Earth as a transducer for ultralight bosonic dark-matter detection

Ultralight bosonic dark matter (UBDM) that couples to electromagnetism can generate an oscillating magnetic-field signal at the Earth's surface. This is referred to as the "Earth transducer" effect, as the Earth converts UBDM into a detectable magnetic field. Similar DM-induced fields in laboratory experiments typically scale with the size $L$ of the experiment. Because the Earth transducer signal instead scales with the large radius of the Earth, $R$, it is one of the most powerful direct probes of UBDM with masses $m_\mathrm{DM}\lesssim1/R\sim3\times10^{-14}\,\mathrm{eV}$. It has many other favorable properties, such as high spatial and temporal coherence and robustness to atmospheric modeling. In this review, we derive the Earth transducer effect and its properties for multiple UBDM models, and discuss current and future prospects to detect it.