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Peter, thanks for sharing this plot! The
intense and well defined peak would indeed suggest something like a focal point
sweeping over the receiver. The 1999 eclipse must have had almost ideal
geometry, with the moon's shadow crossing the middle of the path
from Prangins to Kiel.
Especially on longer paths, we may actiually have
two distinct effects: One is the decreasing D-layer absorption, opening the way
to lossless refraction in higher layers. This is surely also the
explanation of the "quasi-nighttime" boost of MF signals. On the other
hand, there may be that proposed lense-effect, creating a localized and
moving focus point for the reflection off the bottom layer conductivity
gradient.
Taking another look at yesterday's 100 kHz
plots http://df6nm.bplaced.net/LoranView/eclipse/Loran_Eclipse_150320_plots.png,
we may actually find an indication for both effects: Three traces from northerly
stations (Jan Mayen, Eide, Bo) exhibit a large and unsymmetrical hump (10 to 12
dB), beginning around 9:20 UT when the penumbra begins to invade the
North Sea. But on top of the Jan Mayen hump there rides a
little "hat", a small extra peak (~ 2 dB) at 9:53, just when the central shadow
crosses this path. No such thing appears on the Ejde trace, and maybe a
very small effect on Bo at 9:59.
I had first thought that the small peak was
insignificant and might have been caused by a solar flare, but there was none at
the time. I would now propose that the small peak is due to the focusing
effect.
In principle, arbitrary maxima and minima can also
be caused by a variable-phase skywave relative to a fixed groundwave; this has
been included in Micha's simulation. However at these much larger
ranges (2000 km and up), the groundwave contribution at 100 kHz
is already rather weak. In addition, Loran pulses are relatively wideband,
and the applied "total energy" postprocessing incoherently sums
up differently delayed pulse components, reducing the effect of
relative phase.
All the best,
Markus (DF6NM)
----- Original Message -----
Sent: Saturday, March 21, 2015 9:28 AM
Subject: LF: Radio Focussing by Solar Eclipse?
Just came across at an excellent web paper by
Micha Sanders (PC4M, PA3BSH) http://misan.home.xs4all.nl/eclipse.htm,
dealing with observations of the HBG 75 kHz time signal during the 1999
solar eclipse. Most observers found a characteristic W-shaped
fieldstrength curve, with a central maximum preceeded and followed by two
minima. This was found to agree very well with a simulation based on local
D-layer height variation around the moon's shadow.
I've been wondering whether this couldn't be
interpreted intuitively as a focussing effect. An upward indent on the
lower ionosphere could act as a concave mirror, leading to
convergence of radio waves into a focus area, surrounded by a
radio shadow.
At lower VLF frequencies, we tend to think in
two-dimensional waveguide modes rather than vertically separated discrete
rays. An analogous interpretation would be an area of slower
phase velocity near the center of the eclipse,
which would laterally bend radio waves towards a focus area,
acting very much like an optical lens.
Best 73,
Markus (DF6NM)
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