Physicists in Japan have turned Earth's own magnetic field into a planet-sized dark matter detector, using a decade of geomagnetic data to hunt for one of the lightest particles proposed to make up the universe's missing mass. The team, based at Kyoto University with colleagues at Hiroshima University and Nihon University, published its results this month in two companion papers in Progress of Theoretical and Experimental Physics.
The candidate particle is the axion, a hypothetical particle far lighter than an electron that, if it exists, would couple weakly to light in the presence of a magnetic field, producing a faint, steady electromagnetic signal at a frequency set by the axion's mass. Rather than building a laboratory instrument, the researchers treated the space between Earth's surface and its ionosphere as a natural resonant cavity that could amplify such a signal. They developed a theoretical framework that, for the first time, accounts for how the atmosphere's electrical conductivity affects the signal, extending reliable predictions from below 1 hertz up to roughly 30 hertz, with a notable boost in sensitivity near 8 hertz.
Applying the model to about a decade of geomagnetic measurements, from 2012 through 2022, recorded at the British Geological Survey's Eskdalemuir Observatory in Scotland, the team filtered out known instrumental and environmental noise and searched for the kind of narrow, persistent frequency spike an axion signal would leave behind. The analysis produced axion-photon coupling constraints roughly 100 times tighter than the best previous ground-based laboratory experiments in this mass range, and competitive with limits derived from astrophysical X-ray observations, according to a summary of the study.
Importantly, the search also turned up 65 persistent signal candidates with a statistical significance high enough to stand out from background noise. The researchers are careful to note that these are not confirmed detections of dark matter — the origin of the signals remains unexplained, and could turn out to be unmodeled terrestrial or instrumental noise rather than new physics. "We asked ourselves whether we could use the Earth itself as a giant detector in the search," said Atsushi Taruya, the study's corresponding author, describing the motivation behind the approach, according to reporting on the project.
Axions were first proposed decades ago to solve an unrelated puzzle in particle physics and have since become one of the leading dark matter candidates, alongside heavier, more traditionally sought particles known as WIMPs. Because axions would barely interact with ordinary matter, most experiments look for indirect electromagnetic traces like the one this team pursued, typically using small, purpose-built laboratory cavities. Repurposing Earth's own magnetic field sidesteps the scale limitations of tabletop experiments, though it also means the result depends on modeling a far messier, less controlled environment than a laboratory. The authors say follow-up analysis of the unexplained signal candidates, and comparison against data from other geomagnetic observatories, will be needed to determine whether any hold up as genuine physics rather than statistical noise.
