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Date: Sat, 08 Mar 2014 10:29:58 +0000
From: Paul Nicholson <vlf0403@abelian.org>
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Subject: Re: LF: Daytime 29.499 kHz
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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
--