VHF PROPAGATION Via INTENSE Es OVER THE CONTINENTAL UNITED STATES M.S. Wilson EDMAC Associates, Inc. 17 Van Cortland Drive Pittsford, New York 14534 ABSTRACT Midlatitude Intense Es has been observed by means of oblique angle VHF radio propagation by Radio Amateurs since the mid-1930's and, because of their geographical distribution, their data is of value in the study of the occurrence and movements of intense Es clouds. By plotting this data motion of the intense Es clouds can be established with fair accuracy. An unusual, off season, intense Es was observed on 8 November 1970 for a duration of eight hours and extending over almost 90° of longitude and 20° of latitude, and this data was chosen for a detailed analysis. Data over a frequency range of 50 to 144 MHz are shown for the area over the continental United States. The data show the suddenness of the formation of intense Es, and that individual clouds of intense Es continue to be generated after sunset at the height of the E layer. A technique for establishing the birthplace of intense Es and for tracking individual clouds is shown. A model for the 8the of November 1970, locating birthplaces, cloud tracks, and cloud velocities is suggested. I Introduction Intense Es has been observed by means of oblique angle VHF radio propagation by amateurs since the mid 1930's. Their dedication, geographical distribution, and almost continuous observation make available valuable data for the study of the occurrence and motion of intense Es. Early analysis of these data by Pierce7 (1938) established the geographical location of contours of ionization based on equivalent horizontal dis- tribution assuming the secant law to be applicable. Conklin1,2 (1939) continued the plotting of this data and established the skip distance distribution. Wilson9 (1941) suggested possible wave paths via patches of intense Es rather than contours of ionization, and indicated a westward motion of these patches. Ferrell3 (1944) drew isopleths of equivalent ionization and found motion of the intense Es normally moving to the northwest at 40-150 meters per second, and suggested a circulatory motion. Later4 he established that intense Es forms suddenly, and identified high density cells. He suggested that the "heart" of the cloud diffuses but that large reflecting areas were maintained. A cooperative research program was initiated by Ferrell in 1949 under sponsorship of the Geophysical Research Division of the U.S.Air Force. This effort was called the Radio Amateurs Scientific Observations. Gerson5,6 (1950) plotted this RASO data using the midpoint location of all transmission paths and reported that the contours of equivalent ionization changed in both shape and size. He found that the intense Es moved anticyclonically first to the southwest and then to the northwest at a speed of about 50 meters per second. The advent of TV increased the ranks of amateur VHF propagation observers, and Smith8 (1953) using this data plotted the occurrence of intense Es for the summer maximum of 1950. He related specific days with data from the NBS ionospheric station at Washington, D.C. A second cooperative research program,called the Propagation Research Project, was established in 1959 and was again sponsored by the U.S.Air Force and coordinated by the American Radio Relay League during the IGY. Most recent work by Wilson10 (1970) suggests discrete birthplaces for the sudden appearance of intense Es, small individual patches or clouds of intense Es, and for the particular day studied (20 June 1968) approximately straight line cloud tracks to the northwest at a speed of about 90 meters per second. II Observations by Amateurs Extensive experimental observations of intense Es by amateurs have been recorded and a wealth of data is available. These observations should be a welcome and valuable source of data in the study of intense Es. Figure 1 shows the observed days of intense Es for the early years of observation during the summer maxima for the years 1935 - 1942. The increase of observed days during these early days clearly reflects the increase in sophistication of equipment used. It should be noted that by 1938 probably most instances of intense Es in the U.S. were observed,and good data is available from that time to date except for the war years.
This data shows that the occurrence of intense Es over the continental United States varies considerably from year to year, both in longitude and latitude, as well as in duration. Propagation paths via intense Es are reported almost every day from late April to mid August each year. Although the diurnal variation appears to peak twice a day,once in the morning and again in the early evening, on any particular day this is not usually true. Figure 2 shows the number of daily reports of occurrence of intense Es for the summer of 1950. The bars represent the number of reports of transmission distance greater than 1250 miles, indicating a large geographical extent. These data are compared to the NBS Washington D.C. ionosonde data selected for returns of greater than 7.8 Mhz. The lack of correlation of the observed 50 Mhz. propagation and the NBS data merely reflects the fact that the intense Es occurrence was not always over the eastern part of the country,although the correlation for days of double skip, or wide geographical area, is much better.
