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.
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 layer||Postulated Es|
|November - December 1979||June, July 1980|
|October 1980 - January 1981||June, July 1981|
|April 1981||May - July 1982|
|October 1981 - February 1982||May - June 1983|
|October - December 1982||June 1985|
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? Figure
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.
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.
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)
Correlation of transatlantic events with solar and geomagnetic indicesIn
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
|Date||Time start||Time end||Heard by||Sunspot No||Ap|
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).
turn next to the relation with the planetary geomagnetic index, Ap, the
average daily amplitude, prepared by the University of Gottingen.
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%
Comparing the distribution of Ap for the "FY7" days only, there is a significant difference at 90% confidence.
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 1986This
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.