Automated tracking of more than 31 million small magnetic concentrations in 11 years of magnetograms from the Helioseismic and Magnetic Imager (HMI) on NASA's Solar Dynamics Observatory (SDO) (2011-2021, a full activity cycle) revealed that the amplitude of their minute-timescale horizontal oscillations, and the polarization of those perturbations between the east-west and north-south directions, are modulated in phase with the 11-year solar activity cycle, peaking at activity maximum (Stangalini et al. 2025). This demonstrates that the global solar dynamo influences the dynamics of small-scale magnetic fields, but the physical mechanism responsible is unidentified. The candidates are weak modulation of the near-surface convective flows by the large-scale magnetic field, dynamical coupling through deep-rooted relics of decayed active regions, or a combination of these or another process, which must be named and physically justified. The latitudinal and disk-position dependence of the modulation is also unmeasured, since the discovery analysis was confined to a 400 x 400 arcsec^2 region at disk center; this is background context for the mechanism question, not a separate deliverable. The problem is to determine the mechanism that couples global-scale solar magnetism to the minute-scale horizontal dynamics of photospheric magnetic elements.

Background

The Sun's magnetism is organized on two widely separated scales. On the global scale, the solar dynamo - a magnetohydrodynamic self-excitation process driven by convection and differential rotation in the solar interior - regenerates the large-scale magnetic field with an approximately 11-year periodicity, the solar activity cycle, most visibly marked by the cyclic appearance and decay of sunspots and active regions. On the small scale, the quiet photosphere (the Sun's visible surface layer) is carpeted with isolated magnetic flux concentrations known as magnetic elements: kilogauss-strength magnetic flux tubes, typically 100-300 km across, confined to the dark intergranular lanes between convective granules and partly organized into the photospheric network (a web of enhanced magnetic flux outlining the boundaries of the largest near-surface convective cells). These elements are continuously advected and buffeted by granular plasma flows, which excite horizontal, kink-type oscillations of the flux tubes (swaying motions transverse to the tube axis) with periods of a few minutes. These oscillations act as conduits for wave energy into the outer solar atmosphere and are studied through long time series of magnetograms, i.e. maps of the line-of-sight magnetic field derived from polarized light.

Whether the small-scale field is coupled to the global cycle is a long-standing question. Studies of the photospheric flux budget across past cycles have variously found in-phase, anti-phase, or no correlation between the amount of small-scale flux and the sunspot cycle, and mean-field modeling by Karak & Brandenburg (2015) showed that a turbulent small-scale dynamo (field generation by turbulence acting independently of the global dynamo) can weaken or remove such a correlation; Jin et al. (2011) documented the cycle dependence of small-scale magnetic elements in cycle 23. These works concern the amount of small-scale magnetic flux, not the dynamics of its motions.

Stangalini et al. (2025) added a new, dynamical datum. Using the Helioseismic and Magnetic Imager (HMI) on NASA's Solar Dynamics Observatory (SDO), which has provided uniform 45-second-cadence magnetograms in the Fe I 617.3 nm spectral line since 2010, they tracked magnetic elements with the automated SWAMIS feature-tracking algorithm (detecting concentrations above 40 G) in a 400 x 400 arcsec^2 (290 x 290 Mm^2) region at the center of the solar disk. They analyzed 40-minute sequences every three days from 1 January 2011 to 31 December 2021, covering a complete activity cycle, for a total exceeding 31 million tracked concentrations with effective radii peaking near 255 km and a mean lifetime of about 20 minutes. The amplitude of each element's horizontal (kink-like) perturbation was taken as the standard deviation of its horizontal velocity along the east-west and north-south axes.

The result was that this amplitude is not stationary: it varies on the timescale of the activity cycle, smallest at cycle minimum and increasing toward maximum, and the perturbations are polarized, with the east-west amplitude increasing faster than the north-south amplitude, indicating a preferred direction of the underlying driver. Both the amplitude variation and the polarization are strongest at activity maximum and reduced at minimum, tracking the sunspot cycle - behavior that cannot be produced by a purely local driver such as granular buffeting alone, and which indicates a global-scale dynamical influence on minute-scale motions. The authors note a formal resemblance to torsional oscillations, the cyclic, latitude-dependent perturbations of the Sun's differential rotation that are thought to be driven by the oscillating large-scale magnetic field in the solar interior; the new observation suggests this dynamical influence of the global field extends down to the smallest photospheric scales. The tracked elements sit at the resolution limit of HMI (about 1 arcsec) and are therefore mostly network fields or fields associated with active-region emergence, a fact relevant to how the coupling might operate.

Problem Statement

Determine the physical mechanism by which the global-scale solar magnetic cycle (the 11-year activity cycle and the dynamo that regenerates it) modulates the minute-timescale horizontal dynamics of photospheric magnetic elements - specifically the solar-cycle variation of the horizontal-oscillation amplitude and its east-west/north-south polarization reported by Stangalini et al. (2025). The dependence of this modulation on disk position and latitude is unmeasured, the discovery analysis having been confined to a 400 x 400 arcsec^2 region at disk center; the source notes that extending a similar analysis at higher latitudes could provide useful insights, also in preparation for the polar observations of Solar Orbiter in the coming years. The mechanisms to be discriminated are: (i) the near-surface convective flows that buffet the magnetic elements are weakly modulated by the large-scale solar magnetic field, producing the observed amplitude and directional changes; (ii) at least some of the tracked elements are relics of active-region decay with roots deep enough to be dynamically coupled to the interior large-scale field, so that its cycle variation is imprinted on their motions - the explanation the source considers 'arguably a more likely explanation'; or (iii) a combination of these or another process. A resolution must identify which mechanism operates and name the physical pathway by which it produces the observed cycle variation of the amplitude and the east-west/north-south polarization; an appeal to a bare 'combination' of the candidates, or to a process that is not named and physically justified against them, does not by itself constitute a resolution, and the proposed mechanism must be accompanied by an observation or simulation that would discriminate it from the remaining alternatives. This mechanism identification, with its named pathway and discriminating observation or simulation, is the complete required deliverable; the latitudinal and disk-position dependence noted above is background context and an avenue for future work, not a condition of resolution. The source flags higher-resolution observations (e.g., telescopes of the Daniel K. Inouye Solar Telescope (DKIST) class, resolving scales below 100 km), analysis at higher latitudes, and further advancements in simulations as promising avenues for progress on the question.

References

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