  Dark photons, aside from constituting non-relativistic dark matter, can also
be generated relativistically through the decay or annihilation of other dark
matter candidates, contributing to a galactic dark photon background. The
production of dark photons tends to favor specific polarization modes,
determined by the microscopic coupling between dark matter and dark photons. We
leverage data obtained from previous searches for dark photon dark matter using
a superconducting radio-frequency cavity to explore galactic dark photon
fluxes. The interplay of anisotropic directions and Earth's rotation introduces
a diurnal modulation of signals within the cavities, manifesting distinct
variation patterns for longitudinal and transverse modes. Our findings
highlight the efficacy of superconducting radio-frequency cavities,
characterized by significantly high-quality factors, as powerful telescopes for
detecting galactic dark photons, unveiling a novel avenue in the indirect
search for dark matter through multi-messenger astronomy.
