Earth as a transducer for axion dark-matter detection

We demonstrate that ultralight axion dark matter with a coupling to photons induces an oscillating global terrestrial magnetic-field signal in the presence of the background geomagnetic field of the Earth. This signal is similar in structure to that of dark-photon dark matter that was recently pointed out and searched for in [Fedderke et al. Phys. Rev. D 104, 075023 (2021)] and [Fedderke et al. Phys. Rev. D 104, 095032 (2021)]. It has a global vectorial pattern fixed by the Earth's geomagnetic field, is temporally coherent on long timescales, and has a frequency set by the axion mass m(a). In this work, we both compute the detailed signal pattern and undertake a search for this signal in magnetometer network data maintained by the SuperMAG Collaboration. Our analysis identifies no strong evidence for an axion dark-matter signal in the axion mass range 2 x 10(-18) eV. ma. 7 x 10(-17) eV less than or similar to m(a) less than or similar to 7 x 10(-17) eV. Assuming the axion is all of the dark matter, we place constraints on the axion-photon coupling g(a gamma) in the same mass range; at their strongest, for masses 3 x 10(-17) eV less than or similar to m(a) less than or similar to 4 x 10(-17) eV, these constraints are comparable to those obtained by the CAST helioscope.

Automated summary

We demonstrate that ultralight axion dark matter with a coupling to photons induces an oscillating global terrestrial magnetic-field signal in the presence of the background geomagnetic field of the Earth. Our analysis identifies no strong evidence for an axion dark-matter signal in the studied mass range. These constraints on the axion-photon coupling are comparable to those obtained by the CAST helioscope.