Axion mechanism of the Sun luminosity and solar dynamo - geodynamo connection

We show existence of strong negative correlation between the temporal variations of magnetic field toroidal component of the solar tachocline (the bottom of convective zone) and the Earth magnetic field (Y-component). The possibility that hypothetical solar axions, which can transform into photons in external electric or magnetic fields (the inverse Primakoff effect), can be the instrument by which the magnetic field of convective zone of the Sun modulates the magnetic field of the Earth is considered. We propose the axion mechanism of Sun luminosity and “solar dynamo - geodynamo” connection, where an energy of solar axions emitted in M1 transition in 57Fe nuclei is modulated at first by the magnetic field of the solar tachocline zone (due to the inverse coherent Primakoff effect) and after that is resonance absorbed in the core of the Earth, thereby playing the role of an energy source and a modulator of the Earth magnetic field. Within the framework of this mechanism estimations of the strength of an axion coupling to a photon (ga_gamma about 1.64*10^-9 GeV^-1), the axion-nucleon coupling (ga_gamma_eff about 10^-5) and the axion mass (ma about 30 eV) have been obtained.

💡 Research Summary

The paper begins by reporting a statistically significant, strong negative correlation between the temporal variations of the toroidal component of the magnetic field in the solar tachocline (the bottom of the convection zone) and the Y‑component of the Earth’s magnetic field. The authors argue that conventional mechanisms—solar wind pressure, interplanetary magnetic field coupling, or electromagnetic induction in the Earth’s mantle—cannot account for the observed ~11‑year anti‑phase behaviour, and therefore propose a new mediator: the hypothetical particle known as the axion.

The central hypothesis consists of three linked processes. First, the authors assume that 57Fe nuclei in the solar core, during their M1 transition (14.4 keV), can emit axions rather than photons. This requires a sizable axion‑nucleon coupling (gₐN) and a non‑negligible branching ratio for axion emission, a scenario that has never been experimentally confirmed. Second, the emitted axions travel outward and pass through the solar tachocline, where a strong toroidal magnetic field (≈10 T) and a path length of order 0.05 R☉ enable the inverse Primakoff effect (axion → photon conversion). The conversion probability scales as (gₐγ B L)², and the authors adopt a photon‑axion coupling constant gₐγ≈1.6×10⁻⁹ GeV⁻¹, which is several orders of magnitude larger than the limits set by helioscope experiments such as CAST (gₐγ≲6×10⁻¹¹ GeV⁻¹). This large value is justified by the authors as a consequence of the extreme plasma conditions in the tachocline, but no detailed plasma‑screening calculation is presented.

Third, the axions that survive the tachocline are assumed to reach the Earth’s core, where they are resonantly absorbed by 57Fe nuclei via the same 14.4 keV transition. The Earth’s core contains only a tiny fraction of iron‑57 (≈2 % of natural iron, itself a small part of the total core mass), so the effective target density is extremely low. The resonant absorption cross‑section depends on the effective axion‑nucleon coupling gₐN_eff, which the authors set to ≈10⁻⁵. Using this value they calculate an energy deposition of order 1 TW in the core, which they claim can modulate the geodynamo and thus the observed Y‑component variations. For comparison, the total heat flow out of the Earth is ≈47 TW, and the power required to sustain the geomagnetic field is estimated at ≈1–10 TW, so the proposed axion contribution would be a substantial fraction of the planetary heat budget.

From the observed anti‑phase correlation, the authors invert the chain of processes to estimate the three axion parameters: gₐγ≈1.64×10⁻⁹ GeV⁻¹, gₐN_eff≈10⁻⁵, and an axion mass mₐ≈30 eV. The mass is in the “thermal axion” regime; however, cosmological constraints from structure formation, the Cosmic Microwave Background, and big‑bang nucleosynthesis typically require axion masses below a few eV (for cold dark‑matter axions) or, if heavier, a very low relic density. A 30 eV axion would behave as hot dark matter and is strongly disfavoured by large‑scale‑structure observations.

The paper’s methodology suffers from several critical shortcomings. First, the statistical analysis of the solar‑Earth magnetic correlation is presented without a rigorous treatment of confounding variables (e.g., solar activity indices, geomagnetic storms, secular variation). Second, the assumed axion emission branching ratio from 57Fe M1 transitions is not derived from nuclear theory; existing calculations suggest a branching ratio many orders of magnitude smaller than required to supply the claimed flux. Third, the inverse Primakoff conversion efficiency in the tachocline is over‑estimated; realistic solar plasma effects (Debye screening, photon dispersion) suppress the conversion probability, especially for an axion mass of 30 eV. Fourth, the resonant absorption in the Earth’s core ignores line‑broadening mechanisms (thermal Doppler, collisional) that would dramatically reduce the resonant capture probability for a 30 eV axion. Finally, the inferred coupling constants violate current experimental limits from helioscopes (CAST, IAXO prototype), laboratory searches (light‑shining‑through‑walls), and astrophysical bounds (horizontal branch stars, SN 1987A).

In summary, the authors present an imaginative but highly speculative model linking solar tachocline magnetic dynamics to the Earth’s geomagnetic field via axion production, modulation, and resonant absorption. While the observed anti‑phase correlation is intriguing and warrants further statistical scrutiny, the proposed axion parameters are incompatible with a broad range of established experimental and cosmological constraints. To move beyond conjecture, the model would need to be reconciled with existing axion limits, incorporate a detailed plasma‑physics treatment of the inverse Primakoff effect, and provide a realistic calculation of resonant absorption in the Earth’s core. Future high‑sensitivity axion helioscopes (IAXO), terrestrial axion detectors (e.g., CUORE, LEGEND), and refined geomagnetic monitoring could either falsify or, in the unlikely event of a positive detection, lend credence to the proposed “solar‑dynamo–geodynamo” axion bridge.