Spectropolarimetric inversions of GREGOR/GRIS data (the 1.5 m GREGOR solar telescope and its GRIS infrared spectropolarimeter; 2018 August to 2021 August) showed for the first time a clear variation of the averaged magnetic field strength of the internetwork (IN) quiet Sun — the weakest, most ubiquitous magnetic component of the solar photosphere — both with heliographic latitude and with the solar activity cycle: the averaged field decreases toward the equator-side regions and increases suddenly near the poles, the polar variation is in phase with the sunspot cycle while the disk-center variation is almost anticorrelated with it, and the field strengths range over roughly 40–200 G with a mean of 109 G. These findings challenge the prevailing picture that quiet-Sun magnetism is produced by a purely local (small-scale turbulent) dynamo and is therefore spatially uniform and acyclic, but they rest on only about three years of data, leave the polar enhancement physically unexplained, and rest on a conjectural origin scenario. The open problem is threefold and joint: (1) confirm the latitude/time pattern over a full solar activity cycle using a homogeneous observing and inversion strategy; (2) identify the mechanism responsible for the sudden increase of the averaged IN field strength at high latitudes; (3) quantitatively test the proposed scenario that IN fields emerge as mixed-polarity flux supplied by the global dynamo and are redistributed by meridional flow and turbulent motions, against the alternative of local-dynamo dominance.

Background

The quiet Sun is the portion of the solar photosphere outside active regions (sunspot belts and their plages); it is covered by the magnetic network (fields concentrated along supergranular boundaries, so called because supergranulation is the large-scale convective pattern with cells about 30,000 km across) and, in the cell interiors, by the much weaker and predominantly mixed-polarity internetwork (IN) fields, which are the smallest and faintest magnetic elements currently observable. Because IN fields occupy most of the solar surface at any time, whether they are generated in situ by a local dynamo (amplification of magnetic fluctuations by near-surface turbulent convection, as in small-scale-dynamo simulations such as Rempel 2014) or are partly recycled products of the global dynamo (the large-scale mechanism that produces the 11-year sunspot cycle) is a long-standing open question with conflicting observational answers. Studies of the IN field with the Hinode space telescope found no significant solar-cycle variation over 2006–2012 (Buehler, Lagg & Solanki 2013), while a Fourier spectral analysis of Hinode polarization maps found only a marginally significant difference between the 2007 activity minimum and the 2013 maximum, with weaker small-scale spatial fluctuations at maximum (Faurobert & Ricort 2015); latitude trends from Hinode showed the transverse (horizontal) flux density decreasing with latitude except for a substantial increase right at the poles (Lites, Centeno & McIntosh 2014), and Hinode documented the polar field reversals during cycle 24 (Shiota et al. 2012). A helioseismic analysis of the solar surface-gravity (f-) oscillation mode independently indicated quiet-Sun flux emergence timing consistent with the global cycle (Korpi-Lagg et al. 2022); a broad observational review is Bellot Rubio & Orozco Suárez (2019). Against this backdrop, Trelles Arjona, Martínez González & Ruiz Cobo (2023) analyzed GREGOR Telescope (a 1.5 m solar telescope on Tenerife)/GRIS infrared spectropolarimeter observations around 1.565 μm at disk center and near both poles (2018 August to 2021 August, i.e., roughly three years), deriving the height-dependent average magnetic field strength through Stokes inversions with the SIR code (an inversion technique that fits the four Stokes profiles of spectral lines to infer the stratification of the atmospheric parameters). After validating the inversion strategy on synthetic spectra from magnetohydrodynamic simulations, they reported: mean IN field strengths between about 40 and 200 G (mean 109 G) varying with time and position; an increase of the averaged field toward the poles; long-term variations at disk center and at the poles that are almost anticorrelated; polar variation in phase with the activity cycle, with the disk-center rise starting at least one year before the activity minimum. The authors state that the sudden increase of the field at the poles is still a matter for further research, that completing the whole solar cycle with the same observing and inversion strategy is advisable to confirm the disk-center decaying phase, and that their proposed origin scenario — global-dynamo-supplied mixed-polarity flux disrupted by convection into IN fields and transported poleward by the meridional flow (the large-scale circulation from equator to poles at the surface and back at the base of the convection zone) — is speculative and requires testing against the local-dynamo alternative.

Problem Statement

Determine the physical origin of the reported latitude and solar-cycle dependence of the quiet-Sun internetwork (IN) magnetic field strength, jointly through: (1) a full-cycle observational confirmation — extend the homogeneous GREGOR/GRIS-style multi-epoch spectropolarimetric observations at disk center and high-latitude (polar) pointings, reduced and inverted with one fixed strategy, across a complete solar activity cycle, to establish whether the averaged IN field strength really shows the disk-center rise-and-decay and the in-phase polar variation seen in 2018–2021, including the phase relations between disk center and poles; (2) a mechanism for the polar enhancement — identify and quantitatively verify the physical mechanism responsible for the sudden increase of the averaged IN field strength toward the solar poles, discriminating among the candidate causes (projection and inclination-dependent visibility of highly inclined IN fields, poleward meridional transport and accumulation of decaying active-region flux, magnetic-field expansion with height, and latitude dependence of any local small-scale dynamo); and (3) a test of the origin scenario — quantitatively evaluate the conjecture that IN magnetism is supplied by the global dynamo as mixed-polarity emerging flux redistributed by meridional flow and turbulent motions, against the alternative that a local turbulent dynamo dominates, using coordinated observations and modeling (surface flux-transport simulations, radiative-MHD/small-scale-dynamo simulations at multiple latitudes and cycle phases, and helioseismic constraints on subsurface flux emergence). These three components jointly support one objective — establishing what fraction of the quiet-Sun IN magnetism is cycle-modulated global-dynamo flux versus locally generated field, and why its averaged strength increases at the poles.

References

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