.net barcode reader free 2-26A Solar event that can affect radio propagation on earth in Software

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2-26A Solar event that can affect radio propagation on earth
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40 Radio-wave propagation Sunspots A principal source of solar radiation, especially the periodic forms, is sunspots (Fig 2-26B) Sunspots can be as large as 70,000 to 80,000 miles in diameter, and generally occur in clusters The number of sunspots varies over a period of approximately 11 years, although the actual periods since 1750 (when records were first kept) have varied from 9 to 14 years The sunspot number is reported daily as the statistically massaged Zurich Smoothed Sunspot Number, or Wolf Number The number of sunspots greatly affects radio propagation via the ionosphere The low was in the range of 60 (in 1907), and the high was about 200 (1958) Another indicator of ionospheric propagation potential is the solar flux index (SFI) This measure is taken in the microwave region (wavelength of 102 cm, or 28 GHz), at 1700 UTC at Ottawa, Canada The SFI is reported by the National Institute for Standards and Technology (NIST) radio stations WWV (Fort Collins, CO) and WWVH (Maui, Hawaii) The ionosphere offers different properties that affect radio propagation at different times Variations occur not only over the 11 year sunspot cycle, but also diurnally and seasonally Obviously, if the sun affects propagation in a significant way, then differences between nighttime and daytime, and between summer and winter, must cause variations in the propagation phenomena observed The ionosphere is divided, for purposes of radio propagation studies, into various layers that have somewhat different properties These layers are only well defined
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EM wave propagation phenomena 41 in textbooks However, even there you find a variation in the precise altitudes of the layers above the earth s surface In addition, the real physical situation is such that layers don t have sharply defined boundaries, but rather fade one into another instead Thus, the division into layers is somewhat arbitrary These layers (Fig 2-27) are designated D, E, and F (with F being further subdivided into F1 and F2 sublayers) D layer The D layer is the lowest layer in the ionosphere, and exists from approximately 30 to 50 mi above the surface This layer is not ionized as much as the higher layers, because all forms of solar energy that cause ionization are severely attenuated by the higher layers above the D layer The reason for this is that the D layer is much more dense than the E and F layers, and that density of air molecules allows ions to recombine to form electroneutral atoms very quickly The extent of D layer ionization is proportional to the elevation of the sun, so it will achieve maximum intensity at midday The D layer exists mostly during the warmer months of the year because of both the greater height of the sun above the horizon and the longer hours of daylight The D layer almost completely disappears after local sunset Some observers have reported sporadic incidents of D layer activity for a considerable time past sunset The D layer exhibits a large amount of absorption of medium-wave and shortwave signals (to such an extent that signals below 4 to 6 MHz are completely absorbed by the D layer) E layer The E layer exists from approximately 50 to 70 mi above the earth s surface, and it is considered the lowest region of the ionosphere that is important to radio communications Like the D layer, this region is ionized only during the daylight hours, with ionization levels peaking at midday The ionization level drops off sharply in the late afternoon, and almost completely disappears after local sunset During most of the year, the E layer is absorptive and it will not reflect radio signals During the summer months, however, E layer propagation does occur A phenomenon called short skip (ie, less than 100 mi for medium wave and 1000 mi for shortwave signals) occurs in the E layer during the summer months, and in equatorial regions at other times A propagation phenomenon associated with the E layer is called sporadic E propagation This phenomenon is caused by scattered zones of intense ionization in the E layer region of the ionosphere The sporadic E phenomenon varies seasonally, and it is believed to be caused by the bombardment of solar particles Sporadic E propagation affects the upper HF and lower VHF region It is observed most frequently in the lower VHF spectrum (50 to 150 MHz), but it is also sometimes observed at higher frequencies The VHF bands occasionally experience sporadic E propagation Skip distances on VHF can reach 500 to 1500 miles on one hop especially in the lower VHF region (including the 6-m band) F layer The F layer of the ionosphere is the region that is the principal cause of long-distance shortwave communications This layer is located from about 100 to 300 mi above the earth s surface Unlike the lower layers, the air density in the F layer is low enough that ionization levels remain high all day, and decay slowly after local sunset Minimum levels are reached just prior to local sunrise Propagation in the F layer is capable of skip distances up to 2500 mi on a single hop During the day there are actually two identifiable and distinct sublayers in the F layer region, and these are designated the Fl and F2 layers The F1 layer is found approximately
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