Radio Science, Volume 7, Number 3, pages 351-353, March 1972

Fifty-megahertz Es, 1964-1970

Patrick J. Dyer

Office of Telecommunications, Institute for Telecommunication Sciences
Boulder, Colorado 80302

(Received October 28, 1971.)

Using amateur signals in the 50-MHz band to indicate the presence of oblique-incidence     
Es paths, the diurnal, seasonal, and magnetic relationships of the phenomena for the years 
1964-1970 were examined. Summer double diurnal peaks were found along with 'off-season'    
meteor shower influences.                                                                  


    Using commonly available amateur equipment for      
 50-MHz, observations of Es phenomena were con-       
 ducted in the period 1964-1970.                      
      In general, receiver sensitivities were on the order  
 of 1 µv, although the equipment used varied through  
 the years of study. Antenna systems also underwent   
   some variation, with the average gain realized in the  
 7-9 db range. The receiving terminal point was still 
 another unfixed item. Observations were conducted    
 from San Antonio, Texas, from January 1964           
 through early August 1966 and again from No-         
 vember 1969 through the end of December 1970.        
 In the period between August 1966 and November       
 1969, observations were conducted from Austin,       
 Texas (some 85 miles northeast of San Antonio).      
 Equipment outage caused the loss of September-       
 November 1966 data.                                  


   The detection of all instances of 5O-MHz Es-       
 propagated signals was noted for onset and dropout   
 time (with 5-min resolution). A 30-min fadeout       
 period was defined as terminating a given 'opening.' 
   As no automatic recording devices were available,  
 the time that could be spent monitoring 50 MHz       
 varied greatly during the year. The summer periods,  
 however, can in many ways be considered near con-    
   Close monitoring of the 30-50-MHz region for       
 Es enabled the observations on 50 MHz proper to      
 be either intensified or relaxed depending on condi- 
 tions noted. This type of "back-up" system did not   
 come into use until September of 1967. A frequency   
 of 27 MHz (with some 2,000,000 transmitters in       
  this country) was often used, especially at night, to 
 indicate the lack of any Es likely to reach 50 MHz   
   The 50-MHz Es openings were not rated accord-      
  ing to strength, quality, distance, bearing, etc. The 
 wide variations found in those parameters are thus   
  not reflected in these data. As 50-MHz signal sources 
  (amateur stations) are not evenly distributed in time 
 or space, the data are strongly influenced by that   


   Much of the following data analysis is patterned   
 after earlier amateur radio work [Monroe and Mon-    
 roe, 1962, 1964].                                    
   Seasonal.  Figure 1 shows the totals of 50-MHz     
 Es minutes observed each month summed for 1964-      
 1970. The definite minimum occurs in March with      
 an equally definite peak in June (although July      
 of a given year may have exceeded June). A sec-      
 ondary maximum is evident in the month of De-        
 cember. This agrees with the long-known seasonal     
[Figure 1]

Figure 2 presents the yearly sums of 50-MHz Es. No clear relationship to the sunspot cycle is visible, although the largest value in 1965 cor- responded to a period of low solar activity and the smallest value in 1969 came during high solar levels. The large value in 1968 with high sunspot epoch does not agree with the trend found during the down- swing of cycle 19 [Monroe and Monroe, 1962].
[Figure 2]

Diurnal. Figure 3 is a sum of the diurnal varia- tions of the summer months (May-August) for each of the 7 years and a sum of those together. Note the different scale in the last graph. The double- hump maxima show very nicely through these years.
[Figure 3

Figure 4, for December, shows marked contrast to the summer diurnal curves. Only those days are included when both forenoon and afternoon mon- toring were possible in order to avoid biasing the evening results. Even then the preference for De- cember Es to be an after-dark event is strikingly evident.
[Figure 4]

In considering the diurnal behavior (all CST), it must be kept in mind that the actual solar time at the Es patch itself could vary by some two hours since the data make no distinction between clouds east or west of the observer. The 700-mile radio horizon during December, however, is all well into darkness by the time of the peak in Figure 4. Magnetic relationships. At the suggestion of some who attended the 1970 ESSA in-house Es seminar, attempts were made to relate the 3-hour Kp indices to the termination of an Es event in prog- ress. Very inconclusive results were found with Es commencing when Kp's were as high as 7. The 24- hour Kp sum is a much more meaningful parameter to use. Figure 5 compares the daily 50-MHz Es sums with the 24-hour (CST base) Kp sum for May-July of 1965. During this period the most intense mag- netic storm of the IQSY took place, In the period around the storm (June 15-22) one can easily see the depressing effect on Es, followed by a tremen- dous level of Es during the quiet period after. For 3 of 4 days the daily Es sum ran over 800 min., a level not elsewhere duplicated in the 7 years of the study.
[Figure 5]

Meteor influences. Figure 6 shows the October- December daily sums for 1964-197O combined. The abrupt rise in levels in mid-December relates very well to the Geminid meteor shower. Later December effects may be attributable to the Ursids. Likewise the mid-November rise is likely tied to the Leonid shower. Less strong peaks in early and late October may be linked to the Draconids and the Orionids, respectively.
[Figure 6]

CONCLUSIONS By using relatively inexpensive equipment to monitor a VHF band highly populated with signals, many of the commonly observed behavior patterns of an oblique path agree with well-established varia- tions of Es deduced from other modes of radio prob- ing (e.g., vertical incidence, backscatter). Use of the large numbers of active 50-MHz stations should al- ways be kept in mind when considering any Es, in- vestigations related to VHF propagation. REFERENCES Monroe, M., and D. Monroe (1962), 50 Mc propagation effects, CQ, 18 (6), 37-42. Monroe, M., and D. Monroe (1964), 50 Mc propagation effects, CQ, 20 (11), 82-87. Copyright © 1972 by the American Geophysical Union

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December 15, 2003