Hunting axion dark matter signatures in low-frequency terrestrial magnetic fields

We show that Earth’s natural environment can serve as a powerful probe for ultralight axion dark matter. In the presence of global geomagnetic fields, the axions with masses ranging from 10−15 eV–10−13 eV induce electromagnetic waves in the (sub-) extremely low-frequency band (0.3 − 30 Hz) through the axion-photon coupling. We predict the amplitude of induced magnetic fields in the Earth-ionosphere cavity, taking the finite conductivity of the atmosphere into account. This allows us to constrain the axion-photon coupling parameter, gaγ, from the long-term monitoring data of the low-frequency magnetic fields, resulting in a significant improvement from the previous constraints down to gaγ ≲ 4 × 10−13 GeV−1 for axion mass ~ 3 × 10⁻¹⁴\, eV.