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The Ionosphere

The Earth's atmosphere is far more complex than a simple onion-layered picture would suggest. These layers exchange gas and energy all the way out into the depths of the invisible magnetosphere, and deep into the atmosphere. One of the most interesting of these atmospheric layers is the ionosphere: A layer of charged particles surrounding the Earth at an altitude of about 100 kilometers.

Just as a mirror reflects light, a cloud of charged particles can reflect longer-wavelength radio waves. The denser the cloud, the higher the frequency of the wave that can be reflected. In the ionosphere there are typically 100 charged particles per cubic inch, so this means that radio frequencies in the AM radio band are easily reflected, but it also means that the much higher frequency FM signals pass through it very easily. TV signals are at such high frequencies that the ionosphere is completely transparent. As a system, the ionosphere is electrically connected to the ground through the tops of thunder storm clouds, and it is connected to the rest of the magnetosphere through the magnetic lines of force and currents that flow along them.

The ionosphere is a lumpy, cloudy layer, and radio signals actually 'twinkle' like stars because of the changing transparency and location of these cloudlets, in a phenomenon called radio scintillation. Meteors that disintegrate near these layers also add their charged gases to this layer. Radio amateurs and the military alike use these meteor trails to reflect radio signals at very high frequencies; much higher than what the ionosphere can naturally reflect.

Also in this layer, or near its base, we can occasionally see very high altitude clouds forming, which can be seen well after sunset. They are called noctilucent clouds and scientists still don't know how they form or why. Their appearance seems to have something to do with the level of solar activity.

Source:NASA

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