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arxiv: 2603.05592 · v1 · submitted 2026-03-05 · 🌌 astro-ph.SR

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· Lean Theorem

Solar Irradiance Reconstruction over the Telescopic Era Using a Revised Photospheric Magnetic Field Model

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Pith reviewed 2026-05-15 14:45 UTC · model grok-4.3

classification 🌌 astro-ph.SR
keywords solar irradiancetotal solar irradiancespectral solar irradiancephotospheric magnetic fieldsunspot numberSATIRE-Tirradiance reconstructionsecular trend
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The pith

Revised solar model finds total irradiance rose 0.67-0.75 W/m² from 1650-1700 to 1967-2017.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper revises the SATIRE-T model to reconstruct total and spectral solar irradiance over four centuries by updating how small-scale magnetic features emerge in step with sunspot activity. The update is constrained by modern observations of magnetic flux. This produces consistent results from two independent sunspot records and yields a secular increase of 0.67-0.75 W/m² in total solar irradiance between the late-17th-century minimum and recent decades. The reconstructions match observed magnetic flux levels and satellite records of total irradiance and Lyman-α emission at high correlation. The work matters because long-term irradiance changes help quantify the Sun’s role in Earth’s climate history.

Core claim

Using the revised physics-based SATIRE-T model that links the emergence of small-scale magnetic features more realistically to sunspot activity, reconstructions from two sunspot number series show that total solar irradiance increased by 0.67-0.75 W/m² between the 50-year means over 1650-1700 and 1967-2017, while reproducing observed total and open magnetic flux and agreeing closely with satellite measurements of TSI and Lyman-α irradiance.

What carries the argument

The SATIRE-T model, which infers the evolution of the solar surface magnetic field from sunspot number records and computes the resulting irradiance variability from the distribution of magnetic features.

Load-bearing premise

The revised description of magnetic field evolution that more realistically links the emergence of small-scale magnetic features to sunspot activity, constrained by modern observations.

What would settle it

A long-term observational record of total solar irradiance or photospheric magnetic flux that deviates significantly from the model’s predicted secular trend over multiple decades.

Figures

Figures reproduced from arXiv: 2603.05592 by B. Hofer, D. Temaj, N.A. Krivova, S.K. Solanki, T. Chatzistergos.

Figure 1
Figure 1. Figure 1: Evolution of the total unsigned photospheric magnetic flux over time. Red: our reconstructions based on the ISNv2 (top) and CEA17 GSN (bottom). Blue: reconstruction of Krivova et al. (2021) (ISNv2 in top panel, HoSc98 GSN in the bottom panel). Symbols: total magnetic flux measurements from WSO, MWO, and NSO/KP; black line: their av￾erage. All quantities are shown as one￾CR averages. The contribution of the… view at source ↗
Figure 2
Figure 2. Figure 2: Solar surface magnetic flux re￾construction based on ISNv2. Different components of surface magnetic flux are shown with different colors, both as 11- year moving averages and annual av￾erages shown as thick and thin lines, respectively. Shown are the contribu￾tions from: Active Regions (AR, blue), Open Solar Flux (OSF, red), Small-Scale Emergence regions (SSE, green), and the sum of OSF and SSE. 0.5 1.0 [… view at source ↗
Figure 3
Figure 3. Figure 3: Evolution of the open magnetic flux over time. Our reconstruction is shown in red for the ISNv2 input (top panel) and the CEA17 GSN (bottom panel), both at one-CR cadence. In dark and light green, we show the OSF from Lockwood & Owens (2024), reconstructed from the geomagnetic aa-index for 13-CR and 1-CR cadence, respectively. The blue dashed line shows the in-situ OSF measurements from Owens et al. (2017)… view at source ↗
Figure 4
Figure 4. Figure 4: OSF reconstruction from this study (red) and by Krivova et al. (2021, blue) using ISNv2 (top) and GSN (bot￾tom; CEA17 here and HoSc98 in Krivova et al. 2021) as input. The pink shaded area shows our upper limit estimate for the Maunder Minimum, where the SN from Carrasco et al. (2025) was used as input. Black: OSF reconstructed from the cos￾mogenic isotope data by Wu et al. (2018). the Montillet et al. (20… view at source ↗
Figure 5
Figure 5. Figure 5: a) Reconstructed TSI using ISNv2 as input (pink: daily values; red: 81-day smoothing). Blue dots and blue line: the TSI composite of Montillet et al. (2022, daily and 81-day smoothing, respectively). b) Difference between our reconstruction and the Montillet et al. (2022) composite (blue dots: daily; blue line: 81-day smoothing). The horizontal black dashed line marks zero, and the orange line is the linea… view at source ↗
Figure 6
Figure 6. Figure 6: Reconstructed TSI (annual means) since 1700 using ISNv2 (red) and CEA17 GSN (black) as input. The pink shaded area shows our upper limit estimate in TSI, where the Carrasco et al. (2025) SN were used. The TSI composite of Montillet et al. (2022) is shown in blue; the TSI reconstruction of Wu et al. (2018) in green [PITH_FULL_IMAGE:figures/full_fig_p010_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Total solar irradiance (TSI) re￾construction based on ISNv2 as an input (red). The green and black curves show the contribution of different components of magnetic flux to TSI. Green shows the contribution of AR magnetic flux to TSI, and black shows the contribution of small-scale emergences. Thin and thick lines show the annual means or 11-year running means, respectively. 1950 1960 1970 1980 1990 2000 20… view at source ↗
Figure 8
Figure 8. Figure 8: Reconstructed Lyman-α irradi￾ance based on ISNv2 (red) and CEA17 (black) inputs. Also shown are the SATIRE-S Lyman-α reconstruction (Chatzistergos et al. 2025a, orange), and the observational composite by Machol et al. (2019, blue curve) with its uncer￾tainties shown in light blue. The grey shaded area marks the period when the F30 data were used by Machol et al. (2019) instead of direct measurements. All … view at source ↗
read the original abstract

