Detecting Solar Axions Using Earth’s Magnetic Field

We show that solar axion conversion to photons in the Earth's magnetosphere can produce an x-ray flux, with average energy ⟨ω⟩4 keV, which is measurable on the dark side of the Earth. The smallness of the Earth's magnetic field is compensated by a large magnetized volume. For axion masses mₐ10^-4 eV, a low-Earth-orbit x-ray detector with an effective area of 10⁴ cm², pointed at the solar core, can probe the photon-axion coupling down to 10^-11 GeV^-1, in 1 yr. Thus, the sensitivity of this new approach will be an order of magnitude beyond current laboratory limits.