Hi Tom,
Always good to hear from you. But by your remarks, I fear that
you do not understand that I am not trying to deal with the LP
question using any ionospheric model or software. More to the
point, it is just simple geometry, the difference between great-
circle paths for the terminator and the path from N7DD to the A45.
Looking at the geometric trace of a path and that of the
terminator is one thing. One can use the numerical details of the
path, with the aid of a Nautical Almanac, to work out the solar
elevation along the path. But just looking at the terminator and
talking about a heading is something else, open to considerable
error and misinterpretation of the path relative to sunlight.
I have used various locations in Oman but always seem to come up
with the same result, a major part of the long-path is in sunlight
(due to summer in the southern hemisphere). Using Muscat, Oman
I find that the path reaches 80 South Latitude, some 7 degrees
further south than the terminator. At that latitude, for that
date and time, the solar elevation is +6.4 degrees (above the
horizon). That is sunlight, not any sort of twilight, so one is
looking at serious ionospheric absorption there, for sure.
But if one follows the whole path, after leaving N7DD's QTH it
goes from his dawn, into sunlight around the equator at 117
West Longitude and then back into darkness around 28 South
Latitude, 68 East Longitude. So for an interval covering 175
degrees of longitude in the southern hemisphere, the path had
solar illumination on it, peaking at 80 South Latitude and
displaced poleward from the terminator by about 777 km.
As for signal skewing, one can look at Cary Oler's article in CQ to
see how small the angular deviation is in a dark ionosphere. Beyond
that, all attempts that I know of which tried to find a path that
follows the terminator for any distance have failed, no matter what
the season. So skewing, say 5 degs or more, is often due to large
ionospheric electron gradients in the auroral zones.
For the date and time of the A45 contact, NOAA reports low magnetic
activity, with a K-value of 2. If one uses that and goes to the DXAID
mapping program to look at the position of the auroral zone (based on
NASA auroral satellite data) relative to the long-path, one finds that
the path approaches the southern auroral zone almost at normal incidence.
Thus, the electron density gradient which was transverse to the
path was quite small and one would not expect much skewing on the
occasion. (The optical analogy would be light normally incident
on a pane of glass, with transmission and reflection but little
sideways refraction.) So without a reason to expect any skewing, the
previous discussion which is based on geometry would still apply, with
a long-path in sunlight.
All in all, as I look at the two paths, 13,800 km short-path in
darkness the entire way as compared to 26,200 km long-path in
sunlight a good part of the distance, I cannot escape the simple
conclusion that it was not a long-path contact. That conclusion is
bolstered by my earlier analyses of the VK0IR and ZL7DK logs.
The large number of contacts in the VK0IR logs showed that the amount of
weak daylight that can exist on a long-path drops from about 3,000 km
on 40 meters and to 1,500 km on 80 meters. Of course, that decrease
is to be expected as the ionosphere is a dispersive medium, with a strong,
inverse-square frequency dependence for both absorption and refraction.
With the additional loss in going to 160 meters, plus the absorption of
half the radiated power in the extra-ordinary wave, it is not surprising
that there were no LP contacts on 160 from either of those two locations,
not to the 20,000 km limit of definition nor 6,000 km beyond. So one
cannot add another number to that pair for the amount of daylight that
can be tolerated for a LP contact but I suspect it's close to ZERO km.
73,
Bob, NM7M
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