But propagation is non-linear through the ionosphere. The velocity
varies with the electron density. Otherwise the wave wouldn't bend.


This is not nonlinearity, it is refraction: velocity varies with location in
the medium.

Nonlinearity is if the velocity varies with the amplitude of the wave. Like
in acoustics where the positive (high-pressure) peaks propagate faster than
the negative peaks. It leads to waveform distortion and creation of
harmonics. The modern cardiology ultrasound scanners ( 5 years old) usually
default to this mode these days, transmitting ultrasound at about 3 MHz,
receiving at 6 MHz, as it gives better image quality than the fundamental
mode.


Sverre
www.qsl.net/la3za