Occurrence Probability of Large Solar Energetic Particle Events: Assessment from Data on Cosmogenic Radionuclides in Lun
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Occurrence Probability of Large Solar Energetic Particle Events: Assessment from Data on Cosmogenic Radionuclides in Lunar Rocks G.A. Kovaltsov · I.G. Usoskin
Received: 8 March 2013 / Accepted: 20 May 2013 © Springer Science+Business Media Dordrecht 2013
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 timescales: directly measured SEP fluences for the past 60 years; estimates based on the terrestrial cosmogenic radionuclides 10 Be and 14 C for the multimillennial (Holocene) timescale; and cosmogenic radionuclides measured in lunar rocks on a timescale of up to 1 Myr. These three timescales yield a consistent distribution. The data suggest a strong roll-over of the occurrence probability, so that SEP events with a proton fluence with energy > 30 MeV greater than 1011 (protons cm−2 yr−1 ) are not expected on a Myr timescale. Keywords Cosmic rays, solar · Flares, energetic particles
1. Introduction Advanced knowledge of the occurrence probability of extreme events related to solar energetic particles (SEPs) is very important and acute (Hudson, 2010). This is important from different aspects: from purely astrophysical questions of the highest possible energy released in solar flares (Schrijver et al., 2012) to the geo-environment (Thomas et al., 2013), and even to the technological risk assessments (Shea and Smart, 2012). Direct observations of SEPs cover the past six decades with ground-based and space-borne instruments.
G.A. Kovaltsov Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russia I.G. Usoskin () Sodankylä Geophysical Observatory (Oulu Unit), University of Oulu, 90014 Oulu, Finland e-mail: [email protected] I.G. Usoskin Department of Physics, University of Oulu, 90014 Oulu, Finland
G.A. Kovaltsov, I.G. Usoskin Figure 1 The cumulative OPDF of SEP events (the occurrence probability of events with > 30 MeV fluence greater than the given F30 ). Points with error bars (90 % confidence interval) correspond to the data for the space era since 1955 (triangles) and cosmogenic radionuclides in terrestrial archives for the Holocene (circles). Open/filled symbols correspond to the measured data and upper estimates, respectively (modified after Usoskin and Kovaltsov, 2012). Curves depict best fits of the high-fluence event tail, obtained in this work from lunar data, for two models—power law [panel (a)] and exponential [panel (b)]. The curves are numbered in the legend, and the numbers correspond to the lines in Table 1. All curves converge at the point corresponding to P0 = 0.1 and F0 = 5 × 109 protons cm−2 yr−1 .
The cumulative occurrence probability distribution function (OPDF) for the measured proton (> 30 MeV) annual fluences (Shea and Smart, 1990; M. Shea, 2012 private communication) is shown in Figure 1 as triangles with error bars. The average annual S
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