Search for ultralight dark matter in the SuperMAG high-fidelity dataset

Ultralight dark matter, such as kinetically mixed dark-photon dark matter (DPDM) or axionlike-particle dark matter (axion DM), can source an oscillating magnetic-field signal at Earth's surface. Previous work searched for this signal in a publicly available dataset of global magnetometer measurements maintained by the SuperMAG collaboration. This "low-fidelity" dataset reported measurements with a 1-minute time resolution, allowing the search to set leading direct constraints on DPDM and axion DM with Compton frequencies fDM≤1/(1\,min) [corresponding to masses mDM≤7×10⁻¹⁷\,eV]. More recently, a dedicated experiment undertaken by the SNIPE Hunt collaboration has also searched for this same signal at higher frequencies fDM≥0.5\,Hz (or mDM≥2×10⁻¹⁵\,eV). In this work, we search for this signal of ultralight DM in the SuperMAG "high-fidelity" dataset, which features a 1-second time resolution, allowing us to probe the gap in parameter space between the low-fidelity dataset and the SNIPE Hunt experiment. The high-fidelity dataset exhibits lower geomagnetic noise than the low-fidelity dataset and features more data than the SNIPE Hunt experiment, making it a powerful probe of ultralight DM. Our search finds no robust DPDM or axion DM candidates. We set constraints on DPDM and axion DM parameter space for 10⁻³\,Hz≤ fDM≤0.98\,Hz (or 4×10⁻¹⁸\,eV≤ mDM≤4×10⁻¹⁵\,eV). Our results are the leading direct constraints on both DPDM and axion DM in this mass range, and our DPDM constraint surpasses the leading astrophysical constraint in a narrow range around mA'≈2×10⁻¹⁵\,eV.