Hello again Tom,
Thanks for the reply. Let's see if I can shed some light on your
question: about absorption on 160 meters being independent of the
things that control MUF.
Let's go to the basic matter, signal absorption along a path.
That is due to collisions of electrons with neutrals, sort of a
mechanical thing where energy is imparted to the atmosphere. When
you talk about an increase in the MUF, say as 10 meters opens up,
that means solar radiation has increased the electron density up
in the F-region. Whether the increase in ionization down in the
D-region is in direct proportion is another matter.
The figure in Davies that I referred you to shows a typical foF2
critical frequency change in the F-region is about a factor of
2 when the SSN goes from 0 to 200 while the change in foE is about
0.33 for the same change in SSN. That has to do with different
parts of the solar spectrum, the more energetic radiation reaching
the E-region, even lower, while the softer and less energetic
radiation is effective at F-region heights. So we're not talking
about the same solar radiation affecting both regions and being at
different parts of the spectrum, they may not be exactly in some
sort of lock-step. That spectral variability is one way that
ionization in the D-region may change at a different rate than in
the F-region and obviously it would have an effect on absorption.
Similar effects are seen in auroral events, soft and hard parts of
the incoming electron spectrum giving quite different effects at
120 km and 60 km.
The ion-chemistry in the D-region is important too in that it is
the thing that determines the competition between increases and
decreases of the electron population. The murkiest part of that
topic is low in the D-region where electrons may become attached
to neutral molecules and thus removed from the absorption process.
That region is too high for study using high-altitude balloons,
too low for satellite work and passed through briefly on rocket
flights. In short, it is a tough place to dig out information.
So electron attachment at those altitudes is poorly known, only
that the negative ion of molecular oxygen is not the major player
it was thought to be some 30-40 years ago. Thus, instead of
visible radiation being a major factor in the detachment process,
it is now apparent (and my LF work clearly shows it) that UV is
important. I see that through the involvement of ozone in the
recovery of the D-region at sunrise.
But ozone is only one of the minor constituents down there and
those, say coming from pollution or whatever, are quite notorious
in their temporal and spatial variability. So the formation of
exotic negative ions down deep, where the collision frequency is
the highest, may be another source of variability in absorption,
even at a constant level of solar radiation.
These matters, and perhaps others, are responsible for what is
called "excess system loss", departures between what is observed
and the best calculations that were possible at the time. There
is a CCIR document on this topic; I passed my copy on to K9LA so
maybe he can add to this discussion. But in any case, that book
by Davies discusses an engineering calculation on 15.3 MHz (see
page 456-458). There, the excess system loss is 9 dB; I think you
can understand that if the problem were attempted at 1.8 MHz, the
number of dB between observation and theory might be much larger,
and with greater variability in day-to-day observations.
But back to the original question. If we say that solar radiation
is sufficient to open the 10 meter band, I have tried to show you
that there are some other factors which would affect the electron
density in the D-region where absorption takes place. So for a
"soft" solar flux that ionizes up in the F-region, the ionospheric
absorption may vary on 160 and not all the reasons can be readily
identified on a given day.
Of course, I have not touched on magnetic/auroral activity which
is related to the solar wind going by, not the instantaneous solar
radiation. That is another potential source of ionization down
deep, even at mid-latitudes. The best way to look at that is on
the NOAA websites which show the spatial distribution of electrons
coming down the field lines on the satellite tracks. Of course,
those aspects of the problem were not known 30-40 years ago when
the CCIR document was prepared so they did not make their way into
the engineering literature.
I think I have said all that comes to mind; if I missed anything,
perhaps K9LA can check the CCIR document and add more to the
discussion.
73,
Bob, NM7M
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