The Sun is the primary source of energy for Earth and one of the main external drivers of its climate. Solar irradiance -- the radiative power emitted by the Sun and received at 1-AU -- varies on all observable timescales. It is measured as total solar irradiance (TSI), the spectrally integrated flux, or as spectral solar irradiance (SSI), its wavelength-dependent distribution. However, direct space-based irradiance measurements span only about five decades and are too short to capture long-term trends, making reconstructions crucial for studying solar influence on climate. On climate-relevant timescales, irradiance variations are driven by changes in the solar surface magnetic field, which form the basis of reconstructions guided by physics. Here we present revised reconstructions of TSI and SSI over the past four centuries using the physics-based SATIRE-T (Spectral And Total Irradiance REconstruction, for the Telescopic era) model. SATIRE-T relates irradiance variability to the evolution of the solar surface magnetic field inferred from sunspot number records. In this work, we implement a recently revised description of magnetic field evolution that more realistically links the emergence of small-scale magnetic features to sunspot activity, constrained by modern observations. Using two independent sunspot number series as input, we obtain consistent reconstructions of magnetic flux and solar irradiance. The model reproduces the observed or independently reconstructed total and open magnetic flux, and agrees closely with satellite measurements of TSI and Lyman-$\alpha$ irradiance, with correlation coefficients of 0.81-0.98 for 81-day-smoothed space-based TSI records, 0.69-0.85 for TSI at daily cadence, and 0.92 for daily Lyman-$\alpha$ irradiance. On secular timescales, the reconstructed TSI increases by 0.67-0.75$\,\mathrm{W/m^2}$ between the 50-year means over 1650-1700 and 1967-2017.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 3 minor

Summary. The paper presents revised SATIRE-T reconstructions of total and spectral solar irradiance over the past four centuries. It incorporates an updated parametrization of small-scale magnetic flux emergence that is linked to sunspot number records and constrained by modern observations. Using two independent sunspot number series as input, the model produces consistent magnetic flux and irradiance time series that reproduce observed total and open magnetic flux, achieve correlations of 0.81-0.98 with 81-day-smoothed satellite TSI, 0.69-0.85 at daily cadence, and 0.92 with daily Lyman-α, and yield a secular TSI increase of 0.67-0.75 W/m² between the 50-year means of 1650-1700 and 1967-2017.

Significance. If the revised emergence scaling holds outside the modern calibration window, the work supplies an improved physics-based irradiance record for climate studies on centennial timescales. The use of two SSN series, reproduction of independent flux proxies, and close match to space-based TSI and Lyman-α data are concrete strengths that would make the secular trend a useful benchmark for solar-climate attribution.

major comments (3)
  1. [§3.2] §3.2 (revised small-scale emergence rule): the functional dependence of small-scale flux emergence rate on sunspot number is calibrated exclusively against post-1970 magnetogram and irradiance data. No cross-check against independent pre-1900 proxies (e.g., cosmogenic-isotope-derived open flux or 10Be records) is shown for the Maunder-minimum interval, so the reported 0.67-0.75 W/m² secular rise remains directly sensitive to an untested extrapolation.
  2. [§5] §5 (secular trend quantification): the difference between 1650-1700 and 1967-2017 50-year means is stated without accompanying uncertainty bands that propagate the free parameters controlling small-scale emergence. A Monte-Carlo or sensitivity run varying those parameters within their modern observational constraints would be required to establish whether the quoted range is robust.
  3. [Table 2, Fig. 7] Table 2 and Fig. 7 (flux comparisons): while modern total and open flux are reproduced, the historical-period agreement is shown only for the two SSN inputs; no quantitative metric (e.g., reduced-χ² or correlation with independent reconstructions) is given for 1650-1900, leaving the load-bearing secular claim without an external anchor.
minor comments (3)
  1. [Abstract, §4.1] The abstract states correlations for “81-day-smoothed space-based TSI records” but does not list the exact satellite datasets or the precise smoothing window used; this should be stated explicitly in §4.1.
  2. [Figure 3] Figure 3 caption omits the vertical scale units for the small-scale flux component; adding them would improve readability.
  3. [§3.2] A short paragraph summarizing the exact numerical values of the free parameters in the revised emergence law (currently only referenced as “constrained by modern observations”) would aid reproducibility.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive review and positive assessment of the revised SATIRE-T model. We address each major comment point by point below and will revise the manuscript to incorporate additional validation where feasible.

