Sporadic E, also known as Es (i.e. E sporadic) is a form of E layer ionisation that occurs randomly in the ionosphere. It can affect frequencies normally affected by ionospheric propagation, but as the levels of ionisation can rise very high, it can affect frequencies much higher than would be expected by normal E region ionisation. It is not uncommon for frequencies in the VHF FM band to be affected and it can extend as high as 150 MHz, and more on rare occasions.
In view of the fact that Sporadic E can affect amateur radio bands such as 2 Metres (144 MHz) and even 1.25 Metres 220 MHz (N American allocation), it creates a significant amount of interest amongst radio hams, enabling contacts to be made on these bands that would not otherwise be possible. Nevertheless it is also an important form of propagation that can be used on the 10 and 6 metre amateur radio bands where it can enable long distance communications during periods of low sunspot activity when ionospheric propagation would not be possible.
Sporadic E basics
Sporadic E arises when very intense clouds of ionisation start to build in the lower reaches of the E region of the ionosphere. The level of ionisation may be up to five times greater than those normally achieved at the peak of the sunspot cycle and this is the reason why signals well into the VHF region of the radio spectrum can be reflected. In view of the very high levels of ionisation, the levels of loss are particularly low and even low power stations can be heard at good strength.
In view of the altitude of the sporadic E ionisation, the maximum distances that can be achieved are generally around 2000 kilometres. However double hop skip has been noted on lower frequency on many occasions on the lower frequencies where sporadic E is more common.
The ionisation causing sporadic E is found to build up steadily. Initially only frequencies lower in the radio spectrum are affected, but as the level of ionisation increases so do the frequencies that are affected. The highest frequencies that may be affected vary from one "cloud" to another with some affecting radio frequencies only as high as about 30 MHz whereas others may affect frequencies that extend well into the VHF portion of the spectrum. It is also found that the sporadic E clouds become opaque below a certain frequency, dependent upon the state of the cloud.
In view of the much higher levels of ionisation that are required for reflecting signals into the VHF portion of the radio spectrum, some of the openings that occur high in frequency, e.g. on the 144 MHz amateur radio band may only be very short lived. Sometimes they may last an hour or more, but at others they may only last a few minutes. It is also found that the openings that affect the higher frequencies are much less frequent than those that only affect the lower frequencies.
The ionised clouds that give rise to sporadic E vary greatly in size, but they are very much thinner than the E region in which they occur. Typically their dimensions are measured in metres, in comparison with the E region that is measured in tens of kilometres. As a result the reflections from sporadic E clouds occur as a result of an extremely sharp change in electron density. The shape in other planes also varies considerably with some clouds being very approximately circular while others may be relatively long and thin. In many respects the shapes are not of great importance but they do help explain 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. They are also surprisingly thin.
It is found that the sporadic E clouds are very mobile. The winds in the region of the atmosphere where they are formed can reach speeds of 400 km per hour. As a result the ionised clouds can move very quickly, and this can lead to the areas where the signals are reflected to changing quite rapidly.
Sporadic E often enables radio signals well into the VHF portion of the spectrum to propagate over great distances. It also provides a means of radio propagation at the top end of the HF spectrum when normal ionospheric propagation may be at the low point as a result of the sunspot cycle. While sporadic E can enhance communications in most instances it can also degrade them in others. The very high levels of ionisation in the clouds will reflect any signals in the HF radio bands and thereby prevent them from reaching the higher F region where they might also be reflected, but reach greater distances because of the greater altitude. As a result when sporadic E is in existence short-range signals might be detected when longer range signals would be expected. However the sporadic nature of them and the mobility of the clouds means that any effects are likely to be relatively short lived.
Predicting Sporadic E radio propagation
As the name suggests, the occurrence of sporadic E radio propagation is not easy to predict. However a large amount of statistical data has been collected and from this it is possible to judge the times when it is likely to occur.
In temperate regions, i.e. those in the mid latitudes between the equatorial regions and Polar Regions, sporadic E 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, and as a result it is found that the two Metre openings generally only occur in June and July. Surprisingly a small peak of sporadic E occurrences in general is also noticed in December, although these openings rarely affect the higher VHF bands. A similar pattern is also apparent in the equivalent months, November to February in the southern hemisphere.
The time of day also has a significant effect on sporadic E. There are two main peaks that occur. The first is around midday and there is another at around 7 pm. It is found that there are fewer occurrences in the afternoon and particularly in the morning. There are also very few at night.
The occurrence of sporadic E is rather different outside the temperate regions. In equatorial regions the occurrence of sporadic E is primarily a daytime phenomenon, and because of the proximity to the equator there is little difference the over the course of the year. In Polar Regions what is often termed Auroral sporadic E occurs and again there is little difference between the seasons but it is found that the ionisation for the sporadic E usually occurs in the morning.
Using Sporadic E
One of the main keys of being able to take advantage of a Sporadic E opening is to be able to be available when the opening occurs. Obviously the methods of operation tend to be slightly different dependent upon the band in use. Radio contacts, for example on the ten-metre amateur radio band will tend to follow the normal format for a DX band. The radio contact will tend to be short and to the point as conditions may change. However there is probably little difference in the operation compared to that undertaken for normal ionospheric radio propagation. In addition to this, as the ten metre amateur radio band is relatively low in frequency, the openings will last for longer than higher in frequency.
On amateur radio bands such as two metres where the sporadic E openings may be short lived, activity tends to increase very rapidly as news of the opening spreads. Contest style contacts are normally made, with exchanges consisting of just callsign and report. Additionally it is likely to be found that the area to which the opening exists will change as the Sporadic E clouds are blown about in the upper atmosphere. Radio amateurs who are operating should be prepared to change the headings of their antennas to follow the opening.
The rewards of operating on the VHF bands during a sporadic E opening can be very high. Accordingly it is worth monitoring band conditions as much as possible during periods when openings are possible. In this way it can be possible to make the most of any sporadic E openings that may occur and the maximum number of contacts with other radio amateurs can be made.