An Earth ionosphere dark matter search conducted by researchers from Kyoto University, Hiroshima University, and Nihon University claims to have placed constraints on ultralight axions roughly 100 times tighter than the previous best result from a ground-based experiment, while also turning up several unexplained signals that could, the team says, have a dark matter origin. The findings were published across four papers in 2025 and 2026.
Dark matter is estimated to account for about a quarter of the universe’s total energy content, yet its composition remains unknown. Two of the lighter candidates are ultralight axions and dark photons. In the mass range examined by this team, those particles would be extraordinarily light (roughly 19 to 21 orders of magnitude lighter than an electron) which makes them very difficult to probe with conventional laboratory equipment.
Why the Earth ionosphere dark matter search works in principle
Most axion experiments attempt to convert axions into photons by exposing them to strong magnetic fields inside laboratory settings. The constraint is physical: even powerful lab magnets cover only a small region of space. The team asked whether the planet itself could substitute. ‘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.’
The cavity between Earth’s surface and the ionosphere resonates naturally with electromagnetic waves, functioning in a manner loosely analogous to a large resonant chamber. That property made it potentially useful for detecting the kind of steady, narrowband signal that dark matter candidates are expected to produce.
An earlier theoretical limitation had confined reliable predictions to frequencies below 1 Hz, leaving most of the relevant range unexplored. The team developed a new framework that incorporates the electrical conductivity of the atmosphere, extending reliable predictions up to about 30 Hz and identifying an amplification peak near 8 Hz. According to a paper deposited on arXiv, axions with masses between 1×10⁻¹⁵ eV and 1×10⁻¹³ eV passing through Earth interact with the global geomagnetic field and generate electromagnetic waves in the extremely low-frequency range of 0.3–30 Hz through axion-photon coupling. That mass range maps directly onto the frequency window the new framework was designed to probe.
The model also predicts a spatial distinction between the two particle types. Axion signals should vary by location, with the strongest expected in Southeast Asia, whereas dark photon signals should appear at roughly uniform strength globally. That predicted difference gives researchers a way to begin discriminating between candidates if signals are observed.
A decade of geomagnetic data, and what it produced
The team applied its framework to approximately 10 years of geomagnetic measurements collected between 2012 and 2022 by the British Geological Survey‘s Eskdalemuir Observatory. Artificial noise sources were removed from the dataset before the search for narrowband, long-duration signals consistent with dark matter candidates began. The results were then subjected to statistical analysis.
For axions, the new limits are competitive with constraints inferred from astrophysical X-ray observations by observatories such as Chandra and NuSTAR, though the researchers acknowledge those astrophysical limits depend on certain theoretical assumptions. The ground-based improvement of around 100-fold over the previous best is the more direct comparison.
The dark photon results are the part of this work that will attract most scrutiny. Unlike axions, dark photons can produce electromagnetic waves without a magnetic field being present, so the researchers searched the same dataset for a different signature. Several signal candidates emerged that the team says could potentially have a dark matter origin. The source of those signals is, at present, unknown. They have not been confirmed as evidence of dark matter, and the team is careful not to claim otherwise.
That caution is appropriate. Unexplained signals in physics datasets have a long history of resolving into mundane sources once more data or independent replication arrive. The dark photon candidates identified here remain exactly that: candidates requiring explanation, not a detection.
What the work does establish more firmly is that the Earth’s natural electromagnetic environment can serve as a practical tool for this kind of search. The new theoretical framework, which the team says can be extended to higher frequencies, opens up a range that was previously inaccessible to ground-based analysis. Whether the unexplained dark photon signals survive that additional scrutiny is the question the field will now be asking. The papers were published in Progress of Theoretical and Experimental Physics and Physical Review D in 2025 and 2026.
