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arxiv: 2606.26283 · v1 · pith:BIDUOD3Snew · submitted 2026-06-24 · 🌌 astro-ph.SR

Six decades of TIGRE and Mount Wilson chromospheric monitoring in the H and K lines: The quest for an understanding of solar-type activity

J. H. M. M. Schmitt , K.-P. Schr\"oder , M. Mittag , A. Hempelmann , J. N. Gonz\'alez-P\'erez , Dennis Jack This is my paper

Pith reviewed 2026-06-26 01:04 UTC · model grok-4.3

classification 🌌 astro-ph.SR
keywords chromospheric activityS-indexstellar magnetic cyclessolar-stellar connectionMount Wilson surveyTIGRE observationscool main-sequence starsMaunder minimum
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The pith

Stars more active than the Sun show cyclic variability more often and with greater regularity.

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

The paper combines more than five decades of consistently calibrated S-index measurements from Mount Wilson and TIGRE observations of the Sun and over one hundred solar-like stars. These long time series enable examination of the long-term stability of stellar magnetic cycles and possible transitions to other activity patterns. Analysis of a representative sample of cool main-sequence stars shows that stars exceeding solar activity levels are more likely to exhibit cyclic variability, and that more active stars display more regular cycles. The Sun's mean S-value lies at the lower end of the range for cyclic stars, which matches the occurrence of historical Maunder minimum episodes and suggests the solar cycle can become unstable. The study also identifies eighteen stars with lower activity and variability than the Sun, offering a view of the Sun's possible distant future.

Core claim

The combined Mount Wilson and TIGRE S-index time series, now spanning more than five decades, allow probing of long-term stability of solar-type magnetic cycles. Stars more active than the Sun are more likely to show cyclic variability than stars below solar activity. More active stars tend to have more regular cycles, while the solar mean S-value fits the lower end of the S-index range found for cyclic stars. This pattern is consistent with the occurrence of solar Maunder minima episodes. Eighteen stars with even lower activity and lower variability than the Sun provide a preview into the Sun's distant future.

What carries the argument

The S-index, a chromospheric activity indicator from H and K line monitoring, tracked over multi-decade time series to classify cycle regularity and compare activity levels.

If this is right

  • Stars exceeding solar activity are more prone to exhibit cyclic variability.
  • Higher activity levels correlate with greater cycle regularity.
  • The Sun's activity level sits at the lower end of cyclic star S-index values, consistent with possible Maunder minimum episodes.
  • Eighteen identified stars with sub-solar activity and variability illustrate a possible future state for the Sun.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Stellar cycle data may help forecast whether the Sun will enter a prolonged low-activity phase within centuries.
  • The activity threshold separating regular cycles from irregular or minimal states could be tested by extending monitoring to additional low-activity stars.
  • If the pattern holds, models of stellar magnetic evolution would need to incorporate activity-dependent stability of cycles.

Load-bearing premise

The combined Mount Wilson and TIGRE S-index time series are free of uncorrected instrumental drifts or calibration offsets that could affect cycle detection and grading over the full multi-decade baseline.

What would settle it

Discovery of systematic calibration offsets between the Mount Wilson and TIGRE datasets that change the cycle grade or presence for several stars above or below solar activity levels.

Figures

Figures reproduced from arXiv: 2606.26283 by A. Hempelmann, Dennis Jack, J. H. M. M. Schmitt, J. N. Gonz\'alez-P\'erez, K.-P. Schr\"oder, M. Mittag.

Figure 1
Figure 1. Figure 1: S-index distributions of the stellar sample observed and [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Mean yearly S-index for the Mount Wilson stellar sample [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 4
Figure 4. Figure 4: S-index errors vs. the derived solar S-indices for all TI [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: Histogram representation of the solar S-index variability; [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 5
Figure 5. Figure 5: Solar S-index vs. time measured by TIGRE from 2013 [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 7
Figure 7. Figure 7: Comparison of TIGRE derived S-index measurements [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: While obviously quite a bit of variability at a given cycle [PITH_FULL_IMAGE:figures/full_fig_p007_8.png] view at source ↗
Figure 11
Figure 11. Figure 11: S-index variability for HD 81809 between 1966 and [PITH_FULL_IMAGE:figures/full_fig_p008_11.png] view at source ↗
Figure 10
Figure 10. Figure 10: Demonstration of analysis procedures. Top: GLS peri [PITH_FULL_IMAGE:figures/full_fig_p008_10.png] view at source ↗
Figure 12
Figure 12. Figure 12: S-index light curve for HD76151 between 1966 and [PITH_FULL_IMAGE:figures/full_fig_p010_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: Mean S-index distribution for stars without any detected [PITH_FULL_IMAGE:figures/full_fig_p011_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: Relative dispersion vs. mean S-index for all sample [PITH_FULL_IMAGE:figures/full_fig_p012_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: HR diagram of sample stars with low activity and sam [PITH_FULL_IMAGE:figures/full_fig_p013_15.png] view at source ↗
Figure 14
Figure 14. Figure 14: We also show evolutionary tracks (computed for Z [PITH_FULL_IMAGE:figures/full_fig_p014_14.png] view at source ↗
Figure 14
Figure 14. Figure 14: Their G-type inactive equivalents (group II of Fig. 14) [PITH_FULL_IMAGE:figures/full_fig_p015_14.png] view at source ↗
read the original abstract

