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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