The Ionosphere and Radiowave Propagation

- an overview or tutorial about the ionosphere, and how it affects radiowave propagation and radio communications.

As electromagnetic waves, and in this case, radio signals travel, they interact with objects and the media in which they travel. As they do this the radio signals can be reflected, refracted or diffracted. These interactions cause the radio signals to change direction, and to reach areas which would not be possible if the radio signals travelled in a direct line.

The ionosphere is a particularly important region with regards to radio signal propagation and radio communications in general. Its properties govern the ways in which radio communications, particularly in the HF radio communications bands take place.

The ionosphere is a region of the upper atmosphere where there are large concentrations of free ions and electrons. While the ions give the ionosphere its name, but it is the free electrons that affect the radio waves and radio communications. In particular the ionosphere is widely known for affecting signals on the short wave radio bands where it "reflects" signals enabling these radio communications signals to be heard over vast distances. Radio stations have long used the properties of the ionosphere to enable them to provide worldwide radio communications coverage. Although today, satellites are widely used, HF radio communications using the ionosphere still plays a major role in providing worldwide radio coverage.

The ionosphere extends over more than one of the meteorological areas, encompassing the mesosphere and the thermosphere, it is an area that is characterised by the existence of positive ions (and more importantly for radio signals free electrons) and it is from the existence of the ions that it gains its name.


Basics

The free electrons do not appear over the whole of the atmosphere. Instead it is found that the number of free electrons starts to rise at altitudes of approximately 30 kilometres. However it is not until altitudes of around 60 to 90 kilometres are reached that the concentration is sufficiently high to start to have a noticeable effect on radio signals and hence on radio communications systems. It is at this level that the ionosphere can be said to start.

The ionisation in the ionosphere is caused mainly by radiation from the Sun. In addition to this, the very high temperatures and the low pressure result in the gases in the upper reaches of the atmosphere existing mainly in a monatomic form rather than existing as molecules. At lower altitudes, the gases are in the normal molecular form, but as the altitude increases the monatomic forms are more in abundance, and at altitudes of around 150 kilometres, most of the gases are in a monatomic form. This is very important because it is found that the monatomic forms of the gases are very much easier to ionise than the molecular forms.


Ionisation

The Sun emits vast quantities of radiation of all wavelengths and this travels towards the Earth, first reaching the outer areas of the atmosphere. In creating the ionisation it is found that when radiation of sufficient intensity strikes an atom or a molecule, energy may be removed from the radiation and an electron removed, producing a free electron and a positive ion. In the example given below, the simple example of a helium atom is give, although other gases including oxygen and nitrogen are far more common.


Ionisation of molecules by solar radiation

Ionisation of molecules by solar radiation


The radiation from the Sun covers a vast spectrum of wavelengths. However in terms of the effect it has on the atoms of molecules it can be considered as photons. The electrons in the atoms or molecules can be considered as orbiting the central nucleus consisting of protons and neutrons. Electrons are tied or bound to their orbit around the nucleus by electro-static forces, the electron is negatively charged and the nucleus is positively charged. There are equal numbers of electrons and protons in any molecule and as a result it is electro-statically neutral.

When a photon strikes the atom, or molecule, the photon transfers its energy to the electron as excess kinetic energy. Under some circumstances this excess energy may exceed the binding energy in the atom or molecule and the electron escapes the influence of the positive charge of the nucleus. This leaves a positively charged nucleus or ions and a negatively charged electron, although as there are the same number of positive ions and negative electrons the whole gas still remains with an overall neutral charge.

Most of the ionisation in the ionosphere results from ultraviolet light, although this does not mean that other wavelengths do not have some effect. Additionally, each time an atom or molecule is ionised a small amount of energy is used. This means that as the radiation passes further into the atmosphere, its intensity reduces. It is for this reason that the ultraviolet radiation causes most of the ionisation in the upper reaches of the ionosphere, but at lower altitudes the radiation that is able to penetrate further cause more of the ionisation. Accordingly, extreme ultra-violet and X-Rays give rise to most of the ionisation at lower altitudes. This reduction in these forms of radiation protects us on the surface of the Earth from the harmful effects of these rays.

The level of ionisation varies over the extent of the ionosphere, being far from constant. One reason is that the level of radiation reduces with decreasing altitude. Also the density of the gases varies. In addition to this there is a variation in the proportions of monatomic and molecular forms of the gases, the monatomic forms of gases being far greater at higher altitudes. These and a variety of other phenomena mean that there are variations in the level of ionisation with altitude.

The level of ionisation in the ionosphere also changes with time. It varies with the time of day, time of year, and according to many other external influences. One of the main reasons why the electron density varies is that the Sun, which gives rise to the ionisation is only visible during the day. While the radiation from the Sun causes the atoms and molecules to split into free electrons and positive ions. The reverse effect also occurs. When a negative electron meets a positive ion, the fact that dissimilar charges attract means that they will be pulled towards one another and they may combine. This means that two opposite effects of splitting and recombination are taking place. This is known as a state of dynamic equilibrium. Accordingly the level of ionisation is dependent upon the rate of ionisation and recombination. This has a significant effect on radio communications.


A simplified view of the layers in the ionosphere over the period of a day

A simplified view of the layers in the ionosphere over the period of a day


Other effects like the season and the state of the Sun also have a major effect. Sunspots and solar disturbances have a major impact on the level of radiation received, and these effects are covered in other articles on this website on Sunspots and Solar Disturbances. The season also has an effect. Again this is covered in other articles on the Radio-Electronics.Com website. However very briefly, the radiation received from the Sun varies in the same way that heat from the Sun varies according to the season, and accordingly the level of ionisation and free electrons changes. However this is a very simplified view as other facts also come into play.


