  We show that solar axion conversion to photons in the Earth's magnetosphere
can produce an x-ray flux, with average energy \sim 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 < 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 one year. Thus, the sensitivity of this new approach will be an order of
magnitude beyond current laboratory limits.
