Re: What determines if a wave is reflected or absorbed?

At VHF the actual absorption is very low, but there can be substantial 
refraction, even amounting to total reflection, depending on the angle.

Electromagnetic propagation in ionized media such as the ionosphere 
results in modification of the phase that is caused by changes in the path 
length traversed by the wave. As occurs at optical frequencies, deviation 
of the ray path results as the wave passes through media with different 
refractive indices. In the ionosphere, the refractive index n is found as 
a function of several parameters, the most important being the 'free' 
electron density. The function describing the refractive index within the 
ionosphere is known as the Appleton-Hartree equation, a complicated 
expression in its full form. 

Neglecting absorption and geomagnetic effects, a simple approximation is 
Eq 1, where n is the index of refraction, w is the angular freq, wn the 
plasma freq.

The angular plasma freq is given by Eq 2, where N is the electron density, 
e is the electron charge, m is the electron mass, and e0 is the 
permittivity of free space.

Effects of the medium include polarization rotation (Faraday rotation), 
refraction, and scintillations due to non-uniformity within the 
ionosphere. 

The structure of the ionosphere is complex and time variable, with several 
distinct layers.  Verious attempts to model it computationally have met 
with limited success.  The  IONCAP model, embedded in several commercial 
packages, currently represents the state of the art in ionospheric 
propagation forecasting.  These models require a recent history of solar 
flux measurements, the time, path coordinates, and frequency for the 
forecast.  One of the parameters produced is the MUF (Maximum Useable 
Frequency) for the path, that is the highest frequency that a wave tangent 
to the Earth surface will be reflected.  The MUF rarely exceeds 100 MHz, 
so the main gross at 127 MHz effects are refraction and rotation.