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 $10^{-15}\,{\rm eV}-10^{-13}\,{\rm eV}$ induce electromagnetic waves in the (sub-) extremely low-frequency band ($0.3-30\,{\rm Hz}$) 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, $g_{\rm a\gamma}$, from the long-term monitoring data of the low-frequency magnetic fields, resulting in a significant improvement from the previous constraints down to $g_{\rm a\gamma} \lesssim 4\times10^{-13}\,{\rm GeV}^{-1}$ for axion mass $\sim 3 \times 10^{-14}\,{\rm eV}$.