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 $\sim 3 \times 10^{ -14}\,{ \rm eV}$.
Categories: hep-ph astro-ph.CO
