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Although much attention has been focussed on the recent events on 50MHz involving contacts between North America and Europe, there is evidence to show that such events have occurred every summer since 1980 and quite possibly before that.  In fact not only has propagation to North America been evident, but South America has also been represented, mainly in the form of a regular series of reports of reception of the FY7THF beacon in French Guiana.

The existence of a beacon is extremely helpful in looking at propagation statistics..  If we consider the dates and times of FY7THF reception first, we find the following pattern:

Table 1.  FY7THF reception in Europe
F layerPostulated Es
November - December 1979June, July 1980
October 1980 - January 1981June, July 1981
April 1981May - July 1982
October 1981 - February 1982May - June 1983
October - December 1982June 1985

There is good evidence that the events on the left of Table 1 were F layer, since they coincided with high solar flux and sunspot activity. What then of the events on the right?  fy7thf reception in Europe 1979-1985Figure 1 shows the distribution of times of openings consolidated into half hour periods. The open boxes correspond to the openings in the October to February months, whilst the filled boxes are for the May to July summer months.

The F2 reception events correspond to reports from UK amateurs on 59 days during the four winters involved. The Es reception events correspond to reports from UK, France and Sweden on 24 days.  It can be seen immediately that a quite different time of day is favoured by the summer events, which is another indication that a different mechanism is responsible.  

Map showing boundaries of Es zones






 











Map showing boundaries of Es zones and percentage of the daytime hours during the June solstice months (6am to 6pm, May to August) during which fEs exceeded 5MHz for the period 1948 to 1954 (after Smith, 1957)












temoral zones of Es


Correlation of transatlantic events with solar and geomagnetic indices

In an attempt to establish the cause of the 50MHz events, the sunspot numbers and geomagnetic indices have been examined. Considering first the relation with sunspot numbers, Table 5 shows the sunspot numbers (Provisional Brussels numbers except 1983 and 1984 which are definitive, and 1985 which are provisional Boulder numbers) for days on which FY7THF was heard.

Table 5 Summertime observations of FY7THF
DateTime startTime endHeard bySunspot NoAp
27/06/8020452122G4BPY1853
28/06/8017301936G4BPY1883
29/06/8014471449G4BPY1495
11/07/8020552227SM6PU8710
11/07/8020322034G4BPY
13/07/8020012010G3WBQ1059
13/07/8020322110G3WBQ
13/07/8022002235G3WBQ
14/07/8020022025G3WBQ1289
14/07/8023020010G3WBQ
14/07/8020302040G4BPY
05/06/8118502004G4BPY556
13/07/8118101815G3WBQ1539
13/07/8119211923G3WBQ
13/07/8119401949G3WBQ
13/07/8120292034G3WBQ
25/05/8219001935SM6PU888
05/06/8221402230G4BPY1116
17/06/8220292031G3TCT1366
17/06/8221002125G4GLT
18/06/8217241937G4GLT1347
06/07/8220222113G4GLT3215
07/07/8219202033G4GLT3321
08/07/8220172019G4GLT4915
08/07/8220522054G4GLT
09/07/8218502153G4GLT6110
20/07/8219001940G4GLT18024
16/05/8316001700F0FDB999
06/06/8319181922G4GLT8511
07/06/8310301032G4JCC1046
21/06/8317571905G4GLT11720
11/06/8519091935G4BPY8813
11/06/8519172046G4GLT
16/06/8513081330G4BPY504
16/06/8517201840G3WBQ
21/06/8518001930G3TCU1210

It can be seen that sunspot numbers vary between 12 and 188. Applying statistical tests to the events in June and July 1982, it can be shown that there is no significant difference between the numbers on Es days and a random selection. (The method used is the Kolmogorov-Smirnov goodness of fit test for completely specified distributions - see Elementary Statistical Tables, H R Neave. George Allen And Unwin Ltd 1981).  

We turn next to the relation with the planetary geomagnetic index, Ap, the average daily amplitude, prepared by the University of Gottingen.

Cumulative distribution of ApFig 8 shows the distribution of Ap values for each June and July between 1980 and 1984.  On the same figure is shown the distribution of Ap for days on which transatlantic Es occurred (both W/VE and FY7).  We can see that nearly 70% of Es days have Ap less than 10, compared with only 45% of all days in June and July 1980-84.  Therefore days with low geomagnetic activity are best for transatlantic Es.  Applying the Kolmogorov-Smirnov test as before we can compare the distributions.  The results show that the distribution for all Es days (W, VE, FY7) is significantly different from a random selection at 99% confidence.

Comparing the distribution of Ap for the "FY7" days only, there is a significant difference at 90% confidence.










Conclusion

Transatlantic propagation at 50MHz in the summer is not sunspot related, nor is it transequatorial.  The correlation with low geomagnetic index indicates that auroral-E is not involved.  The most likely explanation is multi-hop Es.


“Transatlantic propagation by Sporadic E at 50MHz” - RadCom, July 1986

This article analysed the incidence of transatlantic openings and showed through a statistical analysis that summer time openings to the UK were not sunspot-related, nor transequatorial, but that multi-hop Es was the most likely explanation.  The following text is an abridged version of the original article.