A Kyoto-led team reanalysed a decade of geomagnetic records from a Scottish observatory, after working out how the Earth-ionosphere cavity would amplify an axion signal.

Dark matter accounts for something like a quarter of the universe’s total energy content and has never been detected in a laboratory. The experiments built to find it are enormous — tonnes of liquid xenon underground, cryogenic crystal arrays — and they are mostly tuned to a candidate particle that has steadily failed to appear.

A study in Progress of Theoretical and Experimental Physics from Kyoto University, with collaborators at Hiroshima University and Nihon University, takes a different route: use the planet. The team worked out how an extremely light dark matter particle would perturb the Earth’s magnetic field, then looked for that perturbation in a decade of existing geomagnetic records.

“We asked ourselves whether we could use the Earth itself as a giant detector in the search,” said Atsushi Taruya of Kyoto University, the corresponding author.

Particles too light for a conventional detector

The candidates here are axions and dark photons at masses roughly 19 to 21 orders of magnitude lighter than an electron, which is a number worth sitting with for a moment. At those masses a particle is so light that its quantum wavelength is enormous, and the dark matter around us behaves less like a hail of individual particles than like a classical field oscillating slowly everywhere at once.

That changes what a detector should be. A xenon tank works by waiting for a particle to strike a nucleus, which a field this diffuse will never do in any detectable way. What such a field can do is couple weakly to electromagnetism and produce a faint oscillating magnetic signal — at a frequency set by the particle’s mass, and in this mass range that frequency falls in the extremely low frequency band, a few hertz and below.

Which is a band the Earth is already monitored in, continuously, by geomagnetic observatories that have nothing to do with particle physics.

The cavity, and the theory needed to use it

The reason this works better than it sounds is a resonance. The space between the Earth’s surface and the ionosphere forms a cavity that behaves as a natural electromagnetic resonator, with a fundamental mode near 8 hertz — the first of the Schumann resonances, ordinarily excited by lightning.

“The Earth-ionosphere cavity acts as a natural resonator that amplifies electromagnetic waves right around the mass range we wanted to probe,” Taruya said.

Exploiting it required new theory. Earlier treatments of this signal handled only frequencies below 1 hertz; the team built a framework that accounts for the electrical conductivity of the atmosphere, which extends reliable predictions to about 30 hertz and brings the resonance into range.

The framework also makes a prediction that distinguishes the two candidates geographically. Axion-origin signals should peak in Southeast Asia, because the coupling depends on the orientation and strength of the local geomagnetic field; dark photon signals should be nearly uniform across the globe.

Ten years of Scottish data

The analysis used about ten years of geomagnetic measurements, from 2012 to 2022, recorded at the British Geological Survey’s Eskdalemuir Observatory in southern Scotland.

For a specific mass range, the resulting limits on axion coupling are roughly 100 times tighter than the previous best ground-based experiment — and they rival constraints derived from Chandra and NuSTAR X-ray observations, which depend on astrophysical modelling assumptions that a terrestrial measurement does not.

That last point is the real contribution. The strongest existing bounds on light axions mostly come from astrophysics, where the inference runs through models of stellar interiors or magnetised plasmas, and a limit of comparable strength obtained from an instrument on the ground reading a field we can measure directly is independent in a way that matters when nobody knows which assumptions are wrong.

The dark photon analysis turned up several signal candidates that might come from dark matter. Their nature is unconfirmed.

How to read a candidate signal

That sentence deserves expansion rather than excitement, and the authors’ own wording is appropriately flat.

A decade of continuous magnetometer data in a band crowded with natural and human-made activity will contain narrow spectral features for many reasons. Power-line harmonics, railway electrification, instrumental artefacts, ionospheric variability and the Schumann resonances themselves all produce structure at low frequencies, and the prior probability that any given peak is dark matter is very low.

The test that would settle it is the geographic one the theory supplies, and it is the test this analysis cannot run. A single observatory in Scotland cannot check whether a signal peaks in Southeast Asia or is globally uniform, which is precisely the signature that separates an axion from a dark photon and either from a local disturbance. Confirming anything would require simultaneous records from stations spread across latitudes, correlated in phase.

The good news is that such records exist. There is a global network of magnetic observatories with long archives, and the natural next step is a multi-station correlation analysis — which makes the candidate signals a reason to look rather than a result.

The merits and the limits of reusing data

The approach has an obvious efficiency. The measurements were made and paid for by geophysicists over a decade, and the contribution here is theoretical: working out what to look for, and where in the spectrum the Earth would amplify it.

It also inherits whatever the original measurements were designed for. An observatory built to monitor geomagnetic variations has a calibration, a noise floor and a sampling regime chosen for that purpose, and sensitivity to a coherent sub-nanotesla oscillation was not among the design goals. A purpose-built instrument would do better, and the point of an analysis like this is partly to establish whether building one is worthwhile.

The honest summary is that nothing has been found, the place where it is not has been mapped a hundred times more precisely than before by a ground-based measurement, and the method is cheap enough to extend. In a field where the leading candidate has failed to appear for forty years, a new way of looking at a previously inaccessible mass range is worth more than another null result from the same apparatus.

The research was funded by the Japan Society for the Promotion of Science. The paper was published on 8 June 2026; the release names only Taruya and does not give the full author list or state any conflicts of interest.

---

Notes

1. “Axion Dark Matter Search from Terrestrial Magnetic Fields at Extremely Low Frequencies.” Progress of Theoretical and Experimental Physics, 2026. DOI: 10.1093/ptep/ptag108.
2. Kyoto University. “Searching for dark matter with the world’s biggest detector.” EurekAlert!, 4 August 2026. https://www.eurekalert.org/news-releases/1138855.

NASA/GSFC/SDO