High-cadence vector magnetograms of the 2015 June 22 M6.5 solar flare (SOL2015-06-22T18:23, NOAA active region 12371), obtained with the Goode Solar Telescope in the deep-photospheric Fe I 1.56 μm1.56\ \mu\mathrm{m} line, revealed a transient, unidirectional, counterclockwise rotation of the transverse magnetic-field azimuth by 1212–20∘20^\circ, cospatial and cotemporal with the propagating H-alpha flare ribbon, with a narrow footprint of about 570 km and an azimuth moving toward the potential-field configuration. The source paper showed the change is not the classical line-profile artifact (Stokes I and V profiles stayed unchanged while Q and U varied) but concluded that no existing model — beam-induced self-magnetic fields, hydrodynamic downdrafts, line-impact polarization, or torsional Alfvén-wave pulses — explains the full set of observed properties. Later work has corroborated the phenomenon and added an idealized shear-Alfvén-wave model and adjacent statistical studies, but the mechanism remains unidentified and no photosphere-tailored quantitative model predicting the amplitude, sign, and temporal evolution of the azimuth perturbation at 1.56 μm1.56\ \mu\mathrm{m} formation heights has been published. The problem is to identify the mechanism and establish it quantitatively against the benchmark observations.

Background

Solar flares are powered by magnetic reconnection in the corona, but the released energy is transported to and deposited in the lower solar atmosphere, where it produces most of the visible radiation and measurable changes in the photospheric magnetic field. A vector magnetogram records the three components of the photospheric field; the direction of the horizontal (transverse) component in the solar surface plane — the field azimuth — is retrieved from the linear-polarization Stokes parameters Q and U (via a Milne–Eddington inversion, a standard fit of the full Stokes profiles under a simplified atmospheric model) and carries a 180∘180^\circ ambiguity that must be resolved with an algorithmic or physical assumption. Flare-related photospheric field changes known before 2018 were predominantly permanent, step-like changes: enhancement of the horizontal field around the polarity inversion line (PIL, the line separating opposite vertical magnetic polarities), penumbral decay, and sudden sunspot rotation, generally during strong flares. Transient changes were rarely reported and widely suspected of being instrumental artifacts, because flare heating can alter the profile of the visible spectral lines used for magnetic measurement (turning absorption into emission), producing spurious polarity-reversed 'magnetic transients'.

Xu et al. (2018) observed the M6.5 flare SOL2015-06-22T18:23 in active region NOAA 12371 (close to disk center) with the 1.6 m Goode Solar Telescope (GST) at Big Bear Solar Observatory and its Near InfraRed Imaging Spectropolarimeter (NIRIS), sampling the Fe I line at 1.56 μm1.56\ \mu\mathrm{m}, which forms very deep in the photosphere where flare heating barely reaches. During the impulsive phase they measured a sudden 1212–20∘20^\circ counterclockwise azimuth rotation, cospatial and cotemporal with the propagating H-alpha (hydrogen Balmer-alpha) flare ribbon, with a footprint of about 570 km full width at half maximum along a ribbon-like feature roughly 13,300 km long. The change was transient — the azimuth returned to its pre-flare value after the ribbon swept through — and the measured azimuth moved closer to that of the extrapolated potential field (a current-free field computed from the observed vertical component), indicating that flare loops became less twisted (untwisted) during energy release. Crucially, the line profile remained in absorption and unchanged, and the Stokes I (intensity) and V (circular polarization) profiles were unaffected while Q and U varied, so the rotation is not the classical line-profile artifact; the same event also produced a sudden sunspot rotation as the ribbon passed (Liu et al. 2016).

