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Sporadic E Propagation
What is Sporadic E propagation?

Sporadic E propagation is a result of highly ionized patches or “clouds” that occasionally form in the E region of the ionosphere at altitudes between approximately 80 and 150 kilometres.

Communication distances of 800–2000 km can occur using a single Es cloud. This variability in distance depends on a number of factors, including cloud height and density. MUF also varies widely, but most commonly falls in the 25 – 150 MHz range which includes the amateur radio 2-meter, 6-meter, and even the 10-meter bands. Strong events have allowed propagation at frequencies as high as 250 MHz.

Sporadic E propagation is probably the most interesting and exciting forms of signal enhancement for the keen VHF operator, or anyone interested in the 28Mhz (10m) 50MHz (6m) and 144MHz (2m) amateur bands.  Sporadic E clouds are usually fairly small in size, but larger clouds or multiple clouds often form during substantial openings.

Sporadic E is a sporadic concentration of the E layer ionisation into thin layers of high ionisation density that are able to reflect radio waves at much higher frequencies than normal.

The E region is defined as that between 90 and 120km above the earth’s surface, and it can be regarded as the transition zone from the earth’s atmosphere into space; there are discontinuities in pressure, temperature and chemical composition at this height. Above about 90km ions predominate over neutral atoms. Most of the short-wavelength ultraviolet rays and less energetic X-rays from the sun are absorbed by ionizing the E layer.

Layers of the atmosphere
Layers of the atmosphere

It’s interesting to note that after almost 70 years of study the true cause for sporadic E is still unknown. There are many different theories as to how and why sporadic E clouds form.

Sporadic E clouds vary greatly in size and in the intensity of the ionisation. Some clouds maybe a few metres across, and some have been more than 200 km across.

It was once believed that the formation of sporadic E clouds was directly related to the eleven year solar (sunspot) cycle. You’ll still see that theory expressed in some text books even though overwhelming evidence suggests that this belief is wrong. There seems to be no correlation between the ionization level or formation of sporadic E clouds and the eleven year sunspot cycle – at least not in the mid latitudes away from the geomagnetic equator and poles. It was noted all the way back in the 1930s and 1940s that the formation and intensity of mid-latitude sporadic E clouds does not substantially vary over the course of the eleven year solar cycle.

There is evidence to suggest that the primary cause of sporadic E cloud formation is wind shear, a purely weather-related phenomenon. Intense high altitude winds, traveling in opposite directions at different altitudes, produce wind shear. It is believed that these wind shears, in the presence of Earth’s geomagnetic field, cause ions to be collected and compressed into a thin, ion-rich layers, approximately one-half to one mile in thickness. The area of these patches can vary from a few square miles to hundreds or even thousands of square miles.

Along the same line is the theory that sporadic E clouds are formed in the vicinity of thunderstorms by the intense electrical activity associated with the storm. There is often, but not always, a correlation between thunderstorm activity and the formation of sporadic E clouds, enough to make this theory very tantalizing.

However, strong thunderstorms often form along frontal boundaries, and intense wind sheer is usually found along the same frontal boundaries that produce thunderstorms. Likewise, strong sporadic E activity often appears when there is no apparent thunderstorm activity along or near the propagation path.

The layers of the atmosphere
This diagram illustrates some of the features found in the various layers of Earth's atmosphere.

Yet another emerging theory suggests that sporadic E clouds are formed by concentrations of meteoric debris. Again, there seems to be a strong correlation between meteor shower activity and the number and intensity of sporadic E clouds.

The point is, nobody has presented a definitive explanation for how and why sporadic E clouds form. There are many excellent papers on the subject. It’s entirely possible, perhaps even likely. that sporadic E clouds are formed as the result of a combination of factors, perhaps involving wind shear, cosmic debris and thunderstorm activity.

Sporadic E is very common on 50MHz (6m) during the summer months – October through to January in VK. From time to time, the intensity of Sporadic E cloud ionization increases to the point where the MUF rises into the 144MHz (2m) band. It is common for the MUF to rise up to and then stop at a particular frequency within the FM band. Distant signals will be heard below the MUF, while only local or tropospherically enhanced signals will be heard above the MUF.

Refraction

Refraction is defined as “…a change in direction of a wave as it crosses the boundary that separates one medium from another.”

The amount by which the path of a radio signal is refracted by sporadic E clouds depends on the intensity of ionization and the frequency of the signal. For a given level of ionization, the signal refraction angle will decrease as the frequency is increased. Above a certain critical frequency, refraction of the signal will be insufficient to return it to the surface of the Earth. This critical frequency is known as the Maximum Usable Frequency or MUF.

Sporadic E skip based on frequency
Sporadic E skip based on frequency

It has been observed over the years that the signal strength of received sporadic E signals will be greatest just below the Maximum Usable Frequency. Also, since the bending angle or angle of refraction decreases as the signal frequency is increased for a given ionization level, we can surmise that the most distant receptions will occur as we approach the MUF. In other words, an Es cloud will support longer signals paths at 100 MHz than it will at 50 MHz.

Geometry of non-tilted Es cloud to cloud propagation
Geometry of non-tilted Es cloud to cloud propagation

In this illustration, neither Es cloud is sufficiently ionized to return a single-hop signal to Earth. However, with the two “weak” clouds working together, the refraction angles of each Es cloud are essentially added. This would have the effect of raising the apparent Maximum Usable Frequency and ultimately returning the signal to Earth at a greater than “normal” Es distance. This is a somewhat more simple (thus more likely) Es cloud configuration than that of the “tilted” cloud theory. It would account for variable path distances which fall between that of “normal” single- and double-hop sporadic E path distances.

The maximum distance for a single-hop sporadic E propagated signal is approximately 2,500 kilometers. However, if multiple, sufficiently ionized patches exist in a line along a particular signal path, it’s possible for a given signal to reflect off the surface of the Earth after the first hop and get refracted back to Earth by a second sporadic E cloud. This can extend the range of E-layer propagated signals out to 5,000 kilometers and beyond.

Various geometries of Sporadic E Signal Propagation
Various geometries of Sporadic E Signal Propagation

This illustration shows three sporadic E clouds. Cloud #1 is more intensively ionized, and is thus capable of refracting signals at a sharper angle, producing a shorter skip distance for a given frequency. Signals being refracted by Cloud #2 are returned to Earth at a lesser angle, thus producing longer skip distances. With clouds #2 and #3 in alignment along the signal path, “double-hop” skip can occur. With this cloud alignment signals from both the “transmitter” and the “Single-Hop Zone” would be heard at the receiver location.

If you’re really keen to learn more, the Australian Department of Defence has a very comprehensive paper called A Description of a New Model of Sporadic E for JORN – Jindalee Operational Radar Network.