University of Oulu

Poluianov, S., Kovaltsov, G., Usoskin, I. (2018) Solar energetic particles and galactic cosmic rays over millions of years as inferred from data on cosmogenic 26Al in lunar samples. Astronomy and Astrophysics, 618, A96. doi:10.1051/0004-6361/201833561

Solar energetic particles and galactic cosmic rays over millions of years as inferred from data on cosmogenic ²⁶Al in lunar samples

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Author: Poluianov, S.1,2; Kovaltsov, G. A.3; Usoskin, I. G.1,2
Organizations: 1Space Climate Research Unit, University of Oulu, Finland
2Sodankylä Geophysical Observatory, University of Oulu, Finland
3Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russia
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 1.7 MB)
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Language: English
Published: EDP Sciences, 2018
Publish Date: 2018-10-26


Aims: Lunar soil and rocks are not protected by a magnetic field or an atmosphere and are continuously irradiated by energetic particles that can produce cosmogenic radioisotopes directly inside rocks at different depths depending on the particle’s energy. This allows the mean fluxes of solar and galactic cosmic rays to be assessed on the very long timescales of millions of years.

Methods: Here we show that lunar rocks can serve as a very good particle integral spectrometer in the energy range 20–80 MeV. We have developed a new method based on precise modeling, that is applied to measurements of ²⁶Al (half-life ≈0.7 megayears) in lunar samples from the Apollo mission, and present the first direct reconstruction (i.e., without any a priori assumptions) of the mean energy spectrum of solar and galactic energetic particles over a million of years.

Results: We show that the reconstructed spectrum of solar energetic particles is totally consistent with that over the last decades, despite the very different levels of solar modulation of galactic cosmic rays (ϕ = 496 ± 40 MV over a million years versus (ϕ = 660 ± 20 MV for the modern epoch). We also estimated the occurrence probability of extreme solar events and argue that no events with the F(>30 MeV) fluence exceeding 5×10¹⁰ and 10¹¹ cm−2 are expected on timescales of a thousand and million years, respectively.

Conclusions: We conclude that the mean flux of solar energetic particles hardly depends on the level of solar activity, in contrast to the solar modulation of galactic cosmic rays. This puts new observational constraints on solar physics and becomes important for assessing radiation hazards for the planned space missions.

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Series: Astronomy and astrophysics
ISSN: 0004-6361
ISSN-E: 1432-0746
ISSN-L: 0004-6361
Volume: 618
Article number: A96
DOI: 10.1051/0004-6361/201833561
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: This work was supported by the Center of Excellence ReSoLVE (Project 272157) of the Academy of Finland.
Academy of Finland Grant Number: 272157
Detailed Information: 272157 (Academy of Finland Funding decision)
Copyright information: © ESO 2018. Published in this repository with the kind permission of the publisher.