Figure 25. nnuF is Highest During Periods of Maximum Sunspot Activity MuF extremes are greatest during periods of high sun- spot adivity. lonospheric losses are at a minimum near the MuE and increase rapidly for lower frequencies, espe- cially during daylight. The recommended upper limit of frequency for maximum circuit reliability is called the Optimum Traffic Frequency and is seleded somewhat below the MuF to provide margin for ionospheric irregu- larities.

ionospheric characteristics, the r.uF can be controlled to an extent by adjustments in effective radiated power and circuit bandwidth. Generally speaking, the LuF can belowered approximately 2 MHz for each 10-decibel increase in effective radiated power.

Long Path Propagation The great circle path is the shortest distance between two points on the earth's surface. Most hf communica- tion is via the great circle route. However, long path propagotion along the reciprocal path is common in the bf region. Generally speaking, the long path travels tlu'ough a zone of darkness while the short path travels through sunlight. The itluminated short path implies high ionospheric absorption in addition to path attenuation. The long path passes through the dark hemisphere so that con- aiderably less signal absorption is encountered. Moreo- ver, the maximum usable frequency (cvtuF), which also depends on the position of the sun, is lower for the long path than for the short path. Representative open hours of the long path from Europe to Australia are shown in Figure 26. The spring months of the year are the best for this circuit, with May showing a long path opening of nearly four hours. The shortest openings occur in the winter months. Many times both the short and long paths between two points are open for communication. When this happens, a distinct echo can be heard on a signal which results from the delay in arrival time of the long path eignal as compared to the arrival time of the short path eignal.

Figure 26. Long Path Openings Opening hours of the long Great Circle path between Western Europe and Australia at 7 MHz. (From the Jour- nal of the ITU)

Multihop Propagation Long distance hf propagation occurs by means of a number of reflections from ionospheric layers known as hops. Radio signals may traverse a path by means of several simultaneous modes involving a different num- ber of hops. The nearer the signal is to the operational ivtuF, the smaller is the number of modes involved. Since there are several ionospheric layers that will support propagation, notably E, and Fz, and to some extent Fr it is possible to have modes which are combi- nations of hops reflected from the various layers and also modes which bounce internally between layers. Because propagation time for each mode is different, a single short radio pulse transmitted at a given time may be manifest as a series of broken, short pulses at different arrival times. Multipath distortion may cause serious errors in frequency-shift keying or frequency modulation, as well as in forms of amplitude-modu- lated transmission.

Cycles in lonospheric Activity

The first recorded observations of sunspot activity were made by Chinese observers more than 2000 years ago (Figure 27). Centuries Iater, in 1901, Marconi was una- ware that his successful spanning of the Atlantic Ocean by radio for the first time was possible only because of the existence of sunspots which, the astronomers of that time thought, might be holes cut in the sun's sur- face by solar hurricanes, exposing the cooler layers below. Experiments conducted by Heaviside (1902), Apple- ton (1924), and Naismith (1927) proved the existence of an electrified reflecting region in the atmosphere, mea- sured the characteristics of it and reached the conclu- sion that the principal solar factor in the production of ionization in the atmosphere was ultraviolet radiation from the sun. Later investigators discovered a direct relationship between the ultraviolet radiation, the de- gree of ionization in the atmosphere, and its relation- ship with long distance radio communication.

Sunspots have been observed and recorded for more than 2000 years. In this U.S. Navy photograph, a large group of sunspots is seen, moving from east to west, as the sun rotates. Sunspot adivity has dired bearing on radio transmission.

Sunspots in Action With the aid of suitable instruments, sunspots can be seen to develop from small dark areas on the brilliant surface of the sun. Studies indicate that the inner por- tion of the sunspot is a depression in the sun's surface having an average depth of several thousand miles. The temperature of the sunspot is several thousand degrees cooler than that of the general surface of the sun and gives off about one-half as much light as the same area of the photosphere, or surface of the sun. Sunspots almost always appear in groups, some spots as large as 80,000 miles (128,000 km) in diameter. The groups move parallel to the equator of the sun in an east to west direction in accord with the sun's rotation. Many terrestrial phenomena which are influenced by localized sunspot activity on the sun tend to occur at intervals of about 27 days, which is the period of rota- tion of the sun.

The Sunspot Cycle The number of sunspot groups, and individual sun- spots, visible on the sun's surface vary between wide limits over a period of time. Sunspot activity follows an approximate 11-year cycle, steadily rising from very few to a maximum amount, then slowly receding to a minimum amount again. The sunspot count is recorded in Zurich Sunspot Numbers on a daily and monthly basis, and 12-month, smoothed running numbers are published in CQ maga- zine and various astronomical publications. The re- cordings began in 1750 and 19 complete cycles have been recorded to date. No two cycles have been exactly alike, although a definite repetitive behavior is estab- lished. Basic characteristics of the cycle, such as dura- tion, height of maximum, depth of minimum and

ascent and descent time are observed, and vary from cycle to cycle. No explanation of the sunspot cycle has yet proven to be completely satisfactory and current estimates of future performance are open to specula- tion. The present search for empirical laws governing solar activity has proceeded according to two different schools of thought, one holding that solar activity is a periodic phenomenon, the other considering each solar cycle as an independent event. Since hf radio transmission is dependent on the iono- sphere, which varies with the sunspot cycle, the action of the cycle is of extreme interest to communicators (Figure 28). When the sunspot count is high, ionization

