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Sporadic E or Es is an unusual form of radio propagation using characteristics of the Earth's ionosphere. Whereas most forms of skywave propagation use the normal and cyclic ionization properties of the ionosphere's F region to refract (or "bend") radio signals back toward the Earth's surface, sporadic E propagation bounces signals off smaller "clouds" of unusually ionized atmospheric gas in the lower E region (located at altitudes of approx. 90 to 160 km). This occasionally allows for long-distance communication at VHF frequencies not usually well-suited to such communication.〔amfmdx.net. Sporadic E Reference Page. http://www.amfmdx.net/fmdx/sporadic-e.html Accessed 3 July 2008.〕 Communication distances of 800–2200 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 27–110 MHz range, which includes the FM broadcast band (87.5–108 MHz), Band I VHF television (American channels 2-6, Russian channels 1-3, and European channels 2-4, the latter no longer widely used in Western Europe), CB radio (27 MHz) and the amateur radio 10- and 6-meter bands. Strong events have allowed propagation at frequencies as high as 250 MHz. As its name suggests, sporadic E is an abnormal event, not the usual condition, but can happen at almost any time; it does, however, display seasonal patterns. Sporadic E activity peaks predictably in the summertime in both hemispheres. In North America, the peak is most noticeable in mid-to-late June, trailing off through July and into August. A much smaller peak is seen around the winter solstice. Activity usually begins in mid-December in the southern hemisphere, with the days immediately after Christmas being the most active period. On June 12, 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. It is still possible (as of April, 2010) for many Americans to see Canadian and Mexican analog stations in this manner when sporadic-E occurs, until those countries do their own analog shutdowns over the following few years. ==Characteristics== 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 Sporadic E signals tend to remain in the original transmitted polarisation. Long single-hop () Sporadic E television signals tend to be more stable and relatively free of multipath images. Shorter-skip () 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 increases with each subsequent Sporadic E hop. Sporadic E usually affects the lower VHF band I (TV channels 2–6) and band II (88–108 MHz FM broadcast band). The typical expected distances are about . However, under exceptional circumstances, a highly ionized Es cloud can propagate band I VHF signals down to approximately . When short-skip Es reception occurs, i.e., under in band I, there is a greater possibility that the ionized Es 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. At polar latitudes, Sporadic E can accompany auroras and associated disturbed magnetic conditions and is called Auroral-E. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sporadic E propagation」の詳細全文を読む スポンサード リンク
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