Radio wave refraction depends on density of free electrons and frequency. Skip distance is tied to refractive index and height.
You probably know that solar rays, mainly UV, create layers of plasma we call an ionosphere. Your main layers are D, E and F. D layer is under 100 kilometers, which F stretches well up above 250 km. You probably also know that most signal refraction takes place in E and especially F layers, while D is mainly a sponge absorbing radio frequencies.
Radio wave refraction depends mainly on the density of free electrons in a layer. This density per cubic meter varies from 106 at D layer to 1012 at F. Because of these different densities and chemical composition of the various layers, you will find a wide range of critical or plasma frequencies in the ionosphere.
Radio waves will only refract (bend) below fc which ranges from 5 – 10 MHz in the F layer, 2-4 MHz at E, and amazingly only 0.3 MHz for the D layer. (Yes, the D layer can reflect VLF/LF signals, but for MF/HF it’s the E and F layers doing the work.)
Each layer has a peak electron density at various heights, shown left above. You can also see, in the middle diagram, that the index of refraction dips below unity as the electron density increases.
As shown in the formula, your frequency also effects the refractive index.
Radio Wave Refraction – Devil in the Details
For long distance communications, you want a refractive index somewhere around 0.8 to 0.95 to bend your signal back to earth. Total electron content in the ionosphere is outside your control.
But you can find the bending sweet spots by adjusting your frequency. You raise or lower your frequency to find the best refractive index. This “best index” frequency is what we call the MUF, or maximum useable frequency.
Similarly, you can also try to adjust your antenna takeoff angle to get the best skip distance, which is typically lower angles. But again, you have a sweet spot tradeoff with the height of the electron density profile.
(Credit for graphic: Figure 1.7 in High Frequency Radio Communications with Emphasis on Polar Problems, The Advisory Group for Aerospace Research and Development NATO, 1967)