The combined time series of the consistently calibrated chromospheric activity indicator, the S-index -- derived from Mount Wilson and TIGRE data -- now spans more than five decades of monitoring observations of the Sun and of more than one hundred solar-like stars. For the first time, these data allow us to probe the long-term stability of solar-type magnetic cycles as well as their possible transition into other behavioral forms of stellar activity in the time domain. Such variability studies are an important input for solar-stellar connection research and for our empirical understanding of the long-term perspective of solar activity as well as of a wider empirical picture of the coevolution of magnetic activity with stellar structure. We here present and discuss a representative selection of cool main-sequence stars, for which such data are available. The excellent quality of our TIGRE data now more than doubles the number of known cyclic stars. Grading them (i.e., excellent, good, fair, and poor) and putting them into an evolutionary perspective, we find that stars more active than the Sun are also more likely to show cyclic variability than stars below solar activity. Furthermore, more active stars tend to have more regular cycles, while the solar mean S-value fits the lower end of the S-index range found for cyclic stars. This is consistent with the occurrence of solar Maunder minima episodes, hinting at a sometimes already unstable solar cycle. We find eighteen stars with even lower activity and lower variability than the Sun, which provide a preview into the latter's distant future.

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

1 major / 0 minor

Summary. The manuscript analyzes a combined, consistently calibrated S-index time series spanning more than five decades from Mount Wilson and TIGRE observations of the Sun and over 100 solar-like stars. It grades activity cycles (excellent/good/fair/poor) and reports that stars more active than the Sun are more likely to exhibit cyclic variability, with more regular cycles at higher activity levels; the solar mean S-value lies at the lower end of the cyclic-star range, consistent with Maunder-minimum episodes, while 18 stars show even lower activity and variability as a preview of the Sun's distant future.

Significance. If the instrumental calibration holds, the work supplies the longest available baseline for solar-type cycle stability, directly constraining the solar-stellar connection and the empirical frequency of grand-minima states; the doubling of known cyclic stars and the activity-cycle regularity trend constitute a substantial empirical advance.

major comments (1)
  1. [Abstract and data-combination description] The central claims on cycle frequency, regularity, and the Sun's position at the low-activity edge rest on the assumption that the Mount Wilson + TIGRE S-index series are free of uncorrected zero-point offsets or slow drifts. The abstract states that the data are 'consistently calibrated,' yet the manuscript provides no explicit overlap statistics, residual scatter after correction, or sensitivity tests showing that cycle grading remains stable under plausible residual offsets (especially near solar S-index levels where cycles are described as marginal).

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their detailed review and constructive feedback on our manuscript. We address the major comment point by point below.

read point-by-point responses

  1. Referee: [Abstract and data-combination description] The central claims on cycle frequency, regularity, and the Sun's position at the low-activity edge rest on the assumption that the Mount Wilson + TIGRE S-index series are free of uncorrected zero-point offsets or slow drifts. The abstract states that the data are 'consistently calibrated,' yet the manuscript provides no explicit overlap statistics, residual scatter after correction, or sensitivity tests showing that cycle grading remains stable under plausible residual offsets (especially near solar S-index levels where cycles are described as marginal).

    Authors: We agree that the presentation of the calibration can be strengthened for greater transparency. The methods section describes the alignment of the Mount Wilson and TIGRE S-index scales using the overlap period on common stars, but we did not include quantitative overlap statistics, rms residuals, or explicit sensitivity tests in the submitted version. We will revise the manuscript to add these elements: the number of overlapping targets, the measured residual scatter after correction, and a short sensitivity analysis confirming that the cycle grades (excellent/good/fair/poor) remain unchanged under offsets comparable to the typical measurement uncertainty. This will be placed in the data-combination subsection and referenced from the abstract. revision: yes

Circularity Check

0 steps flagged

No circularity: purely observational reporting of calibrated S-index time series and cycle classifications

full rationale

The paper presents combined Mount Wilson and TIGRE S-index time series spanning decades, classifies stellar cycles by quality (excellent/good/fair/poor), and reports empirical correlations between activity level and cycle presence/regularity. No equations, model derivations, fitted parameters renamed as predictions, or self-citation chains appear in the abstract or described content. The analysis relies on direct measurements and visual/qualitative grading rather than any self-referential definitions or reductions to inputs. The calibration consistency noted by the skeptic is an external data-quality assumption, not a circular step in any derivation chain. This is self-contained observational work with no load-bearing mathematical steps that reduce by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Review performed on abstract only; no free parameters, invented entities, or non-standard axioms are identifiable from the provided text.

axioms (1)
  • domain assumption The S-index serves as a reliable, consistently calibrated proxy for chromospheric magnetic activity across instruments and decades.
    Invoked implicitly throughout the abstract as the basis for all cycle detections and comparisons.

pith-pipeline@v0.9.1-grok · 5848 in / 1254 out tokens · 23391 ms · 2026-06-26T01:04:53.101635+00:00 · methodology

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Reference graph

Works this paper leans on

2 extracted references

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