What is sporadic E

Irregular scattered patches of relatively dense ionization that develop seasonally within the E region and that reflect and scatter radio frequencies up to 150 MHz. Sporadic E is a regular daytime occurrence over the equatorial regions and is common in the temperate latitudes in late spring ,early summer and, to a lesser degree, in early winter. At high, i.e., polar, latitudes, Sporadic E can accompany Auroras and associated disturbed magnetic conditions. It can sometimes support reflections for distances up to 2,400 km

Sporadic E is a form of propagation that can arise with little warning, and enable radio frequencies of 150 MHz and more to travel over distances of a thousand kilometres and more. Many people experienced it in the days of the old VHF television transmissions. When sporadic E propagation arose, it would result in severe interference to the signals. Even now VHF FM broadcasts in the 88 - 108 MHz band can be affected. In many instances the arrival of Sporadic E can cause unwanted interference as signals that are normally too distant to be heard appear. However for radio amateurs it offers the chance to make contacts over much greater distances than are normally possible.

Sporadic E arises when clouds of intense ionisation form in the region of the E layer. These clouds can have very high levels of ionisation, allowing frequencies up to about 150 MHz to be reflected on some occasions. The clouds are usually comparatively small, measuring only about 50 to 150 kilometres in diameter. Their shape is irregular. Sometimes they may be almost circular, whereas others may be long and thin. They are also surprisingly thin, often only measuring a few hundred metres in depth.

These clouds appear almost at random, although there are times when they are more likely to occur. They form in the day, and dissipate within a few hours. They are also far more common in summer, peaking approximately in mid summer. As they form the level of ionisation gradually builds up, affecting first the lower frequencies, and later higher frequencies as the level of ionisation increases.

Propagation via sporadic E occurs in the same way as normal ionospheric propagation. Signals from the transmitter leave the earth as a sky-wave, travelling towards the ionosphere. Here they are reflected (or more correctly refracted) back to earth where they are heard at a considerable distance from the transmitter. Like normal ionospheric propagation it is the free electrons that affect the signals, causing them to bend back towards the earth. In view of the fact that the sporadic E clouds occur at around the same height as the E layer, similar distances are achieved. Typically the maximum distances are about 2000 km.

It is found that the sporadic E ionisation clouds move. Being in the upper atmosphere they are blown by the winds in these areas and can drift at speeds of up to 300 kilometres per hour. This means that when sporadic E is being experienced, the area from which stations are heard will change over the life of the cloud.

Theories
There are many theories about sporadic E and how it occurs. Some believe that it may be related to thunderstorms, others think it results from the winds in the upper atmosphere. None of these theories have been established, leaving the reasons behind sporadic E a mystery, and predictions of when it will occur have to be left to statistics. However even though the mechanism behind the formation of sporadic E is not fully known it is still possible for radio amateurs to utilise them to enable them to make contacts over long distances

To most radio hams being around when a sporadic E event occurs is something special, you can wait weeks for an opening and then miss it completely as you take a tea break, It is somewhat like going fishing ie you never know what is going to be there when you start and will never know what contacts you will catch on air. Most signals in an 'E' event may be very strong s9+, may vary with 'qsb' greatly, and may fade out without warning and not return " like the one that got away"or sometimes last for hours and provide many contacts at greater distances than usual. If you are using FM as the transmission mode the quality of audio can be very good indeed, or as signals fade can become a challenge to read, If an Auroral event is strong enough radio signals refracted from the moving curtain sound like whispering voices. This combination of events = FUN !

The D layer is the lowest of the layers of the ionosphere. It exists at altitudes around 60 to 90 km. It is present during the day when radiation is received from the sun. However the density of the air at this altitude means that ions and electrons recombine relatively quickly. This means that after sunset, electron levels fall and the layer effectively disappears. This layer is typically produced as the result of X-ray and cosmic ray ionisation. It is found that this layer tends to attenuate signals that pass through it.

E layer

The next layer beyond the D layer is called the E layer. This exists at an altitude of between 100 and 125 km. Instead of acting chiefly as an attenuator, this layer reflects radio signals although they still undergo some attenuation. In view of its altitude and the density of the air, electrons and positive ions recombine relatively quickly. This occurs at a rate of about four times that of the F layers that are higher up where the air is less dense. This means that after nightfall the layer virtually disappears although there is still some residual ionisation, especially in the years around the sunspot maximum. There are a number of methods by which the ionisation in this layer is generated. It depends on factors including the altitude within the layer, the state of the sun, and the latitude. However X-rays and ultraviolet produce a large amount of the ionisation light, especially that with very short wavelengths.

F layer

The F layer is the most important region for long distance HF communications. During the day it splits into two separate layers. These are called the F1 and F2 layers, the F1 layer being the lower of the two. At night these two layers merge to give one layer called the F layer. The altitudes of the layers vary considerably with the time of day, season and the state of the sun. Typically in summer the F1 layer may be around 300 km with the F2 layer at about 400 km or even higher. In winter these figures may be reduced to about 300 km and 200 km. Then at night the F layer is generally around 250 to 300 km. Like the D and E layers, the level of ionisation falls at night, but in view of the much lower air density, the ions and electrons combine much more slowly and the F layer decays much less. Accordingly it is able to support communications, although changes are experienced because of the lessening of the ionisation levels. The figures for the altitude of the F layers are far more variable than those for the lower layers. They change greatly with the time of day, the season and the state of the sun. As a result the figures which are given must only be taken as an approximate guide.
Most of the ionisation in this region of the ionosphere is caused by ultraviolet light, both in the middle of the UV spectrum and those portions with very short wavelengths.

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