Tangent: There is about a terawatt worth of solar energy dumped into the ionosphere, if memory serves correctly. This large DC current can be modulated by varying the conductivity of a channel in the ionosphere. You can do that (temporarily increase the conductivity) with a high power (megawatts effective radiated power) VHF transmitter such as the HAARP array.

If you were to modulate the VHF amplitude or direction at VLF rates, this would result in your VLF frequency, at about a Gigawatt level, ringing the earth.

I suspect this is what Tesla was trying to do with his WardenClyffe Tower. Effectively turning the ionosphere into a MASER.