Solar extreme events

TL;DR

This review explores the potential for extreme solar events beyond historical records, analyzing their frequency, magnitude distributions, and recent observational advances that inform their likelihood and characteristics.

Contribution

It synthesizes current knowledge on solar event distributions and discusses how recent observations impact understanding of extreme solar phenomena.

Findings

Solar flare fluxes follow a power law slightly flatter than S^{-2}.

Radioisotope proxies suggest a rollover to steeper distributions at high magnitudes.

Kepler observations have detected superflares, informing extreme event frequency estimates.

Abstract

Solar flares and CMEs have a broad range of magnitudes. This review discusses the possibility of "extreme events," defined as those with magnitudes greater than have been seen in the existing historical record. For most quantitative measures, this direct information does not extend more than a century and a half into the recent past. The magnitude distributions (occurrence frequencies) of solar events (flares/CMEs) typically decrease with the parameter measured or inferred (peak flux, mass, energy etc. Flare radiation fluxes tend to follow a power law slightly flatter than $S^{-2}$, where S represents a peak flux; solar particle events (SPEs) follow a still flatter power law up to a limiting magnitude, and then appear to roll over to a steeper distribution, which may take an exponential form or follow a broken power law. This inference comes from the terrestrial $^{14}$C record and from the depth dependence of various radioisotope proxies in the lunar regolith and in meteorites. Recently major new observational results have impacted our use of the relatively limited historical record in new ways: the detection of actual events in the $^{14}$C tree-ring records, and the systematic observations of flares and "superflares" by the Kepler spacecraft. I discuss how these new findings may affect our understanding of the distribution function expected for extreme solar events.