finland Grant Number: 321882
Detailed Information: 321882 (Academy of Finland Funding decision)
330064 (Academy of Finland Funding decision)
Copyright information: © The Authors 2023. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.