Geomagnetic Constraints on Millicharged Dark Matter

Millicharged particles are well-motivated dark matter candidates arising in many extensions of the standard model. We show that, despite their tiny coupling e_{m} to photons, millicharged dark matter (mDM) in the Earth's geomagnetic field can generate a quasistatic, monochromatic magnetic signal with angular frequency twice the mDM mass. Using null results from the SuperMAG and SNIPE Hunt collaborations, we constrain the effective charge of bosonic mDM in the mass range 10^{-18}-10^{-14}  eV. The resulting upper bounds exceed stellar cooling constraints by over 13 orders of magnitude, demonstrating the power of this method.