>

Figure 28. Relation Between Observed nnuF and Smoothed Sunspot Number When the sunspot count is high, ionization of the earth's atmosphere is heavy and the nnur is correspond- ingly high, opening up additional frequencies for long- distance communication. Predictions for cycle 22 indi- cate a maximum sunspot count of about 100, thus limit- ing the mur to approximately 40 MHz for the next 15 years.

of the earth's atmosphere is heavy and the tvtuF is corre- spondingly high, opening up additional frequencies for long-distance communication. During cycle 19, which peaked at a count of over 200, the NtuF regularly ex- ceeded 50 MHz. Cycle 20, which ended in 1975, was considerably lower, limiting the nuF to something over 30 MHz at the peak of sunspot activity. Cycle 21 reached its peak of about 150 during the late summer of 1979, making it among the most intense recorded since sunspot observations began. In the twenty cycles observed since 1755, only three have ex- ceeded a smoothed sunspot number of 150; cycle 3 with a peak of 159 in 1778, cycle 18 with a peak of 152 in June,1947 and cycle 19 with a record-breaking level of 201 in November, 1957. An extended prediction indi- cates that sunspot numbers in the vicinity of 100 to 150 may be observed during the following sunspot cycle. Thus, the next 40 years may be characterized by me- dium to high values of sunspot activity comparable to the activity of the last 40 years. The implication of low sunspot activity is that the wtuF will be considerably lower, long-distance propaga- tion will be more infrequent and will occur for shorter

20-16

Figure 27. Sunspots in Action

 RADIATION AND PROPA6ATION

riods of time, and with reduced signal levels. Fre- lencies below 8 MHz, however, may show improve- ent even though the higher frequencies may show arginal performance. Thus communication using ionospheric reflection in e hf bands will continue to react to the influence of e sun and undoubtedly more vital communication rcuits will be switched to satellites to overcome the tgaries of ionospheric reflection.

lonospheric Disturbances

e diurnal, seasonal, and solar cycle variations of the osphere discussed previously are dependent on the Ittlar, more-or-less predictable behavior of the ioniz- I solar radiation. From time to time, however, the rmal behavior of the ionosphere is upset by distur- nces of a transistory or short-duration character. It is Geved that these are the result of abnormal radiations im the sun. These disturbances give rise to abnormal dio propagation conditions, sometimes leading to a nporary "radio blackout," or complete failure of hf dio communications. Ionospheric disturbances fall into two main catego- s: the sudden ionospheric disturbance (SID) and the nospheric storm. The SID commences suddenly and sts from a few minutes to an hour or so. The iono- heric storm develops over a period of a day or two d generally continues for several days. In either case, e normal behavior of the ionosphere is upset, with itical frequencies dropping, and ionospheric absorp- 1n increasing as the intensity of the disturbance in- The SID has a spectacular effect on hf propagation. near-simultaneous radio fadeout occurs over a large irtion of the hf spectrum, from approximately 2 MHz 30 MHz, with even background noise sometimes 6appearing. The only signals that can be heard during l SID are those from stations within the ground-wave Dge. The fadeout lasts for a short period, then condi- 1ns slowly return to normal. It is thought that the SID is a result of a solar flare, a Idden, short-lived, bright eruption on the face of the m. The incidence of solar flares varies with the solar Cle and is most prominent during years of very high Ilar activity. The SID takes place about 11 minutes after a solar ue commences, and occurs only in those areas of the orld in complete daylight. Not all flares produce lDs, indicating that the SID is only one manifestation f the release of solar energy. The typical change in a communication circuit during r SID is shown in Figure 29. Signal drop-off is ap- mumately 40 decibels in a matter of a few minutes, ith the signal returning to normal in about 40

A second type of disturbance is the ionospheric storm. hile not as spectacular as the SID, the storm actually nstitutes a more serious communications problem be- use of its much greater duration. During a storm, hf

Figure 29. SID Signal Drop-Out in a Communication Circuit

Solar flare causes a sudden ionospheric disturbance about 11 minutes later in areas of the world in complete daylight. Signal returns to normal in 30 to 40 minutes after a drop-off of about 40 decibels in strength.

signals (from approximately 3 MHz to 30 MHz) drop to a very low level and may even disappear entirely for periods of several days. Measurements indicate that the F layer is usually at an abnormally great height during the disturbance and is subject to considerable turbu- lence. Unlike the SID, the higher frequencies are most affected, and the storm occurs in both daylight and darkness regions of the world. Ionospheric absorption increases and signals are subject to considerable fading, often of an unusual type known as flutter fading. It is thought that the ionospheric storm is caused by corpuscular radiation of ionized calcium emitted from solar flares at the same time the flare emits ultraviolet and X-ray radiation which produce the SID. Corpuscu- lar radiation travels at a velocity much lower than the speed of light because of its greater energy content and arrives at the earth at a later period of time. The radia- tion is so eonfined that unless the emission is pointing directly at the earth, it may miss the earth entirely (Figure 30). Besides radiant energy, solar flares also emit bursts of electromagnetic energy in the form of radio "noise." These bursts, occurring over a wide range of frequen- cies above about 10 MHz, are strongest in the vhf re- gion of the radio spectrum. They can be received as a hissing sound on a sensitive receiver. The flares also violently disrupt the earth's magnetic field for short periods of time as they disrupt the ionosphere. These magnetic storms are most intense in high latitudes and often last for several days. As satellites and space vehicles probe further into space, many of the secrets of the solar flare, the SID and the magnetic storm will be revealed, and in the future the prediction of these phenomena may be made with greater accuracy than is possible at the present time.

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