[Lowfer] NC Hi-Medfer

[Dexter McIntyre W4DEX]

Larry Putman WB3ANQ wrote:

> Dex,
>
> What caused that Bulge??
>

John Davis described the phenomena in a earlier message pasted below:


WE�H wrote:
>I would think that an ionized 'cloud' in the atmosphere could move so fast.
>Look at e-skip and how fast it can be there and not as the ionized 'cloud'
>moves about which changes the point where your signal drops back to earth.
>....
>If I have the explanation correct, an ionized cloud is not a cloud, as we
>would look at it but groups of ionized particles, which we refer to as a
>cloud. Am I right on this explanation?


John Davis replied:
'Cloud' is actually a very good term for regions of e-layer ionization.
They are roughly the same order of magnitude in size as water vapor or ice
crystal clouds, and are pushed across the sky in the same way by the winds
at those altitudes.

Ionization clouds do not account well for phenomena observed at mid-HF and
MF, though.

Think instead of a swimming pool at night.  If the pool lights shine enough
in an upward direction, waves on the surface reflect complex shifting
patterns of brightness on the floor of the pool.  If we recognize that in
lieu of a thin air/water interface being rippled, we're actually dealing
with undulations of density within the tens or scores of miles thickness of
the ionosphere, then the analogy is a pretty good one.  We illuminate the
underside of this refracting "surface" with a broad front of radio waves,
just as the pool lights shine up at the water's surface.  As the reflector
tilts this way and that at different places, the light and the radio waves
will arrive at any given point on the bottom from a number of slightly
different directions.

Let's say that an undulation in effective height of the reflector is
propagating through the medium, and a portion of that undulation's virtual
"surface" is at the proper angle to reflect electromagnetic waves from the
source toward the receiver, at the same time a more-or less stationary
region is also reflecting the transmitted signal toward the receiver.  The
apparent motion of the reflective surface will represent a Doppler shifted
copy of the original signal.  If repeated undulations of the same magnitude
continue to propagate in the same direction, the result is a persistent
frequency-shifted copy (with fuzz and minor variations as the individual
undulations pass through the region where the angle is only approximately
correct for the "moving" reflection to take place).

Note that it is *not* necessary for the refractive medium itself to travel
for this to occur! --just as water molecules are not transported anywhere,
but are moved in relatively small looping oscillations by the waves on the
pool's surface.

If the Doppler shifts depended on lateral motion of regions of greater
density in the ionosphere (clouds or quasi-clouds), at the speeds computed
for the observed shifts, such clouds would move well out of range in a
matter of minutes.  But we have seen Dopplerized traces that go on rather
consistently for long periods of time, even hours.  Hence, ionized clouds
are not as good an explanation for the observed phenomena as oscillations in
the virtual height of the reflective layer.

John D