The Sun: Its Structure & Impact on Radio Propagation

The structure of the Sun is important because it provides the radiation that gives rise to the ionosphere which enables HF ionospheric radio propagation

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Solar effects on propagation includes:
The Sun & its structure     Sunspots     Solar disturbances     SID sudden ionospheric disturbance     Auroras & propagation    

Ionospheric propagation:   Ionospheric propagation    

The Sun is the main hub for the solar system and it is the sole provider of energy that enables life on Earth and in terms of radio communication, it is the source of radiation that gives rise to the ionosphere.

In this way, the Sun and the state of the Sun is of great importance for HF ionospheric radio propagation.

Having a good idea of its structure and the way phenomena occur inside it and on the surface can help with an understanding of why radio propagation conditions change.

Solar interior

The Sun is enormous and when it is seen in the sky, the its actual size cannot be fully appreciated. To us the Earth seems enormous, but the Sun its mass is more than 333 000 times that of the Earth.

The Sun also has a diameter about 109 times that of the Earth, so in terms of dimensions it is also very much bigger.

In addition to this the Sun contains over 99.85% of the mass of the whole Solar System.

The Sun has a complex internal structure and we know comparatively little about it because it is very inaccessible for obvious reasons.

The Sun from Earth at Sunset
The Sun at sunset

Nevertheless it has still been possible for solar scientists to gain a good understanding of some of the processes that occur and to detail the various internal regions.

Research is ongoing all the time and with the new techniques being employed, the understanding of all the areas of the Sun is continually growing.

A word of warning

Under no circumstances should the sun be viewed directly, even though dark glasses. In the past many people have had their sight damaged by doing this.

The Sun consists of several regions, progressing from the very centre or core outwards. each has its own properties and characteristics.

At the centre is the core where the energy is generated. Once generated energy moves outwards by radiation and convection through what is termed the radiative zone or intermediate interior and through a thin interface called the tachocline to the convection zone. Beyond these are the photosphere, chromosphere and finally the corona.

Each of these regions will be looked at in turn.

The Sun's core

The Sun's core is the central region where the energy is generated and it forms the inner 25% of the radius of the Sun.

The energy is generated as continual nuclear reactions convert hydrogen to helium burning up vast quantities of the element.

The amounts of energy produced are almost unimaginably huge: over 383 billion trillion Watts of energy generated. This is equivalent to the energy generated by 100 billion tons of TNT being exploded every second.

The results of all this nuclear activity and energy generation results in temperatures in this region reaching around 15 000 000 °C.

There are also phenomenal gravitational fields and this leads to the density of this region being enormously high - about 150 g / cm3 which is about ten times that of lead.

The nuclear activity is focussed at the centre of the core, but it decreases towards the edge - a mere 175 000 kilometres or so from the very centre.At the edge of the core, the temerpature has also fallen: to around half the maximum value at the centre and the density is only about 20 g / cm3.

Radiative or intermediate zone

Surrounding the core is a region known as the radiative or intermediate zone. This extends over a large proportion of the radius of the Sun - it extends from the core which extends out to 25% of the radius, to about 70% of the radius.

It is in this region where the energy generated in the core is transferred further outwards mainly by radiation. However the energy bounces around and although it is travelling at the speed of light, because it bounces around so much it is estimated that it could take as much as a million years to exit this zone.

Within this region, the gravitational forces are considerably less and as a result, the density falls from around 20 g / cm3 to around 0.2 g / cm3 which actually less than that of water. In addition to this, the temperature falls from around 7 000 000 °C to around 2 000 000 °C, which is still incredibly hot.

The Tachocline

On the the outer edge of the radiative or intermediate zone, an interface known as the tachocline is found. This interface appears because the radiative zone and the next zone which is called the convection zone transfer energy in very different ways, as their names imply.

It is also thought that the tachocline is where the Sun's magnetic field is generated.

It is believed that shearing flows across the layer can stretch the magnetic lines of force and enhancing them. In addition to this there appear to be sudden changes in the chemical composition across the layer.

Obviously it is difficult to detect all the properties of this interface layer, and new facts are being learned about this all of the time from various observations and calculations about what happens in stars like the Sun.

Convection zone

This is the final region of what may be termed the interior of the Sun.

The temperature at the inner region of this zone is about 2 000 000 °C and this falls to around 5 700 °C at its surface.

The reduction in temperature across this zone result from many of the heavier ions such as carbon, nitrogen and oxygen retaining their electrons.

