Sub-MHz Radio Background from Ultralight Dark Photon Dark Matter

Dark photons are a well-motivated candidate for dark matter, but their detection becomes challenging for ultralight masses with both experimental and astrophysical probes. In this work, we propose a new approach to explore this regime through the dark inverse Compton scattering of ultralight dark photons with cosmic ray electrons and positrons. We show this process generates a potentially observable background radiation that is most prominent at frequencies below MHz. We compute this effect using the latest cosmic ray models and radio absorption maps. Comparing it to observations of the Milky Way's radio spectrum from Explorer 43, Radio Astronomy Explorer 2, and the Parker Solar Probe, we place leading constraints on the kinetic mixing of dark photon dark matter for masses $\lesssim 2 \times 10^{-17} \ \rm eV$.