What can the annual ¹⁰Be solar activity reconstructions tell us about historic space weather?

Abstract

-Context: Cosmogenic isotopes provide useful estimates of past solar magnetic activity, constraining past space climate with reasonable uncertainty. Much less is known about past space weather conditions. Recent advances in the analysis of 10 Be by McCracken & Beer (2015, Sol Phys 290: 305-3069) (MB15) suggest that annually resolved 10 Be can be significantly affected by solar energetic particle (SEP) fluxes. This poses a problem, and presents an opportunity, as the accurate quantification of past solar magnetic activity requires the SEP effects to be determined and isolated, whilst doing so might provide a valuable record of past SEP fluxes. Aims: We compare the MB15 reconstruction of the heliospheric magnetic field (HMF), with two independent estimates of the HMF derived from sunspot records and geomagnetic variability. We aim to quantify the differences between the HMF reconstructions, and speculate on the origin of these differences. We test whether the differences between the reconstructions appear to depend on known significant space weather events. Methods: We analyse the distributions of the differences between the HMF reconstructions. We consider how the differences vary as a function of solar cycle phase, and, using a Kolmogorov-Smirnov test, we compare the distributions under the two conditions of whether or not large space weather events were known to have occurred. Results: We find that the MB15 reconstructions are generally marginally smaller in magnitude than the sunspot and geomagnetic HMF reconstructions. This bias varies as a function of solar cycle phase, and is largest in the declining phase of the solar cycle. We find that MB15's excision of the years with very large ground level enhancement (GLE) improves the agreement of the 10 Be HMF estimate with the sunspot and geomagnetic reconstructions. We find no statistical evidence that GLEs, in general, affect the MB15 reconstruction, but this analysis is limited by having too few samples. We do find evidence that the MB15 reconstructions appear statistically different in years with great geomagnetic storms.

The Sun’s Magnetic (Hale) Cycle and 27 Day Recurrences in the aa Geomagnetic Index

“…There have been several methods proposed to map successive Schwabe and Hale cycles onto a uniform epoch (see for example Thomas et al (2014); Barnard et al (2018)). Chapman et al (2020b) recently proposed a new method to order solar cycle variation on a timebase standardised to the solar cycle.…”

Solar Energetic Particle “Ground-Level Enhancements” and the Solar Cycle

“…There are 68 known events in the continuous neutron monitor record since 1956 (see the International GLE database https://gle.oulu.fi). By eye, they do appear to approximately follow the solar cycle (Shea and Smart, 1990;Barnard et al, 2018), but due to the low number of events, it is not clear whether this is by chance or due to an underlying tendency. Furthermore, many of the GLEs are very small increases above the background level and it is of particular interest whether the few strongest GLEs follow the solar cycle.…”

The Dalton Minimum and John Dalton’s Auroral Observations

“…Their visibility has been intermittently recorded for millennia in various historical documents (Silverman, 1998(Silverman, , 2006Stephenson et al, 2004;Vaquero & Vázquez, 2009). These records provide valuable scientific insights into long-term solar variability and solar eruptions prior to telescopic observations (Silverman, 1992(Silverman, , 2006Willis and Stephenson, 2001;Willis & Davis, 2015;Ebihara et al, 2017;Hayakawa et al, 2017aHayakawa et al, , 2017bHayakawa et al, , 2019aHayakawa et al, , 2019bHattori et al, 2019;Stephenson et al, 2019), particularly when considered in combination with cosmogenic isotope data (Usoskin et al, 2007;Inceoglu et al, 2015;Usoskin, 2017;Barnard et al, 2018;Miyake et al, 2019).…”