Hunting Axion Dark Matter Signatures in Low-Frequency Terrestrial Magnetic Fields

Abstract 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, 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 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, $g_{\rm a\gamma }$, from the long-term monitoring data of the low-frequency magnetic fields, resulting in a significant improvement from the previous constraints down to $g_{\rm a\gamma }\lesssim 4\times 10^{-13}\, {\rm GeV}^{-1}$ for axion mass $\sim 3 \times 10^{-14}\, \rm {eV}$.