Friends in Radio Land -
The purpose of this note is to call your attention to a two-year
study of dawn enhancements by Nick Hall-Patch, VE7DXR. The study
used signals on 1,566 kHz, from the Korean broadcast station, HLAZ,
to Victoria, B.C., and noted over 100 enhancements in the winter
months of '97 and '98. With that body of observational material,
the question is then what can be determined about the enhancement
mechanism, particularly in relation to signal ducting.
Earlier, it was suggested (Brown, 1998) that 160 meter signals
could be trapped in the electron density valley that develops
above the E-region at night. Further, dawn enhancements were
attributed to the gain in signal strength which results when
ducted signals, going great distances without intermediate ground
reflections, return to ground level, adding their strength to
signals from the other, more conventional modes of propagation.
The question then is where and how ducted signals are diverted
downward. The data which follows will show that downward tilts of
the ionosphere in the F-region are the factor which determine when
dawn enhancements peak and come to an end.
Observations -
While no set pattern of enhancements was found in the study, the
records showed that the majority of the signals went through a
gradual build-up, rising out of background or noise in an hour or
less and then peaking. After that, signals went into a steep
roll-off, fading away often in less than 15 minutes.
Frequently, in discussing signal variations such as those, mention
is made of their relation to local sunrise. Unfortunately, there
is no significant ionosphere nor other causative agent for wave
refraction close to ground level so the discussion must turn to
other altitudes and other locations. In that regard, a more
appropriate altitude is where signals on DX paths peak in their
penetration of the ionosphere and a more appropriate location
would be one where signals on the path can descend and reach the
receiver on the ground.
Beyond that, it is expected the time of peaks of dawn enhancements
would shift with the seasons, earliest around the equinoxes and
latest at the winter solstice. The present observations have that
feature but also another remarkable one, showing that throughout
winter the times of the enhancement peaks lie close to the onset of
solar illumination at 200 km altitude and at a location about
1,600 km back along the path toward HLAZ.
Those features are seen by inspecting the figure below which is a
crude plot of some of the date-time pairs for enhancement peaks in
'97 and points on the sunrise curve, at 200 km altitude and 1,600 km
back toward HLAZ from Victoria, B.C. which are marked as "X". The
sunrise curve is obtained by joining the "X" by straight lines.
This plot is adequate to represent the matter but actual data seen on
the web site (http://www3.telus.net/7dxr/nm7m/9799.pdf) show it more
clearly. To read the illustration, you will the need Acrobat reader
(available at http://www.adobe.com/prodindex/acrobat/readerstep.html).
Time
ooo |
-----X------------------|------------------X-----14 UTC
o o | o
o | o
X | oX
o | o
o o | o
X oo | X o
------------------------|------------------------15 UTC
o | o o
X o | o X
o | o
X o | o ooo X
o | o
oX | o o X
------------------X--o--|-----X------------------16 UTC
X X X
|
--+-------+-------+------+------+------+-------+--
'97 SEP OCT NOV DEC 21 JAN FEB MAR '98
o - Enhancement Peak
The 200 km level is around the peak altitude of 1.8 MHz signals
propagated across the night-time ionosphere while the distance of
1,600 km corresponds to where 1.8 MHz signals at a 200 km peak
altitude would reach Victoria with a radiation angle of about
15 degrees. As the earth's shadow sweeps downward, the distances
and times of solar illumination change somewhat with radiation
angle but it is impossible to make finer distinctions than above,
given the continuous nature of the ionosphere and the diffuse
aspect of solar radiation. It can be said, however, that the
sunrise curve for 200 km altitude in the F-region matches the
observations quite well, as may be seen by the actual plot of
data points and the sunrise curve. Certainly, the fit to the
data is far better than for a curve below 100 km, say in the
D-region, which would be considerably later in time.
In summary, the experimental data shows that dawn enhancements
reach a peak and then decay when signals encounter solar
illumination in the F-region. By way of interpretation, that
is consistent with the release of ducted signals by downward
ionospheric tilts which are present in the F-region around the
terminator.
It is hoped that a similar body of observational material will be
obtained in the future to explore the presence and features of
signals coming from the East and reaching a receiver at sunset.
Acknowledgement
I am indebted to Nick Hall-Patch, VE7DXR, for sharing his data
and he is to be commended for the time and effort that he put
into making these observations. Those may be seen in upcoming
publications such as The Low Band Monitor and Top-Band Anthology,
Vol. 2, to be published by the Western Washington DX Club.
Reference
Brown, R.R., Signal Ducting on the 160-Meter Band, Communications
Quarterly, Vol. 8, No. 2, p. 65-82, Spring 1998.
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73,
Bob, NM7M
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