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In our recent companion paper [arXiv:2106.00022], we pointed out a novelsignature of ultralight kinetically mixed dark-photon dark matter. Thissignature is a quasi-monochromatic, time-oscillating terrestrial magnetic fieldthat takes a particular pattern over the surface of the Earth. In this work, wepresent a search for this signal in existing, unshielded magnetometer datarecorded by geographically dispersed, geomagnetic stations. The dataset comesfrom the SuperMAG Collaboration and consists of measurements taken withone-minute cadence since 1970, with $\mathcal{O}(500)$ stations contributing inall. We aggregate the magnetic field measurements from all stations byprojecting them onto a small set of global vector spherical harmonics (VSH)that capture the expected vectorial pattern of the signal at each station.Within each dark-photon coherence time, we use a data-driven technique toestimate the broadband background noise in the data, and search for excessnarrowband power in this set of VSH components; we stack the searches indistinct coherence times incoherently. Following a Bayesian analysis approachthat allows us to account for the stochastic nature of the dark-photondark-matter field, we set exclusion bounds on the kinetic mixing parameter inthe dark-photon dark-matter mass range $2\times10^{-18}\,\text{eV} \lesssimm_{A'} \lesssim 7\times10^{-17}\,\text{eV}$ (corresponding to frequencies$6\times 10^{-4}\,\text{Hz}\lesssim f_{A'} \lesssim 2\times10^{-2}\,\text{Hz}$). These limits are complementary to various existingastrophysical constraints. Although our main analysis also identifies a numberof candidate signals in the SuperMAG dataset, these appear to either fail or bein tension with various additional robustness checks we apply to thosecandidates. We report no robust and significant evidence for a dark-photondark-matter signal in the SuperMAG dataset.

Search for dark-photon dark matter in the SuperMAG geomagnetic field dataset