----- Original Message -----
Sent: Friday, May 03, 2013 6:14 PM
Subject: Re: LF: 136KHz propagation
Hi Alan,
the morning light shining from underneath is an
intruiging notion, and I've been wondering about it since you
first mentioned it a while ago.
To me, one conceptual problem seems to
be that the ultraviolet radiation must have passed the
same layer on the way down further east, at the same slant angle. So far
my understanding has been that ionisation of different molecules
selectively takes out certain wavelengths. How can the D layer be transparent
to a ray on the way down, and then opaque on the way up?
Best 73,
Markus (DF6NM)
Sent: Friday, May 03, 2013 11:56 AM
Subject: Re: LF: 136KHz propagation
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, radiation starts
to penetrate deeper and ionize lower layers. So the conductivity
of the D-layer increases so much that it starts to 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
