Radio wave propagation in ionosphere Radio communication is based on the concept that a radio signal incident on the ionosphere is reflected if the frequency of the wave matches the plasma frequency.
But what exactly happens? Is it based on the electrons in the ionosphere absorbing and re-radiating the energy if the frequencies match?
 A: Exactly the same happens as when light reflects off a metal surface. In both cases you have an electron gas that interacts with the light. In the case of a metal it's a dense (almost) free electron gas, and in the ionosphere you have a very dilute electron gas formed by ionisation of air molecules.
The incoming electromagnetic wave causes the electrons to oscillate, and as the electrons oscillate they emit EM radiation. If the forward direction the induced radiation emitted by the electrons interferes destructively with the incoming wave, and in the reverse direction induced radiation emitted by the electrons interferes constructively with the incoming wave. The result is that the wave is reflected.
The dense electron gas in a metal interacts with the incoming EM radiation so strongly that even a micron thick layer of metal is effectively perfectly reflecting i.e. reflects 100% of the incoming light. Because the electrons are so dilute in the ionosphere the reflection is far less efficient, though of course the ionosphere is a lot think than a micron and anyway you don't need perfect relection for radio transmission.
A: Propagation is only supported through the ionosphere in characteristic modes (traveling wave  solutions to Maxwell equations). Typically there is 4 modes at any particular height corresponding to upwardly propagating O and X modes and downward propagating O and X. A bit like light through a calcite crystal if you've ever seen that. As you increase height through an ionospheric layer the electron density generally increases and with it the refractive index of the medium decreases with respect the radio wave. For a vertical incident wave as you approach the plasma frequency height equivalent the radio wave frequency, energy begins to transfer more significantly from upward propagating modes to the downward modes. Eventually the radio wave cannot propagate upwards further as the refractive index no longer supports a traveling wave solution to Maxwell s equations, although an evanescent wave continues on. At the molecular level the wave simply cannot oscillate the elections at the wave frequency rate, as ithe dilectric medium prohibits it. Some of the energy is transferred to neutral particles but the rest must transfer to the downward modes. Ionospheric reflection is more of an increasingly intense coupling between characteristic modes  than a classic boundary reflection. 