This means that the material becomes opaque and this makes it more difficult for the heat to be transferred. As a result the layer starts to "boil" and as a result movement occurs as convection.

in fact the convection zone is very turbulent, but heat is transferred by convection rather than radiation, especially as it becomes opaque.


The photosphere is the region of the Sun that we "see." Obviously it should not be viewed except through specialist dark filters.

The surface is not solid, in view of the temperatures and other factors, but a white hot gaseous region which is only about 100 km thick.

Also being gaseous it is possible to see though it if the right equipment is used, and in these images the Sun looks darker around the edges because the temperature of the photosphere is cooler than the hotter deeper regions.

Images of the surface of the Sun reveal a number of features including the granules and super-granules at the surface of the convection zone. These granules are the result of the motion within the convection zone, where the more dense lower temperature material moves around as hotter material moves to the surface as a result of convection.

The Sun showing its sunspots
The Sun with sunspots visible
Image Coutesy NASA


The chromosphere is an irregular layer that exists above the photosphere. In this region hydrogen emits a reddish light as a result of an increase in temperature to around 20 000 °C that occurs here.


Further out from the photosphere and the chromosphere is the Sun's corona. This can be likened to the outer atmosphere of the Sun, if this term could ever be used for it!

It is the corona that can be seen during eclipses as an area surrounding the Sun.

Within the corona, gases become heated to a temperature of around 1 000 000 °C and at these temperatures the dominant elements of hydrogen and helium atoms become completely stripped of their electrons.

Other elements including carbon, nitrogen and oxygen fare similarly although calcium being much heavier does manager to retain its electrons.

There is a very sharp transition in temperature between the outer reaches of the chromosphere and the corona - the very hot corona can be around 1 000 000 °C while the outer reaches of the chromosphere are at around 20 000 °C.

Sun's rotation

We probably do not think too much about the rotation of the Sun as we are generally more interested in our daily and yearly cycles.

However the Sun, itself rotates in and the period is generally taken to be about 27 days. However the Sun is a gaseous body and as a result it does not all rotate at the same speed. It is found that the equatorial regions rotate faster, taking about 24 days to complete a revolution whereas the Polar Regions take over 30 days. The rotation speeds and times can be judged by monitoring the sunspots and other features that may appear on the surface.

Additionally the axis of rotation of the Sun is tiled at an angle of 7.25 degrees to axis of the Earth's orbit, and this means that more of the Solar North Pole is seen in September and its South Pole in March.

While the rotation of the Sun means that the sunspots appear to move, it also means that the effects of events like CMEs may have some residual effects after the rotational period.

Solar radiation

While the Sun gives off plenty of heat, it also releases plenty of radiation.

This radiation travels to the Earth and causes the upper reaches of the atmosphere to become ionised. The absorption of the radiation protects us from many of the harmful elements of this radiation, but in ionising the high level gasses, the ionosphere is formed.

The ionosphere is able to reflect, or more correctly refract radio signals, aprticaulrly in the MF and HF portions of the radio spectrum giving the possibility of global radio communications.

As the levels of radiation change over the course of a day, so the different regions int he ionosphere change in line with this.

Variations in the ionosphere during the day
Variations of the ionosphere over the day


One form of activity on the Sun is the sunspots that form. These are dark areas, relatively speaking, that can be seen on the surface of the Sun.

These sunspots are areas of intense magnetic activity, and they also emit radiation that causes additional ionisation in the ionosphere. They increase and decrease in number over an approximately 11 year cycle, causing HF ionospheric conditions to mirror this change.

Read more about . . . . Sunspots and their 11 year cycle.

Solar disturbances

In view of the ferocious amount of energy generated by the Sun, it is hardly surprising that it has a number of disturbances.These disturbances can release vast amounts of energy and material causing significant impacts on various aspects of life on Earth.

Coronal Mass Ejections or CMEs, Coronal Holes and Solar Flares are all forms of disturbance that affect us. Vast amounts of material can be flung far into space, particularly by CMEs, and this can cause geomagnetic storms and auroras, but also increases in radio blackouts that can give rise to HF radio blackouts.

Read more about . . . . Solar Disturbances.

With the Sun being our huge energy source at the centre of the solar system, it also controls many aspects of our lives . . . . including the state of the ionosphere and hence ionospheric radio propagation.

Having an understanding of the Sun helps with an understanding of how the ionosphere reacts and how HF ionospheric radio changes with time.

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