III Analyses of Amateur Data Early plotting of amateur data consisted of noting the location of the midpoint of the propagation path and calculating an equivalent electron density for the given frequency and skip distance. Points of equal density were joined to form contours of ionization. After the war Ferrell using RASO data plotted intense Es cloud movement and showed the general directions taken by the clouds. His technique was to draw a straight line between the location of the observer and the location of the station heard, and then marked the midpoint of the path. He developed isopleths for a half hour period and identified a "heart" or intense core. By making such maps for consecutive half hours the motion of the intense Es could be found. He showed a general enlargement of the total area covered by the intense Es. To explain the 144 Mhz. propagation he suggested a high density cell on the leading edge of the core. Using such maps Gerson5 found that the motion of intense Es varied in direction and speed for different days. The difficulty with such a model of intense Es(using contours of ionization)lies in the fact that it does not explain the "negative" reports, by which is meant the case were an active observer within range of the intense Es is unable to hear other stations. The data show that particular propagation paths for a given frequency are limited in range and bearing to an area probably not greater than 50-75 miles in diameter, (although a larger area exists for an enhanced scatter mode.) TV recording data show that for a single cloud crossing normal to the propagation path, the minimum propagation loss occurs for about six minutes, and that the signal intensity changes suddenly some 50 db for both the leading and trailing edges. Such data imply a small patch or cloud of intense Es. When the concept of individual small clouds is applied, and the location of a cloud is not constrained to the midpoint of the transmission path, the data can be explained. Using this technique it becomes apparent that multiple clouds are usually present, often along a line or row, and results in multiple focusing. Many reports of higher frequency propagation paths appear to make use of more than a single cloud. IV Technique of Plotting Small Intense Es Clouds Plotting data to identify small intense Es patches or turbulences employs the usual technique of drawing a line on a map from the observers location to the location of the station heard in order to establish the approximate transmission path. The midpoint of the path is not necessarily used, but arcs of 625 miles are drawn from each location to establish the practical "radio horizon" for each location. This is the major constraint on the location of the intense Es cloud. The tangent distance of 700 miles is not used because the data show that most observers do not attain a zero angle of radiation from their antenna systems. If the arcs do not intersect, there must have been more than a single cloud present, and continued plotting will locate the positions of the clouds. When the arcs do intersect the probability is that the cloud is located within the common area. By plotting the data for a few minutes at a time the location of a single cloud may be found. The initial location, or birthplace, of intense Es can be determined by the beginning of the data and also by the fact that the birthplace will continuously support propagation paths for some time (more than an hour). Signals propagated via a strong birthplace suffer little loss and high levels of intensity continue with no deep fading. The location of a birthplace appears to move at about 25 miles per hour to the east or southeast. Individual clouds or turbulences seem to be shed from the birthplace at discrete times, and travel from the location of the birthplace at a constant velocity as if blown by a wind. Some of the clouds are short lived, but most continue in existence for many hours and have been tracked for over eight hours. Individual clouds which are long lived usually dissipate at some specific latitude for the particular day. Just prior to their dis- appearance their speed decreases. The time spacing between individual clouds shedding vary, but the long lived clouds are generally about a half hour apart. When the birthplace dissipates few if any new clouds are generated. The location of a birthplace of intense Es is often related to weather fronts in the troposhere and many times appear over a squall line or areas of precipitation. The data show that the location of a birthplace for subsequent days moves eastward with the tropospheric weather pattern at an average of 12½ degrees of longitude per day. Hot air from Mexico and the Gulf of Mexico often relates to a birthplace. IV Intense Es Observed 8 November 1970 Intense Es observed on 8 November 1970 was most unusual in that it was of long duration, extended over a wide area, and was off season. Reports from the east coast of the United States to as far west as Hawaii, and from the northern border of the United States to the tip of Florida and Mexico were received.Scattered reports of intense Es were received for the morning and afternoon,but the consistent time span for 50 Mhz. propagation was from 1800 to 0140 EST. The highest frequency of propagation reported was 144 Mhz. and these reports covered a time span of about two hours. The greatest distance reported at 144 Mhz.was 1300 miles, for 100 Mhz 2000 miles, and for 50 Mhz more than 4000 miles. Widespread reports of direct backscatter from intense Es clouds at 50 Mhz. and one report of direct backscatter at 100 Mhz. were received. At least ten different birthplaces were identified across the continental United States and the begining time for all was within a thirty minute period. The first birth- place observed was located near El Paso Texas at 1750 EST and the second near Washington D.C. at 1913 EST. The suddenness of the onset of intense Es is illustrated in figure 3, which is a field strength recording of the amplitude of the video carriers of two channel 4 TV stations. Chart speed is 1 mm per minute, and the amplitude is recorded from noise level (approximately 1 microvolt at receiver input) to amplifier overload (approximately 1600 micro- volts at receiver input) on a non-linear scale. The lower recording shows the signal strength of TV station WTVJ located at Miami, Florida and illustrates the sudden increase of signal strength in about a minute of time as the MUF exceeded the frequency. (67.250 Mhz.). The propagation path distance was approximately 1250 miles. The upper record- ing is the signal strength of TV station WJXT located at Jacksonville Florida at a shorter distance of about 950 miles.
