The Sun's global magnetic field is commonly assumed to be axisymmetric about its rotation axis, an assumption built into most solar dynamo models. Pastor Yabar, Martínez González and Collados (2015) detected a rotation-period ('monthly') modulation of the line-of-sight photospheric field at all latitudes — strongest near the poles — in five years of SDO/HMI magnetograms, implying that the Sun's large-scale magnetic axis is not aligned with its surface rotation axis. Independent estimates from Faraday rotation (about 3.3 degrees at the 1997 minimum), 1996-minimum polar-cap geometry (1–10 degrees, average 4–6 degrees), and polar coronal-hole tracking (about 5 degrees over 2015–2022) corroborate a persistent misalignment of a few degrees. What remains open is a quantitative characterization of the misalignment — its exact magnitude, whether it is steady or varies over the 11-year cycle, and whether its geometry is a fixed tilted dipole or a rotating non-axisymmetric structure — together with its physical origin (a depth-dependent rotation axis, non-axisymmetric dynamo modes, or preferred-longitude flux advection), the helioseismic detectability of an axis tilt with depth, and whether solar dynamo models can self-consistently produce a stable tilted global field.

Background

A magnetogram is a map of the component of the solar photospheric magnetic field along the observer's line of sight, obtained by measuring the Zeeman splitting of spectral lines. Because the Sun's rotation axis is inclined by 7.25 degrees to the plane of the ecliptic (Earth's orbital plane), telescopes in Earth orbit see each solar pole only obliquely, alternating favorably each year; the polar fields themselves are weak, of order a few gauss, close to instrumental sensitivity. The polar fields carry the large-scale 'axial' dipole of the Sun, which reverses polarity near each 11-year activity maximum, while the 22-year magnetic cycle is attributed to a dynamo: the interaction of rotation, convection, and meridional circulation (the slow large-scale flow of plasma along the surface from the equator toward the poles, returning through the interior) that regenerates the global field. Standard solar dynamo theory assumes the magnetic and rotation axes coincide. Throughout this statement, 'the Sun's large-scale magnetic axis' means the axis of the dominant dipole (l=1l=1) component of the global photospheric magnetic field; the observational approaches described below measure this axis through different proxies — a best-fit global dipole from magnetogram analyses, the centres of the polar coronal holes, the coronal topology sampled by Faraday rotation, or the phase of the rotation-period modulation of the longitude-averaged line-of-sight field — which is why the quoted magnitudes and inferred geometries are method-dependent and must be compared together with the proxy underlying each estimate.

Whether this alignment actually holds had been checked before 2015. Studies of the global dipole (for example DeRosa, Brun and Hoeksema 2012) found a high degree of alignment for most of the cycle, with misalignment concentrated around the polar-field reversals. Near the 1996 activity minimum, Norton, Raouafi and Petrie (2008) found from polar-cap and coronal-streamer geometry a tilted dipole of 1–10 degrees (average 4–6 degrees), and Mancuso and Garzelli (2007) inferred a tilt of 3.3 degrees from Faraday rotation (the rotation of the polarization angle of radio waves from extragalactic sources whose line of sight grazes the outer corona, which constrains the coronal magnetic topology), consistent with contemporaneous Wilcox Solar Observatory estimates. These were, however, snapshot estimates, not a continuous measurement.

Pastor Yabar, Martínez González and Collados (2015) analyzed one full-disk SDO/HMI (Helioseismic and Magnetic Imager on the Solar Dynamics Observatory) magnetogram per day from 2010 April 30 to 2015 January 24. They found a modulation of the latitude-binned, longitude-averaged line-of-sight field at the solar rotation period (about 27 days as seen from Earth, hence 'monthly'), present at all latitudes and strongest near the poles. A field that is axisymmetric about the rotation axis cannot produce such a modulation when averaged in the rotating frame; the natural reading is a global (bipolar) field tilted with respect to the surface rotation axis, so that the projected polar signal oscillates as the Sun rotates. Strikingly, the modulation amplitude stayed roughly constant from activity minimum to maximum, suggesting the tilt does not follow the activity cycle. The authors listed candidate explanations: a tilt of the global field produced by a rotation axis whose orientation varies with depth in the convection zone; inherently non-axisymmetric dynamo modes; or coherent advection of magnetic flux toward the poles at preferred longitudes by the meridional circulation — a scenario they themselves deemed unlikely over five years. They also noted that testing the depth-dependent-axis hypothesis helioseismically requires not only the splitting of global oscillation modes into rotationally split frequency components (routinely measured) but also accurate relative amplitudes of those split components, whose accurate measurement is a hard task for helioseismology and may or may not be within reach of present data; and that a persistent tilt would bear on the heliospheric current sheet (the magnetic polarity-separating surface in the solar wind, already known to be tilted and warped), on quasi-periodic 27-day variations of the interplanetary field and solar-wind speed, and on geomagnetic and cosmic-ray recurrences — the sector structure of the interplanetary field is known to depend on the dipole tilt angle (Du, Xu and Feng 2008).

