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Sporadic E (abbreviated EsorSpE) is an uncommon form of radio propagation using a low level of the Earth's ionosphere that normally does not refract radio waves above about 15 MHz.
Sporadic E propagation reflects signals off relatively small ionization patches in the lower E region located at altitudes of about 95~120 km (50~75 miles). The more conventional forms of skywave propagation in the ionosphere's higher F region refract off layers of electrons knocked off of gas atoms and molecules by intense UV light, which are renewed on a regular repeating daily cycle. In both cases, the electrons, when present, refracts (or "bends") radio signals back toward the Earth's surface creating a "bent pipe" path for radio signals.
The Es 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 usually propagate mostly by line-of-sight.[1]
As its name suggests, sporadic E is an unpredictable event that can happen at almost any time; it does, however, display strong seasonal and diurnal patterns. Sporadic E activity peaks predictably near the solstices in both hemispheres. In the mid-latitude of the Northern Hemisphere, activity usually begins in mid-May, with the peak most noticeably beginning in early June. It begins trailing off after mid-July and becomes much less reliable by early August. A much smaller sporadic-E peak occurs during the winter solstice. For the mid-latitudes of the Southern Hemisphere, the timeframes are inversed; the highest activity occurring during their summer solstice.[2]
Communication distances of 800–2,200 km (500–1,400 miles) can occur using a single Es cloud. This variability in distance depends on a number of factors, including cloud height and density. The maximum usable frequency (MUF) also varies widely, but most commonly falls in the 25–150 MHz range, which includes the FM broadcast band (87.5–108 MHz), band IVHF television (American TV channels 2–6, Russian channels 1–5, and European channels 2–4, which are no longer used in Western Europe), CB radio (27 MHz), and the amateur radio 2 meter, 4 m, 6 m, and 10 m bands. On very rare occasions, a MUF of 225 MHz can be attained.[2]
No conclusive theory has yet been formulated as to the origin of sporadic E. Attempts to connect the incidence of sporadic E with the eleven-year Sunspot cycle have provided tentative correlations. There seems to be a positive correlation between sunspot maximum and Es activity in Europe. Conversely, there seems to be a negative correlation between maximum sunspot activity and Es activity in Australasia. Harrison [3] implies that there is a correlation between the formation of sporadic E and iron/magnesium micrometeoroid ablation in the ablation zone, 100 to 140 km above the earth surface. Maruyama discusses this possibility further.[4]
Television and FM signals received via sporadic E can be extremely strong and range in strength over a short period from just detectable to overloading. Although polarisation shift can occur, single-hop Es signals tend to remain in the original transmitted polarization. Long single-hop (900–1,500 miles or 1,400–2,400 kilometres) sporadic E television signals tend to be more stable and relatively free of multipath images.
Shorter-skip (400–800 miles or 640–1,290 kilometres) signals tend to be reflected from more than one part of the sporadic E layer, resulting in multiple images and ghosting, with phase reversal at times. Picture degradation and signal-strength attenuation worsens with each subsequent sporadic E hop.
Sporadic E usually affects the lower VHF band I (TV channels 2–6, E2–E4, and R1–R5) and band II (88–108 MHz FM broadcast band). A 1945 FCC engineering study concluded that Es caused interference issues 1% of the time for a station broadcasting at 42 MHz, but only 0.01% for one at 84 MHz.[5]
The typical expected distances are about 600 to 1,400 miles (970 to 2,250 km). However, under exceptional circumstances, a highly ionized Es cloud can propagate band I VHF signals down to approximately 350 miles (560 km). When short-skip Es reception occurs, i.e., under 500 miles (800 km) in band I, there is a greater possibility that the ionized sporadic E cloud will be capable of reflecting a signal at a much higher frequency – i.e., a VHF band 3 channel – since a sharp reflection angle (short skip) favours low frequencies, a shallower reflection angle from the same ionized cloud will favour a higher frequency. In this case even Es DVB-T reception might be possible if a MUX uses VHF band 3, preferably channel E5, especially if QPSK mode is used, due to its low signal requirements. In addition to that, band 3 signals are more affected by tropospheric propagation which may indirectly increase the actual MUF because the signals only need to be refracted to low enough elevations that they get refracted towards the ground by the troposphere.
Equatorial sporadic E is a regular daytime occurrence over the equatorial regions. For stations located within ±10° of the geomagnetic equator, equatorial E-skip can be expected on most days throughout the year, peaking around midday local time.
At polar latitudes, sporadic E can accompany auroras and associated disturbed magnetic conditions and is called auroral E.
Unlike equatorial or mid-latitude Es, sporadic E propagation over high latitude paths is rare, and supports unexpected contacts between locations surrounding the Arctic, even during periods of low solar activity.[6]
On 12 June 2009, sporadic E allowed some television viewers in the eastern United States to see VHF analog TV stations from other states at great distances, in places and on TV channels where local stations had already done their permanent analog shutdown on the final day of the DTV transition in the United States. This was possible because VHF has been mostly avoided by digital TV stations, leaving the analog stations the last ones on the band.
As of April 2010, it was possible for many in the U.S. to see Canadian and Mexican analog in this manner during sporadic E events; this should continue until all parts of those countries complete their own analog TV shutdowns over the succeeding few years.
In some cases it is even possible to get DTV Es receptions from well over 1,000 miles (1,600 km), since even for DTV, some U.S. stations still use band 1. These signals are characterized for being either extremely clear, or extremely blocky. They are also much easier to identify. Furthermore, ATSC 3.0 could make sporadic E DTV reception easier, due to its usual modulation scheme being more resistant to multipath propagation, as well as impulse noise encountered on those frequencies.[citation needed]
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