The solar magnetic field since 1700: II. Physical reconstruction of total, polar and open flux

TL;DR

This study reconstructs the evolution of the solar magnetic field since 1700 using a flux transport model driven by semi-synthetic sunspot records, confirming correlations with solar cycle strength and analyzing historical minima.

Contribution

It introduces a novel reconstruction method combining semi-synthetic sunspot data with flux transport modeling to extend solar magnetic field records back to 1700.

Findings

Reconstructed total, polar, and open flux agree with observational data.

Confirmed correlation between polar field during minima and next cycle strength.

Identified inconsistency of a hypothesized 'lost' cycle with isotope records.

Abstract

We have used semi-synthetic records of emerging sunspot groups based on sunspot number data as input for a surface flux transport model to reconstruct the evolution of the large-scale solar magnetic field and the open heliospheric flux from the year 1700 onward. The statistical properties of the semi-synthetic sunspot group records reflect those of the observed the Royal Greenwich Observatory photoheliographic results. These include correlations between the sunspot numbers and sunspot group latitudes, longitudes, areas and tilt angles. The reconstruction results for the total surface flux, the polar field, and the heliospheric open flux (determined by a current sheet source surface extrapolation) agree well with the available observational or empirically derived data and reconstructions. We confirm a significant positive correlation between the polar field during activity minimum periods and the strength of the subsequent sunspot cycle, which has implications for flux transport dynamo models for the solar cycle. Just prior to the Dalton minimum, at the end of the 18th century, a long cycle was followed by a weak cycle. We find that introducing a possibly `lost' cycle between 1793 and 1800 leads to a shift of the minimum of the open flux by 15 years which is inconsistent with the cosmogenic isotope record.