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
|Author:||Poluianov, S.1,2; Kovaltsov, G. A.3; Usoskin, I. G.1,2|
1Space Climate Research Unit, University of Oulu, Finland
2Sodankylä Geophysical Observatory, University of Oulu, Finland
3Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russia
|Online Access:||PDF Full Text (PDF, 1.7 MB)|
|Persistent link:|| http://urn.fi/urn:nbn:fi-fe2018102638852
|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.
Astronomy and astrophysics
|Type of Publication:||
A1 Journal article – refereed
|Field of Science:||
115 Astronomy and space science
This work was supported by the Center of Excellence ReSoLVE (Project 272157) of the Academy of Finland.
|Academy of Finland Grant Number:||
272157 (Academy of Finland Funding decision)
© ESO 2018. Published in this repository with the kind permission of the publisher.