Whether an autonomous local (small-scale, turbulent) dynamo operates in the solar polar caps is an open question raised by the identification of more than 300 bipolar magnetic emergences (BMEs) at latitudes above 60 degrees, whose leading polarities split almost evenly between positive and negative and whose magnetic axes are randomly oriented instead of following Joy's law. Random tilt alone is disputed as evidence, cycle-independent polar flux is only suggestive, and projection effects, magnetograph sensitivity, and catalog incompleteness (the identified BME count is a lower limit) make polar field measurement intrinsically hard. The problem is to confirm or refute that the polar BMEs and the mixed-polarity polar background they belong to are products of in-situ local dynamo operation rather than globally sourced emergence transported or dispersed poleward.

Background

The Sun's large-scale magnetic cycle is usually described by the Babcock-Leighton picture of the global dynamo: toroidal flux in the interior is amplified and rises by magnetic buoyancy, emerging as bipolar magnetic regions whose tilts statistically follow Joy's law (leading polarity closer to the equator), while the decay of these tilted regions, transported poleward by meridional circulation and turbulent diffusion, builds the strong unipolar polar caps that reverse sign roughly every 11 years and seed the next cycle. In parallel, a local or small-scale dynamo — conversion of turbulent convective kinetic energy into magnetic energy on scales comparable to or below the convective scales, without coherent global organization — is thought to operate in the near-surface layers; it would produce fields that are weak, rapidly evolving, and independent of the solar cycle. Whether such a local dynamo acts autonomously in the polar caps, far from the sunspot belts, was historically untestable because the polar regions are seen edge-on and severely foreshortened from the ecliptic plane, and because the weak, mixed-polarity polar fields strain the sensitivity of the magnetographs (the telescopes that record magnetograms, i.e. maps of the solar magnetic field). Using full-disk magnetograms from the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory combined with extreme-ultraviolet imaging at 211 Angstrom from the Atmospheric Imaging Assembly, Jin, Zhou, Zhang and Wang (2020) identified more than 300 transient bipolar magnetic emergences (BMEs, pairs of opposite-polarity flux that appear and separate in magnetograms) at latitudes above 60 degrees during 2010 June to 2011 May. Unlike active-region bipoles, these polar BMEs showed leading polarities split almost evenly between positive and negative, and their magnetic axes were randomly oriented rather than obeying Joy's law, which the authors interpreted as possible evidence for a local dynamo in the polar region (Jin 2019, Proc. IAU). The interpretation remains contested: Martin (2024) notes that small ephemeral bipoles are also randomly oriented in the active belts whenever no overlying large-scale field is present to align them, and the open polar caps contain no closed large-scale field, so tilt randomness alone does not discriminate a local dynamo from globally sourced emergence. Later vector-field observations (measuring all three components of the magnetic field rather than only the line of sight) with the Hinode solar observatory satellite (Yang et al. 2025) show that the nondominant-polarity polar flux stays near 0.5×1022 Mx0.5\times10^{22}\,\mathrm{Mx} throughout cycle 24 while the dominant polarity peaks near 1.3×1022 Mx1.3\times10^{22}\,\mathrm{Mx} at minimum — a cycle-independent background that again only implies, rather than demonstrates, local generation. Meanwhile, small-scale dynamo action in the photosphere in general has strong observational and numerical support (Rempel et al. 2023), but its latitudinal dependence, its autonomy at the poles, and its coupling to the global dynamo remain undetermined, and the first out-of-ecliptic polar vector magnetograms from the Polarimetric and Helioseismic Imager (PHI) aboard Solar Orbiter were obtained only in 2025 (Calchetti et al. 2025), with polar-flux inversions still model-dependent (Yamashiro et al. 2026).

Problem Statement

Determine whether an autonomous local (small-scale, turbulent) dynamo operates in the solar polar region. Concretely, establish whether the observed random tilt distribution and even split of leading polarities (the polarity of the rotation-leading member of each emerging bipole) of polar bipolar magnetic emergences (BMEs; transient pairs of opposite-polarity magnetic flux that appear and separate on the solar surface) reflect in-situ generation by a local dynamo — confirming or refuting the interpretation proposed by Jin (2019) and Jin, Zhou, Zhang and Wang (2020) — rather than globally sourced emergence whose tilts are randomized by surface flows during or after emergence, by transport and dispersal of active-belt flux poleward, or by the absence of an aligning overlying large-scale field. Resolving the task requires establishing the origin of the polar BMEs and of the mixed-polarity polar background they belong to, either as in-situ products of an autonomous local dynamo or as globally sourced emergence transported or dispersed poleward; evidence beyond the tilt and leading-polarity statistics alone is required, so demonstrating that the tilt signature is non-diagnostic does not by itself settle the question.

References

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