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Sporadic E, or Es is a mode of propagation that occurs on occasions. As the name indicates, sporadic E is not easy to predict. It occurs on an occasional basis, and can affect frequencies much higher than would normally be expected - often affecting frequencies into the low end of the VHF spectrum where services such as VHF FM broadcasting, and PMR may experience increased levels of interference.
Sporadic E is not normally used for communications purposes (although radio amateurs use it) because of the sporadic nature of its occurrence, and it cannot be relied upon. Instead its occurrence should be noted as it can result in raised levels of interference as signals are propagated over much greater distances than would normally be expected.
Sporadic E basics
Sporadic E, Es, arises when intense clouds of ionisation form in the E region of the ionosphere. The level of ionisation is up to about five times that of the levels reached during the peak of a sunspot cycle when they would normally be at their highest.
The high levels of ionisation resulting from Sporadic E enable signals well into the VHF region of the spectrum to be refracted by these ionised clouds - frequencies up to 150 MHz may be affected. The levels of ionisation also mean that losses are particularly low - often low power transmitters may be heard via sporadic E.
Sporadic E propagation
When sporadic E ionisation clouds form, the intensity builds up steadily. First this affects frequencies in the lower part of the radio spectrum, and then rises. The highest frequencies that may be affected will depend upon a number of factors including the level of ionisation - this will vary from one cloud to the next.
Another factor that is found with Sporadic E clouds, is that they can become opaque below a certain frequency, dependent upon the state of the cloud.
The level of ionisation for any given cloud will rise steadily, reach its peak and then fall away again. As a result they may affect the higher frequencies for only a short time. At the higher frequencies, signals may be propagated for periods of a few hours, whereas at other times the conditions may only occur for a few minutes.
Sporadic E ionised clouds
Sporadic E clouds vary greatly in size and also in the intensity of the ionisation. Some clouds may be a few metres across, whereas others have been seen that are over 200 km across.
The shape also varies - some are approximately circular having approximately the same dimensions in both directions across them, while others are long and thin. While the actual shapes are not of great importance, they explain to some degree why some stations may experience sporadic E propagation whereas others may not experience it or the areas where stations they can hear are totally different.
The clouds are also remarkably thin. The E region itself spans altitudes of several tens of kilometres. However Sporadic E clouds are typically only a few tens of metres thick. As a result the reflections occur as a result of an extremely sharp change in electron density.
Not only is the formation of the Sporadic E clouds, almost random, but they also move as a result of the winds in the upper reaches of the atmosphere. The winds reach speeds of up to 400 km per hour. This movement can result in the sporadic E skip changing relatively quickly - the source of signals heard / interference will change over a relatively short period of time.
Sporadic E skip distances
Sporadic E clouds form in the lower areas of the E region. As a result the maximum distances over which signals are normally heard is around 2000 km. Obviously shorter distances are more normal, although the minimum distance is governed by the amount of refraction required. For shorter distances, higher angles of radiation re needed and these require a greater amount of refraction for the signals to be reflected back to Earth.
Although sporadic E clouds tend to be random and not as widespread as normal E region or F region ionisation, double hop propagation has been detected, especially on lower frequencies where reflection from clouds is more widespread.
Effect of Sporadic E on HF propagation
Although Sporadic E may appear to give an improvement in some HF communications, while also allowing communications / interference to propagate on frequencies well into the VHF portion of the spectrum, it can also have the effect of degrading some HF communications.
The very high levels of ionisation in the clouds will reflect any signals in the HF portion of the radio spectrum. This may prevent them from reaching the higher F regions, thereby preventing them from being able to achieve much greater distances. Under these circumstances short range signals will be detected when longer range signals would be expected. However the intermittent nature of the sporadic E clouds and the fact that the clouds are very mobile means that any effects are likely to be relatively short lived.
Occurrence of sporadic E
The occurrence of sporadic E is very hard to predict. However a large amount of statistical data has been collected regarding the its occurrence.
It is found that the occurrence of sporadic E varies according to the region of the planet:
- Temperate regions: In temperate regions, i.e. those in the mid latitudes between the equatorial regions, it is found to occur mainly in summer. In the northern hemisphere the months of May to August yield the highest number of openings with a peak in June. A small peak is also noticed in December. A similar pattern is also apparent in the equivalent months, November to February in the southern hemisphere. Generally the frequencies well into the VHF portion of the spectrum are only affected in the middle of the sporadic E season, i.e. mainly in June and July in the northern hemisphere.
- Polar regions: In Polar Regions what is often termed Auroral sporadic E occurs and again there is little difference between the seasons with it usually occurring in the morning.
- Equatorial regions: . In equatorial regions the occurrence of sporadic E is primarily a daytime phenomenon, and as might be expected because of the location, there is little difference the year round. Its occurrence is also more frequent than in temperate regions and as a result it is believed the mechanism behind its formation may be somewhat different.
The time of day also has a major impact on the occurrence of sporadic E. Two main peaks can be seen during the course of a day for temperate zones. One occurs around midday, and the other is around 19.00. In the afternoon there is a slight fall in the number of openings, and in the early morning and at night there are far fewer openings.
Mechanism behind Sporadic E
The mechanism behind sporadic E is not well understood. It is thought that there may be several phenomena that give rise to its formation:
- Meteors: There is some evidence for believing that one phenomena that gives rise to sporadic E is the entrance of meteors into the atmosphere. Typically meteors burn up in the E region, and there could be some connection.
- Electrical storms : These may extend high in altitude and there are electrical effects well above the clouds. It is believed these could supply energy for the formation of sporadic E clouds.
- Auroral activity : The occurrences of Sporadic E in the winter at night have also been linked to auroral activity. This is certainly the case for auroral sporadic E that is the result of energetic electrons entering the atmosphere from the magentosphere.
- Upper atmosphere winds : Some theories suggest that shearing forces caused by the fast moving winds in the upper atmosphere may give rise to these intense clouds of ionisation, particularly in temperature regions.
More data is being collected regarding its occurrence and this is likely to increase our understanding of this phenomenon and enable predictions to be made more accurately.
One interesting link has been noted as it appears that the sunspot cycle has some effect on temperate zone sporadic E. It has been seen that the number of openings increases during the period of the sunspot minima.
By Ian Poole