The detection has since been independently corroborated in outline — Riehokainen, Smirnova, Solov'ev and Strekalova (2025) tracked the centers of polar coronal holes (dark, open-field regions that cap the poles near activity minimum) in SDO/AIA (Atmospheric Imaging Assembly on the Solar Dynamics Observatory) extreme-ultraviolet images over 2015–2022 and found the magnetic-axis proxy deviates from the rotation axis by about 5 degrees on average, rotating around it with a main period of 15–16 days that they attribute to the sector structure of the polar field — but the magnitude remains method-dependent, the inferred geometry (rotating sector structure versus a fixed tilted dipole) differs between studies, and no explanation or dynamo-theoretic treatment exists. Solar Orbiter performed the first out-of-ecliptic polar magnetic-field observations in spring 2025 at 15–17 degrees heliolatitude, an early step toward the sustained high-latitude polar time series the question requires, while dedicated analyses show that even the inference of polar magnetic flux itself remains model-dependent at the tens-of-percent level (Yamashiro et al. 2026).

Problem Statement

Determine and explain the misalignment between the Sun's large-scale magnetic axis and its surface rotation axis, the question left open by Pastor Yabar, Martínez González and Collados (2015):

(1) Observational characterization: establish the magnitude of the misalignment and its temporal behavior — whether the tilt is steady or varies with the 11-year activity cycle or on decadal timescales — from extended, high-sensitivity polar magnetic-field time series covering as much of the activity cycle as available observations permit (the source Letter calls for longer time series with better polarimetric sensitivity spanning a complete 11-year cycle as a future observational effort, which no single facility provides yet), and determine its longitudinal geometry: a fixed tilted dipole, a rotating non-axisymmetric (sector) structure, or a combination.

(2) Helioseismic test: establish whether present helioseismic data and techniques — rotational splitting of global oscillation modes together with accurate relative amplitudes of the split components — are sensitive enough to detect a depth-dependent orientation of the rotation axis in the convection zone, or whether new instrumentation is required.

(3) Physical origin: identify whether the misalignment arises from a depth-dependent variation of the rotation axis in the convection zone, from inherently non-axisymmetric dynamo modes, from persistent preferred-longitude advection of magnetic flux by the meridional circulation, or from a combination of these processes.

(4) Theoretical incorporation: demonstrate self-consistently whether and how a solar dynamo model can produce a stable tilted global field and what physical mechanisms set the magnitude and temporal evolution of the tilt.

(5) Implications: assess, as implications raised in the source Letter, the consequences a persistent tilt would have for the heliospheric current sheet, solar-wind modulation, and coupling to interplanetary and terrestrial magnetospheric phenomena on rotation-period timescales.

These five deliverables jointly serve one scientific objective — a quantitative, causally explained characterization of the Sun's magnetic-axis misalignment. All five must be addressed for a complete resolution: deliverable (1) may be satisfied by a quantitative synthesis and comparison of the published estimates where the sustained full-cycle polar time series it calls for does not yet exist, and deliverable (5) may be assessed from published work rather than modelled in detail.

References

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  2. Norton, A. A.; Raouafi, N.-E.; Petrie, G. J. D. (2008). 'The Tilted Solar Dipole as Observed and Modeled during the 1996 Solar Minimum'. The Astrophysical Journal 682, 1306–1314. DOI: 10.1086/589684. https://doi.org/10.1086/589684
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