Thermal Emission of Dark Photons from Earth's Core

Dark photons in the sub-eV regime may be produced by the Earth’s hot core, representing a much less extreme environment than stellar cores. We consider this possibility and estimate constraints on the kinetic mixing parameter $\epsilon$ that governs dark photon coupling to charged particles, using Earth core cooling arguments, as well as dark matter direct detection bounds from SENSEI and DAMIC-M experiments. Our estimates suggest that the current results from these experiments constrain new dark photon parameter space. We also find that the proposed Oscura experiment may reach two to three orders of magnitude below existing bounds on $\epsilon$, for dark photon masses $\sim {10}^{-4}$ eV, depending on the assumed parameters characterizing the Earth core.