The paper confronted four candidate mechanisms and found each inconsistent with at least one observed property. (i) The magnetic self-field of precipitating electron beams: by Ampère's law, the upward current equivalent of downward-moving negatively charged electrons induces a magnetic field around the ribbon; the required current, about 1.6×1011 A1.6\times10^{11}\ \mathrm{A} (an electron flux of 1030 s−110^{30}\ \mathrm{s^{-1}}, a tiny fraction of the >20>20 keV electron flux of order 1035 s−110^{35}\ \mathrm{s^{-1}} inferred from hard X-ray spectra), makes the magnitude feasible, but the geometry predicts an increase–decrease azimuth pattern as the ribbon crosses a sunspot, which was not observed; the opposite-sense rotation behind the ribbon is canceled within the model by the self-magnetic field of the trailing electron beams. (ii) Hydrodynamic downdrafts that amplify twist when plasma sinks into denser layers: the loops were observed to untwist. (iii) Linear polarization of chromospheric lines induced by electron-beam impact: no emission appeared in the 1.56 μm1.56\ \mu\mathrm{m} line, and the observed perturbation was one-sided, unlike the predicted two-sided pattern. (iv) Torsional Alfvén-wave pulses: Alfvén waves are incompressible magnetohydrodynamic waves displacing the magnetic field transverse to the propagation direction, and torsional modes rotate the field about a flux-tube axis; they are naturally excited by reconnection, but torsional oscillations should appear periodic, which was not observed, and existing modeling addressed coronal flux tubes, with no quantitative description of wave effects on photospheric fields. The authors concluded that the observed field change cannot be explained by existing models.

Later work has built around, but not resolved, this question. Wheatland et al. (2018) proposed an idealized two-dimensional model in which a large-amplitude shear Alfvén wave launched by coronal reconnection impacts the photosphere along a line coincident with the flare ribbon, and noted that impulsive driving could in principle produce transient changes of the kind observed, without quantitatively modeling the azimuth amplitude, sign, or time evolution. Liu et al. (2018) reanalyzed the same event with GST and the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory, corroborated the transient azimuth increase at the ribbon front in independent data, and interpreted the permanent changes through the collapse (implosion) of coronal fields, while leaving open why the horizontal field first decreases at newly brightened ribbon sites and whether such changes are generic to all flares. A 2023 review of Alfvén waves in solar flares describes flare-related lower-atmosphere magnetic-field changes as likely communicated by large-amplitude Alfvénic wave fronts, citing quantitative modeling only for chromospheric heating, not photospheric field rotation. Živanović et al. (2025) simulated large-amplitude coronal Alfvén-type waves reaching the photosphere and found the delivered energy flux orders of magnitude below what white-light-flare photospheric heating requires — a challenge to Alfvénic transport to the deep photosphere, though posed for heating rather than for the magnetic perturbation itself. Observational follow-ups have addressed adjacent signatures — the statistics of permanent horizontal-field changes (Liu, L. et al. 2022; Yadav & Kazachenko 2023), line-of-sight field changes within flare ribbons (Alanazi & Fletcher 2025), flare-associated pore rotation observed with DKIST, the Daniel K. Inouye Solar Telescope (Yadav et al. 2024), and vortex flows with field amplification in a C9.3 white-light flare attributed to Alfvén wave pulses (Xu, Z. et al. 2025) — but none has reported or explained the transient azimuth rotation itself in another event.

Problem Statement

Identify the physical mechanism responsible for the transient, unidirectional rotation of the photospheric transverse magnetic field observed during the 2015 June 22 M6.5 solar flare (SOL2015-06-22T18:23, NOAA active region 12371), and establish it quantitatively. A complete answer must:

