Swedish Institute of Space Physics, IRF

The Ionosphere I

Snell's law

Refraction and reflection; drag the balls!

Snell's law: An optical medium denser than its environment will bend incomming light towards the normal of its boundary!

For visible light the air is an optically denser medium than vacuum:

The sun looks slightly as an ellips at sunset and at sunrise; "Derivative" of Snell's law.

The blue sky is not due to "Snell's law". It's due to random scattered sunlight; the blue light is scattered most

Think of a glasphere with a spherical cavity filled with air.

The black ball will be used later.
Glass is optically denser than air and according to Snell's law; a light ray starting from the inside should go out but refracted.
For visible light and normal glass it is so. If we start to make the used visible light more and more violet we will at first find that the light will be more and more refracted but for a certain color (far out in the ultraviolet) it will be absorped by the glass and heat it up.
If we make the light still more violet it will be reflected back to the cavity and lastly; if we make it very much violet the light will again be refracted and leave the sphere, but the glass will then apear as optically thinner than the air.

If we transmitt low frequency, some kHz to some MHz, radiowaves from the earth up into the sky they will come back.
If we increase the frequency it will take longer time for them to come back, and if we increase the frequency very much, above some 10 MHz, the radiowaves will never come back.
One can not explain this with Snell's law; sending "light" from the inside of a dens medium (air) to an outside thin medium (vacuum) -- it has to be a reflecting layer high up in the sky; we call it the Ionosphere.

If the Ionosphere should reflect radiowaves it has to be an optically (electrically) thinner medium than air. Let us compair it with our glassphere enclosing the cavity and associate the black ball with the earth.
It should be reflections if the ionosphere behaves for the radiowaves as the glassphere behaved for the colors which were more violet than the absorption color; but not too violet


Lightwaves can be considered as radiowaves but with very high frequency, visible light will then have lower frequency than ultraviolett light. If we do so we can compair how the electrons in the two medias will interact with the electric fields from the waves.

First of all there is a difference between the ionosphere and the glassphere: the electrons are bounded in the glassphere but free in the Ionosphere.

Free and bounded electrons forced by an alternating electric field

The electrons in glass are bounded to the atoms by "springs", which can be characterized by a resonance frequency which corresponds to a color (the absorption color above) far out in the ultraviolet.
There is no such resonance frequency in the Ionosphere or we can take it as zero!
Consider the glassphere; for colors less violet than the absorption color the electrons (negativly charged!) will follow the vibrations of the wave very closely and modify the electrical (optical) properties for vacuum as seen by the wave; the lightvelocity in glas will be lessers than lightvelocity in vacuum; glass is a dens medium for these colors.
There are no counterpart for that in the Ionosphere; perhaps if we think of the magneticfield

For more violet colors than the absorption color the electrons can be considered as modified free electrons and behave qualitativly as the free electrons in the ionosphere -- in both media they will then contribute as an additional oscillating mode which will couple to the wave.
For this moderate ultraviolet light in the case of the glassphere and for radiowaves at low frequencies in the case of the ionosphere the coupled systems will not permitt any waves to propagate. An incident wave will be reflected back!

At higher frequencies, above a transition one, it will be possible for waves to propagate, but then in an optically (electrically) thinner media. For the Ionosphere the transition frequency is related to the plasma frequency, an important parameter for both types of media.

The glassphere is a simple very first model of our earth and its ionosphere if we think of very high frequency light; ultra-ultra violet light, simulating the radiowaves which sens it.

A piece of glass is build from microcosmos and its structure is almost independent on its environment. It is a solid almost static body!

The Ionosphere up in the sky consists of free charged particles generated when the sun shines on the atmospheric atoms and split them into ions and electrons. The earth's gravity and magnetic field contribute to the final formation. It changes dynamically; it is a kind of plasma, has own identity but interacts strongly with the near cosmos and our atmosphere.

The ionosphere got its name around 1910-20. It was found from reflexions of radiowaves which were thereafter used for several decades as the only method for its study. The method is called classical-ionosounding and the instrument classical-ionosonde. A lot of long timeseries have been collected and if we will go on with the classical-ionosounding we have within two decades a unigue unified knowledge of the ionosphere; a knowledge of its behavior spanning over one hundred years.

Today we can study the ionosphere with other methods; rockets, satellites and techniques for scattering of light- and radiowaves. But none of these methods are of our concern here. We will continue with the study of the Ionosphere with coherent radiowaves transmitted from ground, refracted and reflected by the ionosphere back to the earth, where they are recieved and registred as ionograms.
The ionograms are then "scaled" to 14 parameters, which are described in this tutorial.

jan 1997 christer juren