What is the natural resonant frequency of the ionosphere?

Question

Also, What is the natural resonant frequency of the earth (mechanical and/or electrical)?

How can I find the natural resonant frequency of the ionosphere if it is not already known?

Answer 1

depends on charge density, configuration of layers, solar activity, magnetic field, etc
f_{critical} = 9 x10^-3 /sqrt{N}, where N is electron density per cm^3

It's around 1-4MHz

Answer 2

I don't think it has a resonance, either mechanical or electrical.

See the Wikipedia article for how the ionosphere affects radio communications. It's an ionization effect, but it's not really a resonance. The maximum usable frequency (MUF) is near 10MHz during the day but drops below 1MHz at night. If I transmit at 7MHz during the day, my signal can take any line-of-sight path. Given the curvature of the earth, that's not very far. The signal aimed upward reaches the ionosphere but just passes on through out to space. At night, that signal is refracted back toward earth by the ionosphere, providing 'skip' communications. Using multiple hops, a signal can propagate around the world. I've done it with 50 watts.