A new method for searching for dark matter from dark photons using measurements of Earth's magnetic field at frequencies below 100 Hz is presented. Coherently oscillating dark photons can induce a monochromatic magnetic field through kinetic mixing with ordinary photons. For dark photon masses around 3×10⁻¹⁴ eV, the signal is resonantly enhanced in the cavity formed by Earth's surface and the ionosphere. The expected signal is calculated taking into account atmospheric conductivity. From years of geomagnetic data, new upper limits on the kinetic mixing parameter ε are obtained. These limits surpass previous ground-based results in the mass range 10⁻¹⁵–2×10⁻¹³ eV.
Invisible dark matter governs the rotation of galaxies — Vera Rubin showed back in the last century that stars move as if pushed by an unseen force. One of the prime suspects is the dark photon, a massive cousin of ordinary light, predicted by the extended equations of Maxwell and not fitting into the Standard Model.
Scientists turned the planet into a giant radio receiver. The space between Earth and the ionosphere (the electrically charged layer of the atmosphere) forms a resonator that greatly amplifies magnetic tremors if their frequency matches the 'radio wave' of a dark photon. It's like one guitar string resonating to a note played on another. Years-long magnetic field records enabled a delicate spectral analysis — fishing out a single frequency from the noise.
For the first time, air conductivity was taken into account, refining the calculation and helping to separate a possible signal from interference. They didn't hear a direct 'hello' from dark matter, but they set the strictest limits on its interaction with magnetism — for a mass range from 10⁻¹⁵ to 2×10⁻¹³ eV. And the sought frequencies below 100 Hz, where whales sing and power lines hum, force scientists to scrub the sounds of the ocean and civilization from the data.
🎯 The sought signal lies in the band below 100 Hz — here whales sing and power grids hum. To catch the cosmic 'whisper,' scientists have to filter out the voice of the ocean and the noise of civilization.