A century ago, magnetic observatories recorded only the slow breathing of Earth's core. Today, their data have become hunting grounds for particle physics. At the Eskdalemuir Observatory in northern England, fluctuations of the magnetic field are recorded every hundredth of a second. Ten years of such records, 9909 eight-hour segments, have become a trap for a ghost: the dark photon.
The dark photon is a hypothetical particle similar to the photon but with mass. It can interact weakly with ordinary light through kinetic mixing. If dark matter consists of such particles, their collective oscillations behave like a classical wave. This wave rocks Earth's magnetic field, turning the planet into a giant antenna tuned to the whisper of dark matter.
The key technique of this work is accounting for atmospheric conductivity. Earlier theoretical predictions of the signal were limited to frequencies below 1 Hz and suffered from spurious infinities. Now Maxwell's equations are solved with a realistic conductivity profile, allowing for the first time the use of the resonance in the Earth–ionosphere cavity. The signal was sought as a narrow spectral line with a signal-to-noise ratio above a threshold and consistency across years.
The result: 342 candidates, but none matched the candidates from the previous SuperMAG search. Most are likely instrumental artifacts. However, upper limits on the mixing parameter ε were obtained: about 5×10⁻¹¹ at a dark photon mass of 3×10⁻¹⁴ eV (frequency ~7.7 Hz). In the mass range 10⁻¹⁵–2×10⁻¹³ eV, these limits surpass all previous ground-based constraints. For masses below 10⁻¹⁵ eV, sensitivity is limited by data noise.
The method opens the way to using the entire network of geomagnetic observatories as a distributed dark matter detector. The analysis can be extended to frequencies above 100 Hz, where a second resonance is predicted around 2 kHz. Joint analysis of data from multiple observatories will allow distinguishing the dark photon signal from local noise and refining the limits. Thus, a planet that has served as a home for life for billions of years becomes a tool for searching for invisible matter.
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 for searching for invisible matter.