We estimate the production rate of axion-type particles in the core of the Earth , at a temperature T \approx 5000 K. We constrain thermal geo-axion emission by demanding a core-cooling rate less than \mathcal { O } { ( 100 ) } K/Gyr , as suggested by geophysics .
This yields a “ quasi-vacuum ” ( unaffected by extreme stellar conditions ) bound on the axion-electron fine structure constant \alpha _ { ae } ^ { QV } \lesssim 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 _ { ae } ^ { QV } .