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It's interesting to observe the ripple pattern on a
19.8 kHz spectrogram, created by superposition of long and short path
propagation from the Australian VLF transmitter NWC
During the European morning hours, the part of
the short path which is lit by sunlight is gradually growing. With
increasing ionisation, the "ceiling" of the waveguide comes
down by a few km. We can deduce the path difference (long minus short)
from the nulls of the spectrogram ripples. The diurnal shift is
normally about two ripple periods (40 Hz here), corresponding to a total
phase difference change of two wavelengths (30 km). The
average Doppler velocity would be around -1 m/s.
Today around 8:10 an M-class solar flare
occured, see attached gif from http://www.swpc.noaa.gov/rt_plots/xray_1m.html .
In the NWC spectrogram, we find
a remarkable effect of the additional D-layer depression, resulting in an
extra path shortening of about 0.4 wavelength or 6 km.
Best 73,
Markus (DF6NM)
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