  We estimate the production rate of axion-type particles in the core of the
Earth, at a temperature T~5000K. We constrain thermal geo-axion emission by
demanding a core-cooling rate less than 100K/Gyr, as suggested by geophysics.
This yields a "quasi-vacuum" (unaffected by extreme stellar conditions) bound
on the axion-electron fine structure constant \alpha_a^{QV} < 10^{-18},
stronger than the existing accelerator (vacuum) bound by 4 orders of magnitude.
We consider the prospects for measuring the geo-axion flux through conversion
into photons in a geoscope; such measurements can further constrain
\alpha_a^{QV}.
