Earth as a transducer for axion dark-matter detection

TL;DR

This paper proposes that Earth's magnetic field can act as a transducer to detect ultralight axion dark matter, and searches for the resulting signals in magnetometer data, setting new constraints on axion-photon coupling.

Contribution

It introduces a novel method to detect axion dark matter via Earth's magnetic field and performs the first search using magnetometer network data, establishing new constraints.

Findings

No strong evidence for axion signals in the analyzed mass range.

Constraints on axion-photon coupling comparable to CAST helioscope.

Detailed modeling of the Earth's magnetic response to axion dark matter.

Abstract

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 [arXiv:2106.00022] and [arXiv:2108.08852]. It has a global vectorial pattern fixed by the Earth's geomagnetic field, is temporally coherent on long time scales, 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 $2\times10^{-18}\text{eV} \lesssim m_a \lesssim 7\times10^{-17}\text{eV}$. 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 $3\times 10^{-17}\text{eV} \lesssim m_a \lesssim 4\times 10^{-17}\text{eV}$, these constraints are comparable to those obtained by the CAST helioscope.