Geomagnetic constraints on Millicharged Dark Matter

TL;DR

This work proposes a novel method to probe ultralight millicharged dark matter (mDM) by detecting its geomagnetic signal on Earth. Treating the mDM field as a classical wave in a natural conducting cavity, the authors derive a quasi-static, monochromatic magnetic signal with angular frequency $\omega = 2 m_\phi$, and show a distinctive $|B| \propto e_m^2/m_\phi^2$ scaling that enhances sensitivity at small masses. They solve the coupled electrodynamics in the weak-coupling regime, model the geomagnetic field using IGRF and a core-current interior, and obtain a closed-form external vector potential in vector spherical harmonics. Reinterpreting null results from SuperMAG and SNIPE Hunt, they set world-leading bounds on the effective millicharge $e_m$ for $m_\phi$ in the range $10^{-18}$–$10^{-14}$ eV, surpassing stellar-cooling constraints by more than thirteen orders of magnitude, and they outline how future magnetometer networks could probe remaining parameter space.

Abstract

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 quasi-static, 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 $10^{-18}$--$10^{-14}\,\text{eV}$. The resulting upper bounds exceed stellar cooling constraints by over thirteen orders of magnitude, demonstrating the power of this method.

Figures (5)

• Figure 1: Middle: Natural Earth's cavity formed between the Earth's surface and the ionosphere. The radial component of the effective current $\vec{J}_{\mathrm{eff}}$ induced by mDM passing through a chosen Amperian Loop. The generated magnetic field $\vec{B}$ can be probed by the magnetometers (orange symbols) placed over the surface of the Earth. Lower Left: The 3D version of the Middle, where the effective current is depicted from Eq. (\ref{['eq:Jeff1']}). Upper Right: The signal magnetic field produced by the millicharged effective current in the geomagnetic field background.
• Figure 2: Sensitivity for millicharged dark matter in the plane of $m_\phi$ and $e_m$. The blue, red and green shaded regions are excluded by the null results from the SuperMAG-1min, SuperMAG-1sec and SNIPE Hunt experiments, respectively. The dashed (solid) lines correspond to $\beta=1$ (4.6) reflecting the uncertainty on the geomagnetic field rms value over the Earth's outer core. The lower edge of our constraint is well below the free-wave boundary $\kappa = 1$ (grey dash-dotted line), as given by Eq. (\ref{['eq:Bsignal2_fin']}). We also show the bound from red giant cooling Fung:2023euv and the projected sensitivity of the SNIPE Hunt experiment using a LEMI-120 sensor with one week of data SNIPEhunt2025. This projected reach fully covers the previously unconstrained region, leaving no lower limit in the parameter space.
• Figure S1: The dependence of $B/\sqrt{2\rho}$ on the charges of mDMs, $e_m$, for different masses, $m_\phi$.
• Figure S2: The profiles for $b_r(x)$, $b_1(x)$, and $b_2(x)$ of the magnetic fields in the Earth interior, with $x=r/R_e$.
• Figure S3: Right: Natural Earth's cavity formed between the Earth's surface and the ionosphere. It shows the tangential component of the dark matter effective current $\vec{J}_{\mathrm{eff}}$ passing through a chosen Amperian Loop. The generated magnetic field $\vec{B}$ can be probed by the magnetometers (orange symbols) placed over the surface of the Earth. This shows schematically how for tangential components the generated magnetic field signal is suppressed by the ionosphere height $h$. Left: 3D version of the Right.