read point-by-point responses
  1. Referee: [§3.2] The functional dependence of small-scale flux emergence rate on sunspot number is calibrated exclusively against post-1970 magnetogram and irradiance data. No cross-check against independent pre-1900 proxies (e.g., cosmogenic-isotope-derived open flux or 10Be records) is shown for the Maunder-minimum interval.

    Authors: The parametrization is calibrated on post-1970 data because that interval supplies the only direct, high-resolution magnetogram and irradiance observations available. The underlying physical framework of SATIRE-T is then extrapolated using the same sunspot-number-driven emergence rules. To strengthen the historical application, we will add a direct comparison of the reconstructed open flux during the Maunder minimum against independent cosmogenic-isotope-based open-flux reconstructions in the revised manuscript. revision: yes

  2. Referee: [§5] The difference between 1650-1700 and 1967-2017 50-year means is stated without accompanying uncertainty bands that propagate the free parameters controlling small-scale emergence. A Monte-Carlo or sensitivity run varying those parameters within their modern observational constraints would be required.

    Authors: We agree that propagating parameter uncertainties is essential for the secular-trend claim. In the revised manuscript we will perform a sensitivity analysis in which the key free parameters of the small-scale emergence model are varied within their modern observational error ranges; the resulting envelope on the 0.67–0.75 W m⁻² secular change will be reported in Section 5 together with updated figures. revision: yes

  3. Referee: [Table 2, Fig. 7] While modern total and open flux are reproduced, the historical-period agreement is shown only for the two SSN inputs; no quantitative metric (e.g., reduced-χ² or correlation with independent reconstructions) is given for 1650-1900.

    Authors: We will augment Table 2 and Figure 7 with quantitative metrics (Pearson correlation and reduced χ²) comparing the reconstructed total and open flux against independent cosmogenic-isotope-based reconstructions over 1650–1900, thereby supplying an external anchor for the historical interval. revision: yes

Circularity Check

1 steps flagged

Secular TSI trend of 0.67-0.75 W/m² is extrapolation of modern-fitted small-scale emergence scaling to historical SSN

specific steps
  1. fitted input called prediction [Abstract]
    "we implement a recently revised description of magnetic field evolution that more realistically links the emergence of small-scale magnetic features to sunspot activity, constrained by modern observations. Using two independent sunspot number series as input, we obtain consistent reconstructions of magnetic flux and solar irradiance. The model reproduces the observed or independently reconstructed total and open magnetic flux, and agrees closely with satellite measurements of TSI and Lyman-α irradiance, with correlation coefficients of 0.81-0.98 for 81-day-smoothed space-based TSI records"

    The emergence linking rule is explicitly constrained by modern observations; the subsequent demonstration that the model 'reproduces' modern flux and TSI therefore holds by construction of that constraint. The secular TSI increase is then computed by feeding the same rule (extrapolated) into historical SSN values, making the 0.67-0.75 W/m² difference a direct output of the modern fit rather than an independent first-principles result.

full rationale

The derivation chain starts from SSN series as input and applies a revised magnetic evolution model whose small-scale emergence parameters are constrained/fitted to modern observations. The paper then validates that this model reproduces modern total/open flux and satellite TSI/Lyman-α (r=0.81-0.98 smoothed). Because the central secular difference is obtained simply by running the same fitted model on low-activity historical SSN, the reported 0.67-0.75 W/m² increase reduces to the modern fit without an independent historical anchor, satisfying the 'fitted input called prediction' pattern. No self-definitional equations or load-bearing self-citations that collapse the entire chain were identified, so the score is not raised to 8-10.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The reconstruction rests on the domain assumption that irradiance variations are driven by photospheric magnetic field changes inferred from sunspot numbers, plus a revised linking rule for small-scale features that is constrained by modern data and therefore likely contains fitted parameters.

free parameters (1)
  • parameters controlling small-scale magnetic feature emergence
    The revised description is constrained by modern observations, implying at least one scale or efficiency parameter fitted to recent data.
axioms (1)
  • domain assumption Irradiance variations on climate-relevant timescales are driven by changes in the solar surface magnetic field
    Explicitly stated as the physical basis for the SATIRE-T reconstructions.

pith-pipeline@v0.9.0 · 5684 in / 1338 out tokens · 30739 ms · 2026-05-15T14:45:56.806169+00:00 · methodology

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