Solar Activity Over Last 9000 Years Sheds New Light On Natural Variability

A new consistent reconstruction of the solar activity offers a more reliable estimate of the long-term evolution of the solar variability

Chi Ju Wu et al. (2018) Solar activity over nine millennia: A consistent multi-proxy reconstruction. Astronomy & Astrophysics 5 April 2018

Chi Ju Wu, I. G. Usoskin, N. Krivova, G. A. Kovaltsov, M. Baroni, E. Bard, and S. K. Solanki

Abstract

Aims. The solar activity in the past millennia can only be reconstructed from cosmogenic radionuclide proxy records in terrestrial archives. However, because of the diversity of the proxy archives, it is difficult to build a homogeneous reconstruction. All previous studies were based on individual, sometimes statistically averaged, proxy datasets. Here we aim to provide a new consistent multiproxy reconstruction of the solar activity over the last 9000 years, using all available long-span datasets of 10Be and 14C in terrestrial archives.

Methods. A new method, based on a Bayesian approach, was applied for the first time to solar activity reconstruction. A Monte Carlo search (using the χ2 statistic) for the most probable value of the modulation potential was performed to match data from different datasets for a given time. This provides a straightforward estimate of the related uncertainties. We used six 10Be series of different lengths (from 500–10000 years) from Greenland and Antarctica, and the global 14C production series. The 10Be series were resampled to match wiggles related to the grand minima in the 14C reference dataset. The stability of the long data series was tested.

Results. The Greenland Ice-core Project (GRIP) and the Antarctic EDML (EPICA Dronning Maud Land) 10Be series diverge from each other during the second half of the Holocene, while the 14C series lies in between them. A likely reason for the discrepancy is the insufficiently precise beryllium transport and deposition model for Greenland, which leads to an undercorrection of the GRIP series for the geomagnetic shielding effect. A slow 6–7-millennia variability with lows at ca. 5500 BC and 1500 AD in the longterm evolution of solar activity is found. Two components of solar activity can be statistically distinguished: the main component, corresponding to the ‘normal’ moderate level, and a component corresponding to grand minima. A possible existence of a component representing grand maxima is indicated, but it cannot be separated from the main component in a statistically significant manner.

Conclusions. A new consistent reconstruction of solar activity over the last nine millennia is presented with the most probable values of decadal sunspot numbers and their realistic uncertainties. Independent components of solar activity corresponding to the main moderate activity and the grand-minimum state are identified; they may be related to different operation modes of the dynamo. […]

6. Conclusions

We have provided a new fully consistent multi-proxy reconstruction of the solar activity over about nine millennia, based for the first time on a Bayesian approach. We used all the available datasets of cosmogenic radioisotopes with sufficient length and quality in terrestrial archives and up-to-date models of isotope production and transport or deposition as well as a recent archeomagnetic model. We used six 10Be series of different lengths from Greenland and Antarctica, and the official INTCAL global 14C series. Earlier reconstructions were based on either individual datasets or on a statistical superposition of them (e.g., Steinhilber et al. 2012). Our new method is based on finding the most probable value of the solar modulation potential that matches all the data for a given point in time, providing also a straightforward estimate of the uncertainties. Prior to the analysis, long 10Be series were formally redated to match wiggles in the 14C data. All employed cosmogenic isotope series were reduced to the reference geomagnetic field conditions.

We have also tested the stability of the two long 10Be series and found that they appear to diverge from each other during the second half of the Holocene, while the 14C series lies in between them. The GRIP-based series appears to anticorrelate with the long-term geomagnetic VADM series, implying a possible undercorrection for the geomagnetic shielding effect for the GRIP location, while EDML and 14C-based reconstructions do not show any significant residual correlation with VADM. This suggests that the applied model of beryllium transport and deposition does not work properly for the Greenland site but is reasonably good for the Antarctic site. A full-size transport and deposition model (e.g., Sukhodolov et al. 2017) needs to be applied

in the future to resolve this issue.

The reconstructed series (Figure 14) shows variability on different timescales. Grand minima of activity are of particular interest. They are visible as strong dips in the time series, in which the level of the decadal SNs sinks below 10–15. Another feature of the long-term evolution of the solar activity is a slow variability on the 6–7-millennia timescale with lows occurring in ca. 5500 BC and 1500 AD. This behavior has been interpreted by Usoskin et al. (2016) as a possible effect of climate influence on the carbon cycle, but the 14C series lies between the two diverging 10Be series on this timescale, which makes this explanation unlikely. The cause of the feature remains unknown.

Most of the time, the solar activity varies slightly around the moderate level of SN≈ 40, which corresponds to the main component of the solar activity. Grand minima form a statistically distinguishable component, however. The existence of this component was first determined for the last three millennia by Usoskin et al. (2014), who interpreted it as special mode of the solar dynamo. Its confirmation here for nine millennia implies that it is a robust feature of the solar activity. At the same time, the possible existence of a component representing grand maxima is indicated (cf. Usoskin et al. 2014), although it cannot be separated from the main component in a statistically significant manner. We speculate that the different components of the activity distribution may be related to different modes of the dynamo operation.

Finally, a new consistent reconstruction of the solar activity (in the form of decadal SNs) was presented that offers a more reliable estimate of the long-term evolution of the solar variability and poses robust constraints on the development of solar and stellar dynamo models as well as solar-terrestrial studies.