Dark matter reveals itself only through gravity, but its nature is unknown. Among the candidates are ultralight bosonic fields that behave as classical waves. The dark photon is a massive vector boson that can weakly mix with the ordinary photon. This mixing turns Earth into a detector: the dark photon field induces a magnetic signal that can be sought in geomagnetic data. This work joins the ranks of dark photon searches using terrestrial and space-based observations, but for the first time accounts for atmospheric conductivity at frequencies above 1 Hz.
We used data from the Eskdalemuir Observatory (UK) from September 2012 to November 2022. Two horizontal channels sampled at 100 Hz were split into 8-hour segments (9909 segments in total). For each segment, we computed the power spectral density, then combined the segments with noise weighting. Frequencies that are integer multiples, where artificial lines appear, were excluded. The signal was sought as a narrow spectral line with a signal-to-noise ratio above threshold and stability across years. The theoretical signal was computed from Maxwell's equations with a radial profile of atmospheric conductivity.
We found 342 candidates with signal-to-noise ratio greater than 2, but none matched candidates from the previous SuperMAG search. Most candidates are likely instrumental. We obtained upper limits on the kinetic mixing parameter ε: about 5×10⁻¹¹ at a dark photon mass of 3×10⁻¹⁴ eV (frequency ~7.7 Hz), corresponding to the resonance in the Earth–ionosphere cavity. In the mass range 10⁻¹⁵–2×10⁻¹³ eV, our limits surpass all previous terrestrial constraints. For masses below 10⁻¹⁵ eV, sensitivity is limited by data noise.
This work extends the frequency range for dark photon searches using geomagnetic data. Including atmospheric conductivity removes spurious divergences in the theoretical signal and makes the prediction reliable. This is the first direct limit on the dark photon in the range 10⁻¹⁵–10⁻¹³ eV that is stronger than cosmological constraints. The method can be applied to data from other observatories and to higher frequencies.
The analysis can be extended to frequencies above 100 Hz, where a second resonance is predicted near 2 kHz. Using a global network of magnetometers (e.g., SuperMAG) would allow distinguishing a dark photon signal from an axion signal by the spatial structure of the field. Accounting for the stochasticity of dark matter and local variations in ionospheric conductivity will refine the limits. A search for correlations between observatories is possible to suppress local noise.
The results affect particle physics (constraints on the dark photon), astrophysics (dark matter search), and geophysics (using geomagnetic data as a detector).
Perform a joint analysis of data from multiple observatories to verify candidates and improve sensitivity. Extend the analysis to frequencies up to 2 kHz, where a second resonance is expected.
For particle physics, the existence of ultralight bosons and their interaction with the photon remains an open question. This work closes part of the parameter space but does not solve the nature of dark matter. For geophysics, it is important to separate the signal from natural variations of the magnetic field and instrumental effects.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study searched for dark photons in 10 years of data from the Eskdalemuir geomagnetic observatory. A theoretical signal was calculated accounting for atmospheric conductivity, narrow spectral lines were identified, and their stability across years was verified. Upper limits were obtained on the mixing parameter ε in the mass range 10⁻¹⁵–2×10⁻¹³ eV, surpassing previous ground-based constraints.
Where its strength is
Accounting for atmospheric conductivity removes divergences and makes the signal prediction reliable, which for the first time allowed strong constraints in this mass range.
Where it could go next
Worth trying
Why this may matter
The method could become a standard tool for searching for ultralight dark matter, and combining data from different observatories could cover new mass ranges.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.58, against a usual 0.64. It looks like few have drilled nearby.
The frequency of 8 Hz, where the signal is maximal, coincides with the first Schumann resonance, a global electromagnetic resonance excited by lightning.
In Liu Cixin's novel 'The Three-Body Problem', aliens use protons unfolded into multidimensional structures to influence physics on Earth. Here, Earth itself becomes a tool to search for invisible matter.