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10 Comparison with Cosmogenic Isotopes

Galactic cosmic rays hitting Earth’s atmosphere generate radionuclides by spallation (Beer et al., 2012Jump To The Next Citation Point). Some of these cosmogenic isotopes are stored in terrestrial reservoirs (notably 10Be in ice sheets and 14C in tree trunks), into which dateable cores can be drilled. Because the flux of cosmic rays is modulated by the heliospheric field (Parker, 1965; Potgieter, 1998, 2013), the abundances of these isotopes gives unique information on the long-term variability of the Sun (O’Brien, 1979; Stuiver and Quay, 1980; O’Brien et al., 1991; Beer, 2000; Muscheler et al., 2007; McCracken and Beer, 2007; McCracken, 2007; Solanki et al., 2004Jump To The Next Citation Point) once the effects of the secular variation in the geomagnetic field (which also shields Earth’s atmosphere from cosmic rays) have been accounted for (Bhattacharyya and Mitra, 1997; Masarik and Beer, 1999). Comprehensive reviews of the methods and the underpinning science are present in Usoskin (2013Jump To The Next Citation Point) and Beer et al. (2012). It is useful to employ both 10Be and 14C because the deposition into their respective reservoirs is completely different and checking for close agreement between the inferred production rates can eliminate the possibility of signals in the record caused by changes in Earth’s climate (Bard et al., 1997). Gleeson and Axford (1968) showed, with some approximations, that cosmic rays behave as if they were modulated by an electric field that shields them away from the inner heliosphere. This led to the concept of the solar modulation potential ϕ which is now thought of as a parameter (in units of MV) which describes the heliospheric modification of the local interstellar spectrum (LIS) of galactic cosmic rays at the Earth (Caballero-Lopez and Moraal, 2004; Usoskin et al., 2005). Note that ϕ increases with increased levels of solar activity such that the fluxes of cosmic rays at Earth fall. An excellent review of long term variability of the Sun and heliosphere, as derived from cosmogenic isotopes is given in the Living Review by Usoskin (2013) and so that material will not be repeated here. However, cosmogenic isotopes do provide an independent way of testing (and extending back in time) the reconstructions presented here and so a brief comparison is worthwhile.
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Figure 34: Long-term variations of 25-year means in the IMF B derived from 10Be cosmogenic isotope data by Steinhilber et al. (2010Jump To The Next Citation Point) (SEA10): the green line is the best estimate and the yellow band the estimated uncertainty. In addition, 11-year running means of the reconstructions from geomagnetic activity, as presented in Figure 26View Image, are shown using the same colour scheme as in that figure. Also shown are the SC07 and SC10 floor estimates for annual means and the Dalton and Maunder sunspot minima are labelled DM and MM. Image reproduced by permission from Lockwood and Owens (2011), copyright by AGU.

