Using the Earth's magnetic field to hunt for axions and dark photons

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Add as preferred source Dark matter's existence is all but certain—astronomers believe it makes up about a quarter of the universe's total energy content—yet its true identity has eluded us for decades. Two of the leading candidates for dark matter are the hypothetical particles ultralight axions and dark photons, which in the range studied here would be some 19 to 21 orders of magnitude lighter than the electron.

Conventional axion searches tend to involve converting them into photons with the help of strong laboratory magnets. However, research in a laboratory inherently limits the space over which such a field can be applied.

A collaborative team of researchers from Kyoto University, Hiroshima University and Nihon University realized that, by contrast, Earth's own magnetic field spans a scale no laboratory could match. Their paper is published in the journal Progress of Theoretical and Experimental Physics .

"We asked ourselves whether we could use the Earth itself as a giant detector in the search," says corresponding author Atsushi Taruya. "The Earth–ionosphere cavity acts as a natural resonator that amplifies electromagnetic waves right around the mass range we wanted to probe."

However, existing theory could only handle frequencies below 1 Hz, leaving the rest of this range unpredicted. This inspired the team to construct a new theoretical framework that accounts for the atmosphere's electrical conductivity, showing that the Earth–ionosphere cavity amplifies signals near 8 Hz and extending reliable predictions up to about 30 Hz.

They expected the axion-origin signals to vary by location, with the strongest signals in Southeast Asia, whereas dark photon signals should look nearly the same everywhere.

Building on their new framework, the team analyzed about a decade's worth of geomagnetic field data from 2012 to 2022 from the British Geological Survey's Eskdalemuir Observatory. They removed artificial noise, then searched for the steady, narrow-frequency signal that dark matter is expected to produce over long time scales, followed by statistical analysis.

The team then extended the same theoretical framework to dark photons, which, unlike axions, generate electromagnetic waves even without a magnetic field present, and searched the same data set for their distinct signature.

Mysteriously, the dark photon analysis also turned up several signal candidates that could potentially originate from dark matter, although their true nature has yet to be confirmed.

For now, the identity of dark matter is still a mystery. But the theoretical framework developed in this study is expected to underpin a new phase of dark matter searches.

Atsushi Nishizawa et al, Axion Dark Matter Search from Terrestrial Magnetic Fields at Extremely Low Frequencies, Progress of Theoretical and Experimental Physics (2026). DOI: 10.1093/ptep/ptag108

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