Occurrence Probability of Large Solar Energetic Particle Events: Assessment from Data on Cosmogenic Radionuclides in Lunar Rocks

TL;DR

This study combines data from recent measurements, terrestrial radionuclides, and lunar rocks to assess the probability of large solar energetic particle events over different time scales, revealing a strong rollover in event likelihood.

Contribution

It provides a unified occurrence probability distribution of SEP events across three distinct time scales, integrating lunar and terrestrial radionuclide data with direct measurements.

Findings

Consistent occurrence probability distribution across three time scales.

SEP events with proton fluence >10^{11} protons/cm^2/yr are unlikely over Myr timescales.

Reveals a strong rollover in the probability of large SEP events.

Abstract

We revisited assessments of the occurrence probability distribution of large events in solar energetic particles (SEP), based on measurements of cosmogenic radionuclides in lunar rocks. We present a combined cumulative occurrence probability distribution of SEP events based on three time scales: directly measured SEP fluences for the last 60 years; estimates based on terrestrial cosmogenic radionuclides 10Be and 14C for the multi-millennial (Holocene) time scale; and cosmogenic radionuclides measured in lunar rocks on the time scale of up to 1 Myr. All the three time scales yield a consistent distribution. The data suggest a strong rollover of the occurrence probability so that SEP events with the fluence of protons with energy >30 MeV greater than 10^{11} (protons /cm2/yr) are not expected at the Myr time scale.