Sporadic E propagation
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Sporadic E propagation is a relatively rare form of propagation where a radio wave bounces off a sporadic E cloud, notated as Es in the E layer region of the ionosphere.
The E region of the ionosphere is located about 90 to 160 km in altitude. The height can vary a little, and, along with electron (ionization) density, depends on solar zenith angle and solar activity. During daylight hours, electron density (a measure of the ionization level) can reach 105 electrons/cm3. At night, when the supply of x-rays from the sun is cut off, ionization levels drop to 103 e/cm3. These ionization densities are expected under normal conditions, absent of sporadic E.
A precise definition of Es can be found in the book titled "Worldwide Occurrence of Sporadic E", written by Ernest K. Smith, PhD, 1957. He defines Sporadic E as "a comparatively strong and protracted transmission (several minutes to several hours) "returned" from the E region of the ionosphere by some mechanism other than the normal reflection process from the daytime E layer."
Single "clouds" or multiple layers of "clouds" have been observed to form, usually spaced by about 6 km. The mechanisms for how these clouds form is still not precisely known. Some possible theorized mechanisms include upper level wind shears and geomagnetic activity.
Communication distances of 500–1400 miles are common from a single Es cloud. The distance variability of Es propagation is due to cloud height and density. Depending on the density of the ionization, frequencies from 27–110 MHz are most common (USA TV channels 2-6, all USA FM channels (88–106 MHz), amateur radio bands 10, and 6 meters). With strong events, frequencies up through 250 MHz have been reflected off the clounds (USA TV Channel 7, amateur bands 144 MHz and 220 MHz).