Tom,
In magneto-ionic theory, applicable on 160 meters where the
frequency (1.8 MHz) is close to the electron gyro-frequency
(0.9-1.6 MHz) in the earth's field, RF is propagated as two
elliptically polarized waves. The nature of the polarizations at
any point in the ionosphere depends on the local field strength,
the electron density and the direction of propagation with respect
to the field. A discussion of this will be found in the books by
Ratcliffe, published in '59, and by Ken Davies, first published in
'65 and then again in '89.
As a result, RF is propagated as ordinary and extraordinary waves,
the electric field vectors rotating relative to the field. The
sense of rotation is anti-clockwise for the ordinary wave when
viewed along the field direction and just the opposite for the
extraordinary wave. The rotation of the electric field vectors is
due to the presence of the ionospheric electrons gyrating around
the field lines and extraordinary waves will ALWAYS rotate in the
same sense as the electrons gyrate around the field, no matter
whether the view is along the field line or opposite. As a
result, Faraday Rotation of HF or VHF signals from satellites with
linear antennas is in different directions in the two hemispheres
when there is a change in electron content along the line of sight
to the satellite.
As long as the ionosphere changes in a smooth, slowly-varying
fashion, the elliptically polarizated waves vary in a continuous
fashion while propagated, going from circular polarization when
the RF goes along a field line, to linearly polarized when it goes
perpendicular to the field and elliptically polarized between
those two extremes. But there are limiting polarizations, at the
points of entrance and exit of RF from the ionosphere. Thus, the
coupling of RF into the ionosphere depends on the polarization of
the wave coming up from the ground and how it compares with the
polarization of magneto-ionic waves at the base of the ionosphere.
This point was made in a recent posting by VE7FPT and has been
treated in some detail by BBC engineers in '65.
The limiting polarization can be quite different at the two
ends of a path. Good examplse would be the recent DXpeditions at
low latitudes (S21XX, P29VXX and XZ1N). Great-circle paths from
those low-latitude sites to the East Coast are toward the north
and with a low magnetic dip, the entrance polarization is close to
being circular. On the other hand, the dip angle on the East
Coast is rather large, around 60 degrees, so the elliptical
polarization of signals leaving the ionosphere is quite different
than on entrance, close to being linear, i.e., long, skinny
ellipses at exit as compared to fat, almost circular ellipses on
entrance.
On 160 meters, the extraordinary wave is heavily absorbed so
only the ordinary wave survives at great distances, like to the
USA in the above examples. Thus, there is no possibility of
Faraday Rotation on 160 meters as that phenomena depends on the
presence of BOTH polarizations and with essentially equal
amplitudes.
In spite of that, there is the possibility of polarization
shifting around the exit point if there is any sharp gradient in
ionization. That point is raised only for completeness, except
that it is known from Sporadic E work that wind shears can produce
gradients in ionization at low altitudes. That would represent a
meteorological influence on Top Band propagation, as I have argued
for from a different direction, i.e, acoustic/gravity waves in the
eutral atmosphere affecting the E-region at night.
All of the above is probably more than you wanted to read but it
is a reasonable condensation of what you'd obtain by going through
Davies' writings. It is tough sledding but worth the trouble.
73,
Bob, NM7M
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