Deacon Dave wrote:
"For an EM wave sharing a common boundary in different media, I offer
that the wave "rolls" in the direction of propagation but does not
create harmonics from some non-linear process."

That sounds right to me.

All those medium wave broadcast signals propagate along the earth`s
surface and the differences of peopagation velocities in earth and air
tend to give the waves a tilt toward the earth as they travel.. It`s as
if the wave were "dragging its feet" as it travels.

Many waves share common paths, but I don`t recall hearing that signals
sharing those paths got mixed by the experience. This is a fair
indication of linearity of the earth/air boundary.

There has been speculation about the "Luxemburg effect". Radio Luxemburg
hit the air with extreme power long ago. Many listeners, to other
signals thought to share the same ionospheric volume with Radio
Luxemburg, reported hearing Radio Luxemburg`s modulation on their
received signals. It was assumed a non-linear effect in the ionosphere
caused the mixing and could be triggered somehow by transmitted power
were it high enough.

The Luxemburg effect interested propaganda broadcasters and their
jammers among others. As far as I know, none has been successful in
exploiting the Luxemburg effect. Sometimes we get lucky.

Best regards, Richard Harrison, KB5WZI