This image shows how the ionosphere is divided even further into layers: D, E, and F layers.
Click on image for full size
Windows to the Universe original image
Regions of the Ionosphere
can be further broken down into the D, E and F regions. The breakdown is based on what wavelength
of solar radiation is absorbed in that region most frequently or on what level of radiation is needed to photodissociate the molecules found in these individual regions.
The D region is the lowest in altitude, though it absorbs the most energetic radiation, hard x-rays. The D region doesn't have a definite starting and stopping point, but includes the ionization that occurs below about 90km (or ionization that occurs below the E region).
The E region peaks at about 105km. It absorbs soft x-rays.
The F region starts around 105km and has a maximum around 600km. It is the highest of all of the regions. Extreme ultra-violet radiation (EUV) is absorbed there.
On a more practical note, the D and E regions (the lower parts of the ionosphere), reflect standard AM radio waves back to Earth. Radio waves with shorter lengths are reflected by the higher F region. Visible light, radar, television and FM wavelengths are all too short to be reflected by the ionosphere. So these types of global communication are made possible by satellite transmissions.
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