Why does the ionosphere reflect radio waves


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The ionosphere

The ionosphere is that part of the upper atmosphere of the earth in which ions, i.e. charged particles, and free electrons can accumulate through hard, short-wave solar radiation. The ionosphere makes up most of the high atmosphere.

The ionosphere begins above the mesosphere at an altitude of about 80 km, reaches its maximum charge at about 300 km and then gradually merges into interplanetary space. It is therefore largely within the thermosphere, with which it is often incorrectly equated. The thermosphere is thus also part of the ionosphere and vice versa.

There is no upper limit to the ionosphere, because the decrease in the density of the atmosphere and thus the number of charge carriers occurs more slowly with increasing altitude. The ionosphere finally changes into the plasma sphere, in which almost all the particles present are ionized. The so-called transition height at an altitude of 1,000 km can be viewed as the boundary between the ionosphere and the plasma sphere.

The ionosphere

The gas molecules of this extremely thin layer of the atmosphere are ionized by the incoming high-energy cosmic radiation (hard ultraviolet and X-ray radiation), i.e. split into ions and electrons. The resulting energy is converted into heat. This process makes the air electrically conductive. Just as a hot street reflects the light in summer, this layer begins to reflect radio waves. In addition, other subsequent processes are also important for the extent of ionization, which is why maximum ion concentrations result at different heights. In ascending order, these regions are called the D, E and F layers. These layers of maximum ionization are especially important for the propagation of radio waves, since they essentially act like a reflector for them and thus enable radio communication.

In the case of short waves, which otherwise could only directly cover short distances quasi-optically, the so-called sky wave is reflected in the ionosphere so that it can circle the entire globe. This ability of the ionosphere to reflect radio waves is heavily dependent on solar activity. As a result, this level of the earth's atmosphere plays an essential role in the propagation of radio waves.

Because solar radiation is a prerequisite for the formation of such ions, the ion concentration is largely dependent on the position of the sun, i.e. the ion concentration has a daily as well as a yearly variation. The reception of shortwave transmitters is therefore significantly better at night than during the day. If there is no or less solar radiation, i.e. at night or in winter, the recombination effect predominates, i.e. negative and positive ions that have formed once again combine to form neutral molecules. In addition to light, a steady stream of charged particles also reaches the foothills of the upper atmosphere: the solar wind. These particles are influenced by the earth's magnetic field and preferentially enter the earth's atmosphere in the polar regions. The deceleration of these up to one million km / h fast particles leads, among other things, to the northern lights (aurora), impressive natural spectacles that can be observed above all in high northern and high southern latitudes.

Northern lights

The solar wind and the earth's magnetic field