  The photon is the paradigm for a massless particle and current experimental
tests set severe upper bounds on its mass. Probing such a small mass, or
equivalently large Compton wavelength, is challenging at laboratory scales, but
planetary or astrophysical phenomena may potentially reach much better
sensitivities. In this work we consider the effect of a finite photon mass on
Schumann resonances in the Earth-ionosphere cavity, since the TM modes
circulating Earth have eigen-frequencies of order $\mathcal{O} (10 \, {\rm
Hz})$ that could be sensitive to $m_\gamma \approx 10^{-14} \, {\rm eV/c}^2$.
In particular, we update the limit from Kroll [Phys. Rev. Lett. 27, 340
(1971)], $m_\gamma \leq 2.4 \times 10^{-13} \, {\rm eV/c}^2$, by considering
realistic conductivity profiles for the atmosphere. We find the conservative
upper bound $m_\gamma \leq 2.5 \times 10^{-14} \, {\rm eV/c}^2$, a factor 9.6
more strict than Kroll's earlier projection.
