Sporadic E propagation

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Sporadic E (abbreviated E_s or SpE) is an unusual form of radio propagation using a low level of the Earth's ionosphere that normally does not refract radio waves.

Sporadic E propagation reflects signals off relatively small "clouds" in the lower E region located at altitudes of about 95~150 km (50~100 miles). The more conventional forms of skywave propagation in the ionosphere's higher F region refract off layers of electrons knocked off of gases by intense UV light, which are renewed on a fairly regular daily cycle. In both cases, the ionized material, when present, refracts (or "bends") radio signals back toward the Earth's surface creating a "bent pipe" path for radio signals.

The E_s propagation often supports occasional long-distance communication during the approximately 6 weeks centered on summer solstice at very high frequencies (VHF), which under normal conditions can only propagate by line-of-sight.^[1]