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The ionosphere is the part of the atmosphere that is ionized by solar
radiation, and too tenuous to be cooled by contact with other air. It forms
the inner edge of the magnetosphere and has practical importance because it
reflects radio waves to distant places on Earth.
The ionosphere is generally recognized to have three, sometimes four layers.
The D layer is the innermost layer, and mostly absorbs radio waves. The E
layer is the middle layer. The F layer combines into one layer at night, but
in sunlight divides into two layers, the F1 and F2. The F layers are
responsible for most skywave propagation of radio, and are thickest and most
reflective of radio on the side of the Earth facing the sun.
Scientists explore the structure of the ionosphere by bouncing radio waves
of different frequencies from it, and using special receivers to detect how
the reflected waves have changed from the transmitted waves.