Ionospheric layers

The traditional view of the ionosphere indicates a number of distinct layers, each affecting radio communications in slightly different ways. Indeed, the early discoveries of the ionosphere indicated that a number of layers were present. While this is a convenient way of picturing the structure of the ionosphere it is not exactly correct. Ionisation exists over the whole of the ionosphere, its level varying with altitude. The peaks in level may be considered as the different layers or possibly more correctly, regions. These regions are given letter designations: D, E, and F regions. There is also a C region below the others, but the level of ionisation is so low that it does not have any effect radio signals and radio communications, and it is rarely mentioned.


The typical electron distribution in the ionosphere

The typical electron distribution in the ionosphere


The different layers or regions in the ionosphere have different characteristics and affect radio communications in different ways. There are also differences in the exact way they are created and sustained. In view of this it is worth taking a closer look at each one in detail and the way they vary over the complete day during light and darkness.


D Region

The D region is the lowest of the regions within the ionosphere that affects radio communications signals to any degree. It is present at altitudes between about 60 and 90 kilometres and the radiation within it is only present during the day to an extent that affects radio waves noticeably. It is sustained by the radiation from the Sun and levels of ionisation fall rapidly at dusk when the source of radiation is removed. It mainly has the affect of absorbing or attenuating radio communications signals particularly in the LF and MF portions of the radio spectrum, its affect reducing with frequency. At night it has little effect on most radio communications signals although there is still a sufficient level of ionisation for it to refract VLF signals.

The layer is chiefly generated by the action of a form of radiation known as Lyman radiation which has a wavelength of 1215 Angstroms and ionises nitric oxide gas present in the atmosphere. Hard X-Rays also contribute to the ionisation, especially towards the peak of the solar cycle.


E Region

The region above the D region is the E region. It exists at altitudes between about 100 and 125 kilometres. Instead of attenuating radio communications signals this layer chiefly refracts them, often to a degree where they are returned to earth. As such they appear to have been reflected by this layer. However this layer still acts as an attenuator to a certain degree.

Like the D region, the level of ionisation falls relatively quickly after dark as the electrons and ions re-combine and it virtually disappears at night. However the residual night time ionisation in the lower part of the E region causes some attenuation of signals in the lower portions of the HF part of the radio communications spectrum.

The ionisation in this region results from a number of types of radiation. Soft X-Rays produce much of the ionisation, although extreme ultra-violet (EUV) rays (very short wavelength ultra-violet light) also contribute. Broadly the radiation that produces ionisation in this region has wavelengths between about 10 and 100 Angstroms. The degree to which all of the constituents contribute depends upon the state of the Sun and the latitude at which the observations are made.


F Region

The most important region in the ionosphere for long distance HF radio communications is the F region. During the daytime when radiation is being received from the Sun, it often splits into two, the lower one being the F1 region and the higher one, the F2 region. Of these the F1 region is more of an inflection point in the electron density curve (seen above) and it generally only exists in the summer.

Typically the F1 layer is found at around an altitude of 300 kilometres with the F2 layer above it at around 400 kilometres. The combined F layer may then be centred around 250 to 300 kilometres. The altitude of the all the layers in the ionosphere layers varies considerably and the F layer varies the most. As a result the figures given should only be taken as a rough guide. Being the highest of the ionospheric regions it is greatly affected by the state of the Sun as well as other factors including the time of day, the year and so forth.

The F layer acts as a "reflector" of signals in the HF portion of the radio spectrum enabling world wide radio communications to be established. It is the main region associated with HF signal propagation.

Like the D and E layers the level of ionisation of the F region varies over the course of the day, falling at night as the radiation from the Sun disappears. However the level of ionisation remains much higher. The density of the gases is much lower and as a result the recombination of the ions and electrons takes place more slowly, at about a quarter of the rate that it occurs in the E region. As a result of this it still has an affect on radio signals at night being able to return many to Earth, although it has a reduced effect in some aspects.

The F region is at the highest region in the ionosphere and as such it experiences the most solar radiation. Much of the ionisation results from ultra-violet light in the middle of the spectrum as well as those portions of the spectrum with very short wavelengths. Typically the radiation that causes the ionisation is between the wavelengths of 100 and 1000 Angstroms, although extreme ultra-violet light is responsible for some ionisation in the lower areas of the F region.


Summary

The ionosphere is a continually changing area of the atmosphere. Extending from altitudes of around 60 kilometres to more than 400 kilometres it contains ions and free electrons. The free electrons affect the ways in which radio waves propagate in this region and they have a significant effect on HF radio communications.

The ionosphere can be categorised into a number of regions corresponding to peaks in the electron density. These regions are named the D, E, and F regions. In view of the fact that the radiation from the Sun is absorbed as it penetrates the atmosphere, different forms of radiation give rise to the ionisation in the different regions as outlined in the summary table below:

Summary of forms of radiation causing ionisation in the ionosphere.


Region Primary Ionising Radiation Forms
C Cosmic
D Lyman alpha, Hard X-Rays
E Soft X-Rays and some Extreme Ultra-Violet
F1 Extreme Ultra-violet, and some Ultra-Violet
F2 Ultra-Violet

The ionosphere is a continually changing area. It is obviously affected by radiation from the Sun, and this changes as a result aspects including of the time of day, the geographical area of the world, and the state of the Sun. As a result radio communications using the ionosphere change from one day to the next, and even one hour to the next. Predicting how what radio communications will be possible and radio signals may propagate is of great interest to a variety of radio communications users ranging from broadcasters to radio amateurs and two way radio communications systems users to those with maritime mobile radio communications systems and many more.

By Ian Poole


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