Yes Jim the 'z' scale is proportional to amplitude (flux
density), not power.
I completely agree with your interpretation of the plot -
- The nighttime window doesn't quite close and even
in high summer we still have couple of useful hours.
- In compensation for the short night, the daytime propagation
improves: better and longer, and by July, midday levels
are about 6dB below the March nighttime.
We should consider 'midday' to be relative to the center of the path.
For comparison, NAA for two more years
http://abelian.org/vlf/tmp/naa_2012a.png
http://abelian.org/vlf/tmp/naa_2011a.png
Looks pretty repeatable year to year.
As Alan mentioned, the midday 'dome' is a feature of most long
range diurnals. For example, NAU (Aguada) at 40.8 kHz for
2012 and 2013
http://abelian.org/vlf/tmp/nau_2012a.png
http://abelian.org/vlf/tmp/nau_2013a.png
Path length to NAU is 6740km, NAA is 4672km.
Going eastwards, TBB (Bafa, Turkey) on 26.7 kHz range 2883km
http://abelian.org/vlf/tmp/tbb_2013a.png
shows that it's not just an east-to-west feature.
During flares, the signal is always enhanced which confirms
that the dome improves with ionisation.
The midday dome is essentially D-layer propagation. D-layer is
formed by solar radiation (mostly nitric oxide ionised by Lyman
alpha) with a reflection height of around 70km. At night this
layer dissipates and reflection occurs from the E-layer at
95 km or thereabouts. Away from midday we have a mixture of
the two, with E-layer reflections being attenuated by passage
through a partially formed D-layer.
The D layer is a good reflector when fully formed but being
lower, more hops are required to cover a path.
--
Paul Nicholson
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