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

Abstract

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.

No signature of extreme solar energetic particle events in high-precision 14C data from the Alaskan tree for 1844–1876 CE

“…Geomagnetic Cliver et al, 2020Cliver et al, , 2022 1.1 ± 0.3 Â 10 10 ( Usoskin et al, 2023) occurs in the northern hemisphere during spring (e.g., Kitagawa et al, 2004;Levin et al, 2010;Leuenberger et al, 2018). According to the atmospheric 14 C data presented by Levin et al (2010), there exists a % 5‰ D 14 C increase from early spring to autumn at mid-high latitude observational points in the northern hemisphere (e.g., at Alert and Jungfraujoch stations), which corresponds to locations at similar latitudes to our Alaskan tree sample.…”

“…In addition to the modern data, the upper limit of the cumulative F 200 in 1859 CE derived herein is exhibited in Figure 3. Here, the upper limit is assumed to be two times the 14 C average error and scaled to the total 14 C increment of the 774 CE event, i.e., 17.6‰ (Brehm et al, 2022), using an estimated F 200 value for the 774 CE event (Usoskin et al, 2023; Table 1). We observed that the upper limit does not deviate from a fitted power function of the modern data.…”

“…As such, these events occur only once every several centuries to 24 centuries (Brehm et al, 2022), thereby rendering their classification as "extreme" extreme SEP events. According to Usoskin et al (2023), both the occurrence distributions of SEP events, i.e., the occurrence frequencies of extreme events observed by cosmogenic nuclides data and SEP events observed in space era, can be fitted using the same Weibul distribution function. This finding signifies that the extreme events are an extension of the events detected in modern observations.…”

“…This finding signifies that the extreme events are an extension of the events detected in modern observations. However, extreme SEP events that have a magnitude in the gap between modern observations and "extreme" extreme SEP events, i.e., once in every to several centuries, have not been extensively investigated owing to several uncertainties involved in the cosmogenic nuclides datastatistical and systematic errors in measurements and insufficient separation from background fluctuations (Usoskin et al, 2020b(Usoskin et al, , 2023Usoskin & Kovaltsov, 2021). Moreover, none of the extreme events detected thus far in cosmogenic nuclides have been definitively determined to originate from extreme solar eruptions by means other than cosmogenic nuclides information (although current knowledge indicates that non-solar origins are unlikely; see Usoskin, 2023).…”

Consistency of the average flux of solar energetic particles over different timescales up to mega-years

“…There are three ways to study the mean SEP flux using direct measurements and indirect proxy [3], as described in the subsequent sections. Here we describe the estimates of the average SEP fluxes on decadal (Section 2), multi-millennial (Section 3) and millions-of-years (Section 4) timescales, and then make an intercomparison between them in Section 5.…”