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 similar or equal to 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 m(a) less than or similar to 10(-4) eV, a low-Earth-orbit x-ray detector with an effective area of 10(4) cm(2), 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.