University Crest RSPPG

Ionospheric Physics

The earth's ionosphere


The ionizing action of the sun's radiation on the earth's upper atmosphere produces free electrons. Above about 60km the number of these free electrons is sufficient to affect the propagation of electromagnetic waves. This "ionized" region of the atmosphere is a plasma and is referred to as the ionosphere.

Longer wavelength radio signals can be "bounced" off the ionosphere allowing radio communication "over the horizon". This is how the long, medium and short wave radio broadcasts reach receivers over long distances. Because the ionosphere is not a nice smooth "mirror" the signal can be scattered in many directions causing loss of signal strength and interference from other transmitters. The ionosphere is particulary disturbed in the auroral regions, and during magnetic sub-storms.

Shorter wavelength radio signals pass through the ionosphere but are affected by it. These shorter wavelengths are used by satellites for imaging the earth, and the ionosphere affects the images rather like the way the atmosphere causes "twinkling" of the stars.

Production of the ionosphere

Solar radiation passing through the atmosphere is absorbed and causes ionisation. This absorbtion means less radiation reaches the lower levels of the atmosphere and the level of ionisation is reduced.
The density of the atmosphere decreases with height. This means there is less gas to ionise and so there is less ionisation. This results in a peak in the level of ionisation called a Chapman layer.

The real ionosphere is more complicated than this. There are several gases present in the upper atmosphere and their concentrations vary differently with height. The different gases also vary in how easily they are ionised.

Ionization is also produced by high energy particles emitted from the sun and in the cosmic background. The amount of ionization produced in this way is generally much less than that produced by electromagnetic radiation. However, at night when there is very little or no solar illumination, at times of high magnetic activity and at the lowest altitudes where electomagnetic radiation cannot reach the ionisation produced by charged particles is important.

The plasma density is a balance between the rate at which electrons are produced and the rate at which they are lost. The electrons are lost mainly in two ways, recombination and diffusion. Again, these rates vary with altitude.

These varying rates of production and loss result in a complex profile of electron concentration with altitude, and several distinct layers are produced.

Far from being isolated by empty space, the earth rotates in a magnetised plasma called the solar wind. This affects the earth's magnetic field and the near-earth environment is very complex.


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