The first out-of-ecliptic measurements of the Sun's polar magnetic field, obtained with the High Resolution Telescope of the Polarimetric and Helioseismic Imager (SO/PHI-HRT) on board Solar Orbiter at heliographic latitudes between 14.9° and 16.7° in spring 2025, show that both polar caps had already reversed polarity in solar cycle 25, but leave the intrinsic structure of the polar field undetermined: the measured flux density decreases toward the poles, small magnetic elements with fluxes below ∼1017\sim10^{17} Mx appear scarce near the poles, and only eight north-pole snapshots are available. Calchetti et al. (2025) could not decide how much of this reflects real physics — a delayed polarity reversal at the highest latitudes driven by poleward flux transport — and how much is observational bias from foreshortening, limited spatial resolution, and signal-to-noise losses. The open problem is to separate these causes and recover the true latitudinal progression of the polarity reversal and the small-scale contribution to the polar net flux, using more extensive out-of-ecliptic observations at larger heliographic latitudes, viewing-angle-controlled measurements, and improved SNR and coverage.

Background

The Sun's global magnetic field reverses its polarity roughly every 11 years. The polar caps (heliographic latitudes above about 70°) hold the dominant share of the Sun's axial dipole, and their signed net magnetic flux is the best-known precursor of the amplitude of the next sunspot cycle. The timing and latitudinal progression of the polar-field polarity reversal are therefore key tests of flux-transport dynamo models, in which magnetic flux of trailing polarity from decaying active regions is carried poleward by the meridional flow (a slow large-scale circulation on the solar surface) until it cancels the old polar field and rebuilds one of opposite sign.

Measuring the polar field has always been difficult because telescopes in near-Earth orbit view the Sun from the ecliptic plane and see the poles at grazing incidence. Two geometric effects dominate: foreshortening, which compresses the image of the surface near the solar limb and degrades the effective spatial resolution, and line-of-sight projection of the mostly vertical magnetic field, which suppresses the measured signal. Since 2008 the spectropolarimeter on the Hinode satellite has provided the highest-quality ecliptic-view polar magnetograms, revealing vertical kilo-gauss magnetic patches concentrated at the poles (Tsuneta et al. 2008), yet the resulting polar fluxes remain subject to projection systematics estimated to reach tens of percent (Gosain and Uitenbroek 2024).

Solar Orbiter is the first mission to observe the poles from out of the ecliptic plane. In March–April 2025 it reached heliographic latitudes between 14.9° and 16.7°, and its Polarimetric and Helioseismic Imager High Resolution Telescope (SO/PHI-HRT, a filter spectropolarimeter measuring polarized line profiles of the Fe I 617.3 nm absorption line) recorded the first direct vector-magnetogram sequences of both poles. Calchetti et al. (2025) analyzed 42 datasets of the south pole and 8 of the north pole and reported magnetic flux (in maxwells, 1 Mx = 10−810^{-8} Wb) and flux density (mean field per unit area) as functions of latitude. Both polar caps had already reversed polarity relative to the first half of cycle 25, with strong north–south asymmetries in flux and flux density.

Two observed features, however, could not be given a definitive physical interpretation. First, the flux density decreases toward the poles; the authors read this as a sign of a delayed polarity reversal at the highest latitudes, consistent with poleward flux migration and transport models, but the measurements themselves cannot exclude a substantial contribution from foreshortening and signal-to-noise (SNR) losses. Second, magnetic elements with fluxes below ∼1017\sim10^{17} Mx are scarcer than expected near the poles, and the average flux of detected elements rises toward the limb (small μ, where μ = cos θ is the cosine of the heliocentric angle of the observation) — exactly the signature expected if weak small elements fall below the detection threshold. The authors state that at present it is not possible to discern whether the scarcity of smaller elements at the pole is a foreshortening effect or a real absence. They also note that the eight north-pole snapshots yield larger uncertainties and no robust temporal characterization, and conclude that limb observations far from the poles and future polar campaigns at even higher heliographic latitudes (larger μ at the poles) will help point to a solution; SO/PHI is expected to remain the only instrument able to measure the magnetic field out of the ecliptic for the coming five years. Independent ecliptic-view work on cycle 24 with Hinode found the polarity reversal completed successively from 70° latitude to the pole (Yang et al. 2024) and argued that small-scale elements contribute only limited amounts to the polar flux imbalance (Yang et al. 2025) — evidence that is suggestive but carries the same projection biases and concerns a different cycle.

Problem Statement

Determine the intrinsic structure and reversal progression of the Sun's polar magnetic fields from out-of-ecliptic measurements, separating physical signal from observational bias, using the available out-of-ecliptic measurements and viewing-angle-controlled comparisons (such as limb observations far from the poles providing matched-heliocentric-angle comparisons). Two deliverables are each mandatory and independently standing, and each is resolved by the same standard: a supported attribution or quantification, or an explicit quantitative demonstration that the available measurements admit no separation. A bare statement that the causes cannot be distinguished does not by itself resolve either deliverable.

