This problem originates in P. Raychaudhuri's 2006 preprint 'Total Solar Irradiance Variability and the Solar Activity Cycle' (arXiv:astro-ph/0601335), which proposed that the solar activity cycle and total solar irradiance (TSI) variability are driven by pulsations of the nuclear energy generation rate in the solar core, and acknowledged several unresolved issues. The open problem is the physical attribution of the residual TSI variability that sunspot, facular, and network modeling leaves unexplained — most prominently the inter-minimum secular changes such as the roughly 0.2 W/m² decline between the 1996 and 2008 activity minima — namely whether it reflects a global structural or thermal change of the Sun, surface magnetism below current proxy thresholds, or instrumental artifacts of the composite record, with any interior mechanism required to be consistent with the neutrino-flux stability bounds.

Background

Total solar irradiance (TSI) is the spectrally integrated solar radiative flux measured at the top of Earth's atmosphere at mean Sun–Earth distance. Space-borne radiometers since 1978 show that TSI varies by roughly 0.1% over the 11-year solar activity cycle, in phase with magnetic activity. The accepted explanation of this cyclic modulation is surface magnetism: dark sunspots reduce the emitted flux, while faculae (small, concentrated magnetic flux tubes that appear brighter than the surrounding photosphere) and the magnetic network (weaker field spread over the quiet solar surface) increase it; semi-empirical models built on sunspot and facular proxies reproduce these intra-cycle changes well. The TSI record must be assembled from successive radiometer missions into composites, whose instrumental degradation corrections and cross-calibrations are themselves debated, and secular (inter-cycle) changes are hard to discern because instrument stability limits the record.

A distinct question is whether any part of irradiance variability originates in the solar interior rather than in surface magnetism. The source of this candidate, Raychaudhuri (arXiv:astro-ph/0601335, 2006), develops a 'perturbed solar model' in which the activity cycle is driven by pulsations of the nuclear energy generation rate in the core: cyclic core energy release powers an internal dynamo and perturbs the Sun's global structure (luminosity, radius, temperature), producing both residual TSI variability and an anticorrelation of the solar neutrino flux with sunspot number, with claimed multiple phases and periods (about 5–10, ~11, and ~30–33 months) across activity indicators. Solar neutrinos are emitted by the fusion reactions that power the Sun; the radiochemical experiments Homestake, SAGE, and GALLEX-GNO and the water-Cherenkov detectors (Super-)Kamiokande and SNO provided time series in which such variability was claimed. In the standard solar model (SSM), the accepted stellar-interior model of the Sun, the core burns steadily and the computed neutrino luminosity is tied to the measured optical luminosity. Mainstream dynamo theory instead locates the solar cycle in the convection zone and the tachocline — the shear layer at its base — through an alpha–Omega dynamo (the combined action of differential rotation and cyclonic convection on magnetic fields), with no cyclic modulation of core nuclear burning. The source paper's claimed five-phase structure of the cycle and its neutrino-periodicity claims rest on short-baseline, low-statistics datasets; dedicated later searches (SNO 2005; Super-Kamiokande 1996–2018) tested exactly this time-variability question and found no significant periodic modulation beyond the annual geometric modulation caused by Earth's elliptical orbit. Meanwhile the residual-TSI question entered the mainstream literature on its own: Fröhlich (2009) documented that TSI at the 2008 activity minimum was more than 0.2 W/m² below the 1996 minimum, noted that sunspots, faculae, and network 'explain well the intra-cycle variability' but that the secular change 'must differ from the effect of surface magnetism', and proposed a global temperature change of the Sun; the review by Solanki, Krivova, and Haigh (2013) records the origin of such secular irradiance changes as unresolved; Kopp (2016) emphasizes that discerning secular variation is limited by instrument stability over the short space-borne record; and Amdur and Huybers (2025) showed that satellite-era reconstructions still disagree in their linear trends by up to 0.17 W/m² per decade, attributing part of the disagreement to unaccounted instrument drifts and proxy insensitivity while leaving a corrected −0.15 W/m² per decade secular decline whose physical origin remains unexplained. Thus the interior-variability hypothesis is heavily constrained from the neutrino side, while the attribution of the residual irradiance signal remains genuinely open.

