Yes, there is a natural frequency for oscillations of potential
between the ionosphere and the ground (Schumann Resonance).
No, you can't use it to efficiently transmit power. The
ionosphere/ground make a rather lossy waveguide, and the
losses to currents induced in the air/ground/ocean will overwhelm
anything one could pull out with receivers. A receiver of practical
size would be able to pull very little power out of these
I do understand the Loonies on KeelyNet say otherwise.