Sporadic E propagation is a relatively rare form of propagation where a radio wave bounces off a sporadic E cloud, notated as E_s 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 E_s 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).