Problem Statement

Identify the physical origin of the component of solar irradiance variability that models based on surface magnetism — sunspot darkening plus facular and network brightening — leave unexplained, most concretely the inter-minimum secular changes in the TSI composites such as the roughly 0.2 W/m² decline between the 1996 and 2008 activity minima, and determine whether any part of this residual reflects a global structural or thermal change of the Sun (as posited by the source paper's perturbed-core framework, in which cyclic nuclear-energy pulsations in the core drive an internal dynamo and perturb the Sun's luminosity, radius, and temperature) rather than surface magnetism below current proxy thresholds or instrumental artifacts of the composite record. Any global-structural or interior mechanism proposed must be quantitatively modeled and consistent with independent diagnostics — in particular the upper limits on solar-neutrino flux modulation established by dedicated searches. The objective is a physically attributed, quantitatively tested account of the non-surface-magnetic part of the Sun's irradiance variability.

References

  1. P. Raychaudhuri, "Total Solar Irradiance Variability and the Solar Activity Cycle", arXiv:astro-ph/0601335 (2006), DOI: 10.48550/arXiv.astro-ph/0601335, https://arxiv.org/abs/astro-ph/0601335
  2. SNO Collaboration (B. Aharmim et al.), "A Search for Periodicities in the 8B Solar Neutrino Flux Measured by the Sudbury Neutrino Observatory", Physical Review D 72, 052010 (2005); preprint arXiv:hep-ex/0507079, DOI: 10.1103/PhysRevD.72.052010, https://doi.org/10.1103/PhysRevD.72.052010
  3. Super-Kamiokande Collaboration (K. Abe et al.), "Search for Periodic Time Variations of the Solar 8B Neutrino Flux between 1996 and 2018 in Super-Kamiokande", arXiv:2311.01159 (2023, revised 2024), DOI: 10.48550/arXiv.2311.01159, https://arxiv.org/abs/2311.01159
  4. C. Fröhlich, "Evidence of a long-term trend in total solar irradiance", Astronomy & Astrophysics 501, L27 (2009), DOI: 10.1051/0004-6361/200912318, https://doi.org/10.1051/0004-6361/200912318
  5. S. K. Solanki, N. A. Krivova, J. D. Haigh, "Solar Irradiance Variability and Climate", Annual Review of Astronomy and Astrophysics 51, 311 (2013), DOI: 10.1146/annurev-astro-082812-141007, https://doi.org/10.1146/annurev-astro-082812-141007
  6. G. Kopp, "Magnitudes and Timescales of Total Solar Irradiance Variability", arXiv:1606.05258 (2016), DOI: 10.48550/arXiv.1606.05258, https://arxiv.org/abs/1606.05258
  7. P. Charbonneau, "Dynamo Models of the Solar Cycle", Living Reviews in Solar Physics 7 (2010), DOI: 10.12942/lrsp-2010-3, https://doi.org/10.12942/lrsp-2010-3
  8. K. Sakurai, H. J. Haubold, T. Shirai, "The Variation of the Solar Neutrino Fluxes over Time in the Homestake, GALLEX(GNO) and Super-Kamiokande Experiments", arXiv:1111.5530 (2011), DOI: 10.48550/arXiv.1111.5530, https://arxiv.org/abs/1111.5530
  9. N. B. Xiang, D. F. Kong, "What Causes the Inter-Solar-Cycle Variation of Total Solar Irradiance?", The Astronomical Journal 150, 171 (2015), DOI: 10.1088/0004-6256/150/6/171, https://doi.org/10.1088/0004-6256/150/6/171
  10. T. Amdur, P. Huybers, "Negative trend in total solar irradiance over the satellite era", Proceedings of the National Academy of Sciences 122 (2025), DOI: 10.1073/pnas.2417155122, https://doi.org/10.1073/pnas.2417155122