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How does the ionosphere reflect or absorb some radio waves?

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The ionosphere acts as a medium that can reflect, refract, or absorb radio waves based on their frequency and the density of the plasma.12

Reflection and Refraction

The ionosphere contains layers of ionized gas—created by solar radiation—that possess a specific plasma frequency. When radio waves encounter these layers, their behavior depends on their frequency:
* Reflection: If a radio wave's frequency is near or below the plasma frequency, the ionosphere acts as a mirror, reflecting the wave back toward Earth. This allows signals to travel far beyond the line of sight, effectively propagating around the planet.13
* Refraction: Because the ionosphere has a different refractive index than the surrounding atmosphere, radio waves are bent (refracted) as they pass through different density gradients.2

Absorption

Absorption occurs primarily when radio waves interact with the plasma, particularly in the lower D region. During the day, this region absorbs medium and lower-frequency radio waves, which limits the range of certain transmissions. Absorption is generally lowest at night because the plasma density in the lower atmosphere decreases as electrons recombine with ions.341