We present a novel search for dark photon dark matter (DM) using terrestrial magnetic field measurements at frequencies below 100 Hz. Coherently oscillating dark photon DM can induce a monochromatic magnetic field via kinetic mixing with ordinary photons. Notably, for dark photon masses $m_{A^{\prime }}$ around $3\times {10}^{-14}\mathrm{eV}$, the signal can be resonantly amplified within a cavity formed by the Earth’s surface and the ionosphere. We compute the expected signal incorporating the effect of atmospheric conductivity, and derive new upper limits on the kinetic mixing parameter $ϵ$ from long-term geomagnetic data. These limits improve upon previous ground-based constraints in the mass range of $1\times {10}^{-15}\mathrm{eV}\lesssim m_{A^{\prime }}\lesssim 2\times {10}^{-13}\mathrm{eV}$.

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• Dark matter
• Dark matter direct detection
• Particle dark matter
• Hypothetical gauge bosons