Lockwood (2001Jump To The Next Citation Point) noted that the open solar flux reconstruction of Lockwood et al. (1999a) overlapped with cosmogenic isotope records, in a way that modern data on cosmic rays from neutron monitors (Simpson, 2000) did not. A good anticorrelation was found by Lockwood (2001, 2003Jump To The Next Citation Point) on both solar cycle and centennial timescales, with the upward drift in open solar flux reflected in the downward drift in cosmogenic isotope abundances in terrestrial reservoirs, and also the drift in results from early ionisation chambers (Forbush, 1958; Neher et al., 1953; McCracken and McDonald, 2001). This trend can also be detected in the cosmogenic 44Ti isotope found in meteorites (Bonino et al., 1995; Taricco et al., 2006; Usoskin et al., 2006Jump To The Next Citation Point) which is significant as it finally removes any possibility that the trend is associated with climate change influence on deposition into terrestrial reservoirs. Usoskin et al. (2006) use the 44Ti isotope data to give strong support to models of the evolution of heliospheric fields based on sunspot number (first introduced by Solanki et al., 2000Jump To The Next Citation Point), as discussed Section 11. This work showed that the well-known Hale cycle variation in cosmic ray fluxes detected using neutron monitors (with alternately peaked and then plateau-like maxima at sunspot minimum) was also well matched by the inverse of the open solar flux variation (see, in particular, Figure 2 of Rouillard and Lockwood, 2004). The anticorrelation with near-Earth IMF had been noted by Cane et al. (1999) and Belov (2000). Furthermore, Thomas et al. (2013) has shown that this feature is also present in the B and FS reconstructions from geomagnetic activity. This raises an interesting question, which remains largely unresolved, as to the relative influences of cosmic ray drifts in the heliosphere and of the open solar flux on the modulation of cosmic rays arriving at Earth, both on decadal and centennial time scales. That the open solar flux is a factor is not a surprise as cosmic rays are scattered off irregularities in the heliospheric field and those irregularities are known to scale in amplitude with the average field value and, as shown for near-Earth space by Figure 29View Image, that field scales with the FS. The drift theory is very well established (e.g., Jokipii et al., 1977; Jokipii, 1991; McDonald et al., 1993) and has some notable successes; for example, the antiphase Hale cycle seen in electrons (Evenson, 1998) and positrons (Clem and Everson, 2002) and their latitudinal variations (Heber et al., 1999). If it is assumed that these drift effects contribute to the Hale cycle but not the secular drift, their effect can be averaged out by taking means over the Hale cycle (Steinhilber et al., 2008Jump To The Next Citation Point). Using ice core records of the abundance of the 10Be cosmogenic isotope and a simple theory of cosmic ray shielding, Steinhilber et al. (2010) (SEA10) have reconstructed 25-year means of the IMF B over the last 9300 years. The results since the Maunder minimum are shown in Figure 34View Image and compared with 11-year running means of B from the reconstructions discussed in Section 9.1.

The general agreement between the geomagnetic and cosmogenic isotope reconstructions is extremely good although there are obvious differences and there may be some timing errors which may turn out to be attributable to dating problems with the ice cores. The agreement is very good after 1900 but less good before then. Between 1850 and 1875 the SC10 reconstruction agrees well with the average level of the SEA10 reconstruction, although showing oscillations that are not found in the SEA10 data. However, SC10 yields higher values of B in the intervals 1875 – 1905 and 1835 – 1850. The 25-year means of the SEA reconstruction of B remain above the SC10 postulated floor level for annual means, even in the Dalton minimum (DM). However, this is not true of the Maunder minimum (MM) where they fell well below it. Even in 25-year means the SEA10 B estimate fell to 1.80 ± 0.59nT by the end of the Maunder minimum, which is still lower than the downward revision of the floor estimate to 2.8 nT by Cliver and Ling (2011Jump To The Next Citation Point). Extending the sequence over 9300 years, SEA10 find 14 grand solar minima in which the reconstructed B fell to even lower values in 25 year means. The SEA10 value of B (and its uncertainty) at the end of the Maunder minimum is marked by the white dot in Figure 29View Image, and using the polynomial fit shown, this yields an estimate of the signed open solar flux at the end of the Maunder minimum of (0.48 ± 0.29) × 1014Wb.

The open flux continuity model discussed in Section 11, which was derived to explain and fit the open solar flux reconstructions from geomagnetic activity data, has been used to estimate the variation of sunspot numbers from the cosmogenic data isotope data for the last millennium (Usoskin et al., 2003; Solanki et al., 2004). These studies found that the recent grand maximum contained unusually high sunspot numbers in the past 11 000 years, a conclusion that generated some debate (Raisbeck and Yiou, 2004; Usoskin et al., 2004; Muscheler et al., 2005; Solanki et al., 2005). Using the composite of cosmogenic isotope data compiled by Steinhilber et al. (2008Jump To The Next Citation Point), Abreu et al. (2008Jump To The Next Citation Point) found that the recent grand solar maximum may not have been the largest in the sequence, but it was the longest in duration.


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