Solar activity over nine millennia: A consistent multi-proxy reconstruction^★

¹ Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
² Space Climate Research Unit, University of Oulu, Finland
e-mail: Ilya.Usoskin@oulu.fi
³ Sodankylä Geophysical Observatory, University of Oulu, Finland
⁴ Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russia
⁵ CEREGE, Aix-Marseille Université, CNRS, NRD, INRA, Collège de France, Technopôle de l’Arbois, Aix-en-Provence, France
⁶ School of Space Research, Kyung Hee University, Yongin, Gyeonggi-Do 446-701, Republic of Korea

Received: 4 September 2017
Accepted: 23 March 2018

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 multi-proxy reconstruction of the solar activity over the last 9000 yr, 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 500–10 000 yr) 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 long-term 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.