Discussion
How does the refractive index of the ionosphere behave for radio waves?
Answer: C
For a plasma the refractive index is below one, and it falls further as the frequency approaches the plasma frequency, so the wave is refracted away from the normal and bends back towards earth. It goes to zero at the plasma frequency, which is where reflection occurs. A refractive index below unity implies a phase velocity above the speed of light, which is permissible because the group velocity remains below it. The counter-intuitive index is the whole basis of sky wave propagation.
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