Figure 4 shows the transmission paths reported at 1900 EST about an hour after the initial appearance of the intense Es. Clouds over the eastern part of the country began to support FM frequencies and short skip, and backscatter was reported from very intense clouds along a line from Washington, D.C. to Western Pennsylvania. TV propagation began from clouds over the central states and TV stations were heard over single skip distance from the southern west coast. 50 Mhz double skip existed from Arizona to Ohio, and southern California continued to report the Kansas area but no double skip. At this time intense Es was present over at least 80 degrees of longitude and observers in Hawaii began to hear stations as far east as West Virginia. The detailed data show that the Hawaiian signals were focused to relatively small areas at any given time, and stations 50-75 miles away from these areas were unable to hear the Hawaiian stations.
By 1930 EST 144 Mhz propagation paths were reported over the southern central states. FM reports increased over the eastern states and pro- pagation paths were reported over the central and western states. 50 Mhz paths covered the country and coast-to-coast propagation was reported. A half hour later at 2000 EST 144 Mhz transmission paths were reported from Texas to southern California and one report from Kansas to Virginia. FM propagation paths disappeared over the eastern section and began from the panhandle of Florida to the Denver, Colorado area which was to last for more than two and a half continuous hours. Another FM path was observed from Iowa to the Arizona-New Mexico area. By 2030 EST the 144 Mhz reports were from Wyoming to Texas and from Nebraska to southern California. FM propagation paths were spreading west to southern California and north to Minnesota. 50 MHz paths remained strong. At 2100 EST a 144 Mhz path was present from Nebraska to southern California, the FM paths had reached Idaho and a double skip path opened from Idaho to Mexico. The 50 Mhz propagation paths which for single skip distance had been creeping up the California coast had reached Santa Maria at this time. A half hour later the last 144 Mhz path was reported from Nebraska to southern California. The FM double skip from Idaho had shortened to southern Texas, and FM double skip was reported from Santa Maria, California to Illinois. The 50 Mhz paths became very short over the northeast section of the country. FM propagation was still being reported at 2200 EST over the central and eastern United States, but was beginning to weaken. The FM path from Idaho continued to shorten and was now into New Mexico. 50 Mhz paths continued to be reported coast to coast from San Diego to New York City. An hour later at 2300 EST intense Es no longer supported the FM frequencies and the 50 Mhz coast-to-coast paths were beginning to creep up the coast of California and continued to go northward over the eastern end of the path. At 0000 EST the coast-to-coast 50 Mhz propagation paths still existed although most of the clouds were beginning to disappear. By 0030 the far west clouds had reached a position to support 50 Mhz paths from Southern California to the northwest corner of the United States. The coast-to-coast path had now reached as far north as San Francisco. An hour later at 0100 EST most of the clouds had dissipated and the coast-to-coast path disappeared. One very persistent cloud had reached upper Michigan, the highest latitude reached for the day, and allowed a Connecticut to Minnesota path to exist. This particular cloud had been responsible for the very short skip at 50 Mhz some two hours earlier when over the Maryland-Pennsylvania border. At this time all the remaining clouds began to dissipate, and by 0130 only one cloud could be found from the data and the last report was at 0157 EST. A model which explains the detailed data for this day is shown in Figure 5. The tracks are required and the birthplaces for nine tracks are shown. The tracks are shown as straight lines on the map but because of map distortion the actual tracks may be somewhat curved, especially over the western half of the country. Note that most of the lead clouds dissipated before reaching 43 degrees north latitude. The lead cloud for the track fourth from the left was tracked for almost seven hours as it travelled from the Oklahoma-Kansas border to northern California. Many other clouds were generated and these clouds followed the lead clouds on each track. Most of the birthplaces had dissipated by 2100 EST, although the Washington, D.C. birthplace did generate some clouds after the continuous phase had ended. Note that the location of the birthplaces starting on the left seem to first be on a line in a northeast direction, then across the country just above 30 degree north latitude, then up the east coast. Figure 6 shows the surface weather map for the 8th of November 1970 and illustrates a typical relation- ship between the location of a birthplace of intense Es and the interface of large air masses on the earth's surface. This coincidence should not be ignored.
Conclusions The volume of simultaneous data collection by a large number of geographically spaced observers makes possible the construction of a model of intense Es, and emphasizes the value of amateur VHF observers' contributions. A model of the birthplaces and movement of intense Es clouds based on amateur observers' data for the 8th of November 1970 has been presented. This analysis of an off season occurrence of intense Es when compared to a like analysis for a summertime occurrence indicates little difference in the movement of the intense Es clouds. Acknowledgement The author wishes to express his appreciation to the hundreds or Radio Amateur Operators and the TV-FM Dx-ers for their data,without which this analysis could never have been accomplished.
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