  1. Specify the mechanism — for example reconnection-driven torsional Alfvén-wave pulses, magnetic self-fields and return currents of precipitating electron beams, hydrodynamic flows, or a combination — and reproduce the observed directionality: a counterclockwise azimuth rotation of 1212–20∘20^\circ that is unidirectional along the eastern flare ribbon and brings the measured azimuth closer to the extrapolated potential-field azimuth, with no impulsive change in total field strength or inclination in the benchmark mid-ribbon regions where the rotation was measured (selected away from the sunspot boundary).
  2. Provide a quantitative model, tailored to the formation heights of the Fe I 1.56 μm1.56\ \mu\mathrm{m} line, that predicts the amplitude, sign, spatial scale (a ~570 km FWHM footprint moving with the ribbon front), and temporal evolution (impulsive onset, non-oscillatory profile, recovery after ribbon passage) of the azimuth perturbation, and account for the observed absence of periodic torsional oscillations.
  3. Satisfy the full set of constraining observables, including the absence of a flare-induced line-profile change in the deep-photospheric diagnostic (the line remained in absorption, with Stokes I and V profiles unchanged in the benchmark regions while Q and U varied), consistency with the current and energy budget (a ~100 G perturbation requires of order 1.6×1011 A1.6\times10^{11}\ \mathrm{A} over the ribbon), and the absence of the increase–decrease azimuth pattern that the simple beam self-field geometry predicts.

The single overarching objective is a physical explanation of the transient azimuth rotation; the mechanism identification, quantitative modeling, and constraint satisfaction are jointly supporting deliverables of that one explanation, benchmarked against the public GST/NIRIS observations of SOL2015-06-22.

References

  1. Xu, Y., Cao, W., Ahn, K., Jing, J., Liu, C., Chae, J., Huang, N., Deng, N., Gary, D. E., & Wang, H. (2018). Transient rotation of photospheric vector magnetic fields associated with a solar flare. Nature Communications, 9, 46. DOI 10.1038/s41467-017-02509-w. https://doi.org/10.1038/s41467-017-02509-w
  2. Liu, C., Xu, Y., Cao, W., Deng, N., Lee, J., Hudson, H. S., Gary, D. E., Wang, J., Jing, J., & Wang, H. (2016). Flare differentially rotates sunspot on Sun's surface. Nature Communications, 7, 13104. DOI 10.1038/ncomms13104. https://doi.org/10.1038/ncomms13104
  3. Wheatland, M. S., Melrose, D. B., & Mastrano, A. (2018). Photospheric Response to a Flare. The Astrophysical Journal, 864, 159. DOI 10.3847/1538-4357/aad8ae. https://doi.org/10.3847/1538-4357/aad8ae (preprint arXiv:1808.03097)
  4. Liu, C., Cao, W., Chae, J., Ahn, K., Choudhary, D. P., Lee, J., Liu, R., Deng, N., Wang, J., & Wang, H. (2018). Evolution of Photospheric Vector Magnetic Field Associated with Moving Flare Ribbons as Seen by GST. The Astrophysical Journal, 869, 21. DOI 10.3847/1538-4357/aaecd0. https://doi.org/10.3847/1538-4357/aaecd0 (preprint arXiv:1810.11733)
  5. Russell, A. J. B., & Fletcher, L. (2013). Propagation of Alfvénic waves from corona to chromosphere and consequences for solar flares. The Astrophysical Journal, 765, 81. DOI 10.1088/0004-637X/765/2/81. https://doi.org/10.1088/0004-637X/765/2/81
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  7. Liu, L., Zhou, Z., Wang, Y., Sun, X., & Wang, G. (2022). On the Nature of the Photospheric Horizontal Magnetic Field Increase in Major Solar Flares. The Astrophysical Journal Letters, 934, L33. DOI 10.3847/2041-8213/ac83bf. https://doi.org/10.3847/2041-8213/ac83bf
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  9. Cannon, B., Jing, J., Li, Q., Liu, N., Lee, J., Cao, W., & Wang, H. (2023). Magnetic Reconnection Rate in the M6.5 Solar Flare on 2015 June 22. The Astrophysical Journal, 950, 144. DOI 10.3847/1538-4357/accf9f. https://doi.org/10.3847/1538-4357/accf9f
  10. Yadav, R., Kazachenko, M. D., Afanasyev, A. N., Cauzzi, G., & Reardon, K. (2024). Photospheric Pore Rotation Associated with a C-class Flare from Spectropolarimetric Observations with DKIST. The Astrophysical Journal Letters, 973, L10. DOI 10.3847/2041-8213/ad758d. https://doi.org/10.3847/2041-8213/ad758d (preprint arXiv:2408.16956)
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