Solar activity during the Holocene: the Hallstatt cycle and its consequence for grand minima and maxim

TL;DR

This study reconstructs nine millennia of solar activity using cosmogenic isotopes and models, revealing a ~2400-year Hallstatt cycle linked to grand minima and maxima, with implications for understanding long-term solar variability.

Contribution

It introduces an improved multi-proxy reconstruction of solar activity over nine millennia, incorporating new geomagnetic field data and analyzing the Hallstatt cycle's influence on grand minima and maxima.

Findings

Identification of a ~2400-year Hallstatt cycle in solar activity.

Grand minima and maxima cluster near the cycle's highs and lows.

New geomagnetic dipole field reconstruction for the last nine millennia.

Abstract

Cosmogenic isotopes provide the only quantitative proxy for analyzing the long-term solar variability over a centennial timescale. While essential progress has been achieved in both measurements and modeling of the cosmogenic proxy, uncertainties still remain in the determination of the geomagnetic dipole moment evolution. Here we improve the reconstruction of solar activity over the past nine millennia using a multi-proxy approach. We used records of the 14C and 10Be cosmogenic isotopes, current numerical models of the isotope production and transport in Earth's atmosphere, and available geomagnetic field reconstructions, including a new reconstruction relying on an updated archeo-/paleointensity database. The obtained series were analyzed using the singular spectrum analysis (SSA) method to study the millennial-scale trends. A new reconstruction of the geomagnetic dipole field moment, GMAG.9k, is built for the last nine millennia. New reconstructions of solar activity covering the last nine millennia, quantified in sunspot numbers, are presented and analyzed. A conservative list of grand minima and maxima is provided. The primary components of the reconstructed solar activity, as determined using the SSA method, are different for the series based on 14C and 10Be. These primary components can only be ascribed to long-term changes in the terrestrial system and not to the Sun. They have been removed from the reconstructed series. In contrast, the secondary SSA components of the reconstructed solar activity are found to be dominated by a common ~2400-yr quasi-periodicity, the so-called Hallstatt cycle, in both the 14C and 10Be based series. This Hallstatt cycle thus appears to be related to solar activity. Finally, we show that the grand minima and maxima occurred intermittently over the studied period, with clustering near highs and lows of the Hallstatt cycle, respectively.