  1. Latitudinal decrease of the flux density. Establish whether the observed decrease of magnetic flux density toward the poles is an intrinsic signature of a delayed polarity reversal at the highest latitudes — as expected from poleward flux transport — or is substantially produced by observational biases (foreshortening and SNR losses).

  2. Scarcity of small-scale elements. Establish whether the scarcity of magnetic elements with fluxes below approximately 101710^{17} Mx near the poles is real or an observational artifact, and quantify the contribution of these smallest-scale elements to the polar net flux.

References

  1. D. Calchetti, S. K. Solanki, J. Hirzberger, G. Valori, L. P. Chitta, J. Blanco Rodríguez, A. Giunta, T. Grundy, K. Albert, T. Appourchaux, F. J. Bailén, L. R. Bellot Rubio, A. Feller, A. Gandorfer, L. Gizon, A. Korpi-Lagg, X. Li, A. Moreno Vacas, T. Oba, D. Orozco Suárez, J. Schou, U. Schühle, J. Sinjan, H. Strecker, J. C. del Toro Iniesta, A. Ulyanov, R. Volkmer, and J. Woch (2025), 'The First out-of-Ecliptic Observations of the Polar Magnetic Field of the Sun', The Astrophysical Journal Letters 995(2), L60, DOI 10.3847/2041-8213/ae2850, arXiv:2512.03611, https://arxiv.org/abs/2512.03611
  2. Y. Li, S. Yang, Y. Zhang, Q. Song, G. Zhou, Y. Deng, and J. Wang (2026), 'Polarity Reversal of the Polar Magnetic Fields in Solar Cycle 25', Solar Physics 301, 98, DOI 10.1007/s11207-026-02699-x, arXiv:2606.15259, https://arxiv.org/abs/2606.15259
  3. R. Wang, J. Jiang, and Y. Luo (2026), 'Prediction of the Solar Polar Fields in 2026: An Unusually Weak Level Across the Last Five Solar Cycles', The Astrophysical Journal 1000(2), 197, DOI 10.3847/1538-4357/ae4d38, arXiv:2512.17359, https://arxiv.org/abs/2512.17359
  4. S. Yang, J. Jiang, Z. Wang, Y. Hou, C. Jin, Q. Song, Y. Luo, T. Li, J. Zhang, Y. Zhang, G. Zhou, Y. Deng, and J. Wang (2024), 'Long-term variation of the solar polar magnetic fields at different latitudes', Research in Astronomy and Astrophysics 24(7), 075015, DOI 10.1088/1674-4527/ad539a, arXiv:2408.15168, https://arxiv.org/abs/2408.15168
  5. S. Yang, C. Jin, Q. Song, Y. Zhang, Y. Li, Y. Hou, T. Li, G. Zhou, Y. Deng, and J. Wang (2025), 'Variations of the vector magnetic structures in the solar polar regions observed by Hinode', arXiv:2509.19739, https://arxiv.org/abs/2509.19739
  6. S. Gosain and H. Uitenbroek (2024), 'Estimation of projection effects in the polar magnetic field measurements from the ecliptic view', arXiv:2407.09010, https://arxiv.org/abs/2407.09010
  7. L. P. Chitta, D. Calchetti, J. Hirzberger, G. Valori, E. R. Priest, S. K. Solanki, D. Berghmans, C. Verbeeck, E. Kraaikamp, K. Albert, T. Appourchaux, F. J. Bailén, L. R. Bellot Rubio, J. Blanco Rodríguez, A. Feller, A. Gandorfer, L. Gizon, A. Lagg, A. Moreno Vacas, D. Orozco Suárez, J. Schou, U. Schühle, J. Sinjan, H. Strecker, R. Volkmer, J. Woch, X. Li, T. Oba, and A. Ulyanov (2025), 'Supergranulation and Poleward Migration of the Magnetic Field at High Latitudes of the Sun', The Astrophysical Journal Letters 993, L45, DOI 10.3847/2041-8213/ae10a3, arXiv:2509.26586, https://arxiv.org/abs/2509.26586
  8. S. Tsuneta, K. Ichimoto, Y. Katsukawa, B. W. Lites, K. Matsuzaki, S. Nagata, D. Orozco Suárez, T. Shimizu, M. Shimojo, R. A. Shine, Y. Suematsu, T. K. Suzuki, T. D. Tarbell, and A. M. Title (2008), 'The Magnetic Landscape of the Sun's Polar Region', The Astrophysical Journal 688(2), 1374–1381, DOI 10.1086/592226, arXiv:0807.4631, https://arxiv.org/abs/0807.4631
  9. X. Zhang, L. Yang, X. Feng, H. Tian, M. Ma, F. Shen, J. He, M. Zhang, Y. Zhou, Z. Wang, X. Ma, and W. Zhang (2026), 'Influence of Solar Polar Magnetic Fields on the Propagation of Coronal Mass Ejection', arXiv:2603.04928, https://arxiv.org/abs/2603.04928
  10. I. A. Bilenko (2025), 'Solar Polar Field Reversals as the Result of the Global Magnetic field Meridional Flows', arXiv:2510.27441, https://arxiv.org/abs/2510.27441