Assessment of different sunspot number series using the cosmogenic isotope ⁴⁴ Ti in meteorites

Abstract

Many sunspot number series exist suggesting different levels of solar activity during the past centuries. Their reliability can be assessed only by comparing them with alternative indirect proxies. We test different sunspot number series against the updated record of cosmogenic radionuclide 44 Ti measured in meteorites. Two bounding scenarios of solar activity changes have been considered: the HH-scenario (based on the series by Svalgaard and Schatten), in particular, predicting moderate activity during the Maunder minimum, and the LL-scenario (based on the R G series by Lockwood et al.) predicting moderate activity for the 18th-19th centuries and the very low activity level for the Maunder minimum. For each scenario, the magnetic open solar flux, the heliospheric modulation potential and the expected production of 44 Ti were computed. The calculated production rates were compared with the corresponding measurements of 44 Ti activity in stony meteorites fallen since 1766. The analysis reveals that the LL-scenario is fully consistent with the measured 44 Ti data, in particular, recovering the observed secular trend between the 17th century and the Modern grand maximum. On the contrary, the HH-scenario appears significantly inconsistent with the data, mostly due to the moderate level of activity during the Maunder minimum. It is concluded that the HHscenario sunspot number reconstruction significantly overestimates solar activity prior to the mid-18th century, especially during the Maunder minimum. The exact level of solar activity after 1750 cannot be distinguished with this method, since both H-and L-scenarios appear statistically consistent with the data.

A history of solar activity over millennia

“…The red and blue colored curves with the 1σ model uncertainties (hatched areas) depict the modelled 44 Ti activity computed for "high" (e.g., Svalgaard and Schatten 2016) and "low" (e.g., Hoyt and Schatten 1998;Usoskin et al 2016b) reconstructions of solar activity, respectively. Modified after Asvestari et al (2017) number reconstructions yielding high solar activity during the 17th and 18th appear inconsistent with the data of 44 Ti in meteorites (Fig. 19).…”

“…Several attempts to test/validate different sunspot reconstructions using indirect proxies yielded indicative results: tests based on cosmogenic radionuclides (Asvestari et al 2017), including 44 Ti in measured in the fallen meteorites (see Fig. 19) as well as geomagnetic and heliospheric proxies (Lockwood et al 2016a, b) favor the 'lower' reconstructions (Hoyt and Schatten 1998;Usoskin et al 2016b) against the 'high' reconstructions (Svalgaard and Schatten 2016).…”

Space climate and space weather over the past 400 years: 2. Proxy indicators of geomagnetic storm and substorm occurrence

“…Hence sunspot numbers are used as a proxy for OSF emergence rate. This has been used a great many times since, in conjunction with the OSF continuity equation and/or photospheric flux transport models, in reconstructions (e.g., Solanki et al, 2002;Schrijver et al, 2002;Lean et al, 2002;Usoskin et al, 2002;Lockwood, 2003;Wang et al, 2005;Vieira & Solanki, 2010;Steinhilber et al, 2010;Demetrescu et al, 2010;Goelzer et al, 2013;Wang & Sheeley, 2013;Karoff et al, 2015;Rahmanifard et al, 2017;Asvestari et al, 2017). However, as noted in the area of climate science, regressions with proxies can lead to errors in reconstructions (e.g., Bürger & Cubasch, 2005) and hence it is important to carry out tests on the predictions and to understand the degree of extrapolation used when the reconstructions extend beyond the range of conditions covered by the regressions.…”

“…Fortunately, there are independent tests that can be applied for most of interval A, provided by the cosmogenic isotopes 14 C (found in tree trunks), 10 Be (in ice sheets) and 44 Ti (in meteorites). Tests using all 3 of these show that the reconstructed OSF predicts well with the variation of their abundances back to, and during, the Maunder minimum (Lockwood, 2001(Lockwood, , 2003Usoskin et al, 2015;Owens et al, 2016b;Asvestari & Usoskin, 2016;Asvestari et al, 2017). Initially these tests used simple linear regressions of OSF and the cosmogenic isotope abundances but, more recently, have evolved to employ relationships between the two based on the physics of cosmic ray shielding by the heliospheric field.…”

Estimating Solar Irradiance Since 850 CE

“…Comparisons of multiple sunspot number records with cosmogenic radionuclides extracted from meteorites suggest that sunspot records such as SILSO may overestimate solar activity prior to the mideighteenth century (Asvestari et al, 2017). This motivates the adoption of an average of the two reconstructions using the different sunspot records as the preferred irradiance specification since 1610.…”