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

Comments: 11 pages, 4 figures. Accepted for publication in PTEP Letters (Progress of Theoretical and Experimental Physics)