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Hi Markus, Spiros, there is another interesting
effect at dawn. The weak rays of the rising sun actually strike the ionoshere
first from underneath !! this means that the first photodissociated
ionisation is very highly absorbing because it is formed at lower levels where
the pressure is higher and the electron mean-free-path is very short. This has
the effect of producing a deep dip in levels at dawn at mid-path. That is dawn
at an altitude of 50km or so, not ground level. This effect is most noticable on
N<>S paths which I think is the direction of the grabbers from Spiros.
An hour or so later the Sun will have risen
sufficiently to illuminate the path from above and the daytime D-layer will
build to provide daytime skywave. The signal levels in daytime usually show a
gentle "dome" shape with the peak at mid-day at mid-path.
in the early days of 136kHz it was though that dawn
would be a good time for DX because the noise was lowest. However we eventually
realised that the noise was low because the distant noise which propagated by
skywave was wiped out at dawn by this effect. Thus the best DX times are often
(but not always :-)) ) early evening or about an hour before
dawn.
Great Fun this Propagation stuff :-))
Alan G3NYK
----- Original Message -----
Sent: Friday, May 03, 2013 8:36 AM
Subject: Re: LF: 136KHz propagation
Hi Spiros,
not really strange: the rising
sun is suddenly ionizing lower ionospheric layers and increasing the
absorption.
Here's a somewhat simplistic explanation of
diurnal propagation: At night, LF is mostly reflected by the lower boundary of
the E-layer, at a height of about 100 km. After sunrise, radiations starts to
penetrate deeper and ionize lower layers. So the conductivity of the
D-layer increases so much that it starts first absorb and later reflect
LF.
This is somewhat similar to a shunt
resistor inserted in a transmission line: During the night it's
a large resistor which will let most of the wave pass through, absorbing
only a small fraction of the power which depends on "conditions" (like
DST index, describing charges leaking down from a storage ring).
At sunrise, the shunt
suddenly conducts more ("closer to 50 ohms") so most of the wave is
being absorbed. At midday when the sun is highest, the conductance
is approaching a "short circuit" which reflects the wave from a
lower altitude (about 80 km).
After sunset, the electrons and ions in the lower
layers recombine quickly so the D-layer becomes transparent again. However at
high altitudes, the gas density is so low that charges take
long to find a partner, thus ionisation of the E and F layers persists
throughout the night.
Best 73,
Markus (DF6NM)
Sent: Friday, May 03, 2013 5:58 AM
Subject: LF: 136KHz propagation
GM to all.
The same strange propagation tonight as every
night.
At around 03.00z my signal disappear from DF6NM, DK7FC, YO/4X1RF
& G4WGT
grabbers.
I have better prop earlier and during the day
with day light.
73, Spiros
SV8CS
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