Propagation Primer Part 3: Why frequency dispersion works
If you were ask a fairly scientifically savvy person what property makes a prism work, he'd probably say, "Refraction." Well, that's not really correct. Refraction is the ability of a material to BEND light rays. However, the DIFFERENT angle of bending for different wavelengths is not refraction per se, but rather DISPERSION. A color corrected telescope lens, for instance, intentionally removes the dispersion while retaining the refractive properties that make the lens work in the first place.
Actually, it's a peculiar property of glass that causes dispersion....it's NOT an inherent property of the refractive index of a given material.
The ionosphere, likewise is both refractive AND dispersive. But even beyond that, the magnetic field of the Earth causes the ionosphere to be BIREFRINGENT....that means is has a DIFFERENT refractive index depending on which way a radio wave is traveling through it...whether the radio wave is parallel to or perpendicular to the magnetic field! As you can see, this can get pretty complicated pretty fast!
The actual frequency DISPERSION of the ionosphere is caused by the fact that the ions are very sluggish...the amount of "wobble" they undergo in response to a radio wave is inversely proportional to the frequency....while the electron acceleration is relatively independent....they respond nearly instantaneously.
This is why Sporadic E reflections are nearly frequency independent over a very wide range of H.F. They consist almost entirely of free electrons, while the F layer is a soup of ions and electrons.
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