Anfinogentov, Stupishin, Mysh'yakov and Fleishman (2019) detected a long-lived optically thick gyroresonance source at 34 GHz above the light bridge of NOAA active region 12673, implying a record coronal magnetic field of roughly 4000 G at the base of the corona — about twice the previously accepted empirical upper envelope — and announced a systematic analysis of the historical Nobeyama Radioheliograph 34 GHz record. That analysis has now been published: Fedenev, Anfinogentov and Fleishman (2023) identified 27 active regions (1995–2017) with bright 34 GHz gyroresonance emission, about 0.6% of all active regions, five of them showing the compact, bright character of optically thick emission, confirming that coronal fields approaching 4000 G recur in both solar cycles 23 and 24 and are strongly associated with X-class flares, sunquakes, and solar energetic particle events. The occurrence-rate part of the original question is therefore resolved. What remains open is the physical origin and predictive value of these extreme fields: which photospheric evolutionary conditions (flux-emergence rate, colliding or rotating sunspots, shear flows, light-bridge formation) build them, and whether the appearance of an optically thick 34 GHz gyroresonance source is a reliable, quantifiable precursor of major eruptive activity.

Background

Active regions (ARs) are areas of the Sun where strong magnetic flux emerging from the interior pierces the photosphere; they host nearly all flares and eruptions. The coronal magnetic field — the field above the transition region, in the million-degree outer atmosphere — stores the free energy powering this activity, but it cannot be measured routinely. It is inferred either by extrapolating photospheric vector magnetograms with the nonlinear force-free field (NLFFF) approximation, which assumes magnetic forces dominate over plasma pressure, or through radio diagnostics. One such diagnostic is thermal gyroresonance (GR) emission: thermal electrons spiraling along the magnetic field radiate at harmonics ss of the electron gyrofrequency fBe=eB/(2πmec)f_{Be}=eB/(2\pi m_e c), so emission at frequency ff originates in thin layers where B≈357 fGHz/sB \approx 357\, f_{\rm GHz}/s G (about 4000 G for f=34f=34 GHz at the third harmonic). At the low harmonics (s=2s=2–33) this emission is optically thick, meaning the plasma radiates like a blackbody at the local electron temperature, so the highest frequency showing a long-lived bright GR source is proportional to the strongest coronal field on the line of sight. Before 2019, compilations suggested no coronal field above roughly 2000 G, with the strongest direct measurements (Brosius & White 2006, using the Very Large Array (VLA) radio interferometer at the solar limb) approaching that level. The Nobeyama Radioheliograph (NoRH), a Japanese radio imaging interferometer, recorded full-disk images at 17 and 34 GHz nearly every day from 1995 through the late 2010s, creating the archive in which such extreme sources can be sought. Anfinogentov et al. (2019) combined vector magnetograms from the Solar Dynamics Observatory's Helioseismic and Magnetic Imager (SDO/HMI) and the Hinode solar satellite's Spectropolarimeter (Hinode/SOT-SP) with NLFFF extrapolation and a parametric thermal model into a 3D magneto-thermal model of NOAA AR 12673 — host of the X9.3 flare of 2017 September 6, the largest of solar cycle 24, formed during one of the fastest magnetic flux-emergence episodes ever recorded (Sun & Norton 2017). Validating the model against NoRH images, they found an optically thick GR source at 34 GHz implying about 4000 G at the base of the corona, the strongest coronal field on record. The same AR displayed extraordinary photospheric fields: a transverse field of 5570 G in the light bridge (a bright lane separating opposite-polarity umbrae within a sunspot) measured with the Goode Solar Telescope (GST), a 1.6-meter ground-based optical solar telescope (Wang et al. 2018), with the authors' own Hinode reduction yielding comparably strong values; super-strong photospheric fields are a recognized phenomenon elsewhere too, reaching about 6.3 kG in sunspots (Okamoto & Sakurai 2018) and 8.2 kG in the light bridge of AR 11967 (Castellanos Durán et al. 2020). Notably, the NLFFF model showed that the 5570 G light-bridge flux closes at the photosphere or low chromosphere rather than extending into the corona, while a broader region of roughly 5 kG field dominates the coronal field — a height-distribution pattern whose mechanism was not explored. The systematic NoRH survey announced in that paper was published by Fedenev et al. (2023): scanning 1995–2017 with brightness and compactness criteria, they found 27 ARs with bright GR emission at 34 GHz (GR-34) out of 4774 (about 0.57%), five of which (including AR 12673) show the compact, slowly varying, bright behavior expected of optically thick GR sources, so coronal fields of roughly 4000 G do occur in other active regions and in both cycles 23 and 24; occurrence tracks the solar-cycle phase, and 17 of the 27 ARs produced at least one X-class flare. Related modeling of AR 12673's non-potential field evolution exists (Yamasaki et al. 2021), the microwave neutral-line source has been discussed as a geoeffectiveness indicator for this single AR (Kudriavtseva et al. 2021), and postflare loops in these extreme-field ARs have been detected at 34 GHz (Alissandrakis et al. 2024) — but no study has established which evolutionary conditions actually build such coronal fields, why the strongest photospheric flux stays confined below the corona, or whether detecting these sources has validated predictive value.

Problem Statement

For the extreme coronal magnetic fields diagnosed by an optically thick 34 GHz gyroresonance source in the Fedenev et al. (2023) GR-34 catalogue — taking as the target class the five active regions (NOAA 9393, 10808, 11515, 11967, 12673) classified as confirmed optically thick GR-34 sources, implying coronal fields of about 4000 G at the base of the corona if the emission is produced at the third gyroharmonic, or about 3000 G if at the fourth (Fedenev et al. 2023), and using the remaining bright GR-34 active regions of the 1995–2017 survey as the comparison sample — determine how such extreme coronal fields form and whether their detection predicts major eruptions. Because single-frequency data cannot confidently exclude other bright GR-34 active regions from being optically thick, the confirmed subset delimits the optically thick class only up to that classification uncertainty. Specifically: (1) identify which photospheric evolutionary conditions (rate and topology of magnetic flux emergence, collision and rotation of sunspots, shear and convergence flows, light-bridge formation) lead an active region to build a confirmed optically thick GR-34 coronal field; (2) establish whether the appearance of an optically thick 34 GHz gyroresonance source is a reliable precursor of major eruptive activity (X-class flares, fast coronal mass ejections, solar energetic particle events), by quantifying the temporal relation between the onset of the optically thick GR-34 source and the onset of that activity — its lead-time distribution, detection rate and false-alarm rate against a control sample of active regions that lack an optically thick GR-34 source and are matched in sunspot area, magnetic complexity and solar-cycle phase — and stating, against an explicit and justified acceptance threshold for "reliable", whether the association already reported by Fedenev et al. (2023) satisfies that criterion.

References

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