Historical Reconstruction of Solar Surface Magnetism from Cycle 1-24 Using the Synthetic Active Region Generator (SARG) and the Advective Flux Transport (AFT) Model

TL;DR

This work tackles the challenge of reconstructing the Sun’s historical surface magnetism by coupling a synthetic active-region generator (SARG) with an advanced Advective Flux Transport model (AFT 2.0). By using the SIDC/SILSO sunspot-number time series (SSN v2.0) as the sole input, the authors generate synthetic AR catalogs for cycles 1–24, seed them into AFT-AR.SYNTH, and produce full-Sun radial-field maps from 1755 to 2020, with cycle-by-cycle tilt adjustments constrained by an ADM-based target. The reconstructed polar fields show strong correlations with observational proxies (polar faculae counts and Ca II K polar network indices), validating the approach, while comparisons with data-driven AFT-DA reveal both fidelity in large-scale features and limitations due to model simplifications. The study provides publicly available daily maps and outlines future work on ensembles and richer AR-physics to quantify uncertainties and further improve long-term solar-cycle reconstructions, enabling broader applications in solar variability studies and climate-relevant irradiance reconstructions.

Abstract

The historical reconstruction of the Sun's surface magnetic field remains a persistent challenge, limiting our ability to investigate the long-term global properties of the Sun, including the evolution of the large-scale magnetic field, solar cycle prediction, reconstruction of total solar irradiance (TSI), and secular solar variability. In this study, we employ the Advective Flux Transport (AFT) model in conjunction with our newly developed Synthetic Active Region Generator (SARG) to construct a catalog of synthetic active regions (ARs) spanning Solar Cycles 1-24 (1755-2020). We use the SIDC/SILSO sunspot number version 2.0 as the sole input governing the properties of the synthetic ARs in this catalog. This SARG catalog is then incorporated into the AFT model, which simulates the emergence of new ARs on the Sun, which are then transported under the influence of surface flows to produce maps of the full-Sun radial photospheric magnetic field over the entire 265-year period. We modulate the active region tilt for each cycle in order to ensure that the polar fields are consistent with the solar cycle amplitudes. We find that the polar fields derived from these simulations exhibit excellent correlation (r > 0.8) with observational proxies, including polar faculae counts and Ca ii K polar network indices. Daily synchronic maps from these simulations for the entire 265-year period are made publicly available to support a wide range of applications beyond those presented in this work.

Figures (9)

• Figure 1: Distribution of magnetic flux (top) and tilt (bottom) for each synthetic AR across 30 SARG realizations. The red dashed curve represent the input empirical distribution of the magnetic flux and tilt of of ar.
• Figure 2: (a) The relationship between ADM at the solar cycle minimum and strength of the following solar cycle. Here the observed ADM is the dipole component calculated from the WSO observations (blue), whereas the estimated ADM values (red) are interpolated/extrapolated using the best fit straight line (dashed) through observed data points. These expected values are the values we target by adjusting the tilt. (b) The initial dipole configuration used in the model at the beginning of Solar Cycle 1.
• Figure 3: Distribution of $<\Delta\gamma>$, applied in each solar cycle to obtain the desired ADM at the end of the cycle. The shaded background colored bar show the relative strength of the solar cycle (mirrored on both side of x-axis for better comparison).
• Figure 4: Two representative photospheric $B_r$ maps from Solar Cycle 19 during a (a) low- and (b) high-activity period.
• Figure 5: The scatter plot showing the correlation between the polar field reconstructed (with non-zero $\Delta \gamma$) here and its correlation with the other proxies of polar field obtained from KoSO Ca II K and faculae counts in the northern hemisphere (a, b) and for southern hemisphere (c, d) .