Let’s understand radio waves, and how they behave is different media including solid wire, free space and plasma.
We will start at the beginning. When your transmitter applies power to an antenna. you create an electric current in the copper. Current is the movement of free electrons which separate from the copper atoms. Your radio signal causes these electrons to wiggle.
The amount of wiggling is tiny from our perspective, but huge at the atomic level. As they wiggle, electrons accelerate and decelerate quickly. Changes in acceleration create electromagnetic waves which leave the antenna, or solid medium.
Electromagnetic waves, shown above top, are very special. They propagate themselves, potentially forever, by shifting energy between closely coupled electric and magnetic fields. Unlike other waves, EM does not need a medium to propagate.
So, you should understand radio waves as a force independent of any medium. Vacuum or free space are just fine.
But when this EM wave hits a plasma, like the ionosphere, it shifts back to moving free electrons for a short distance, before converting back to a wave.
Understand Radio Waves in Plasma
Solar rays heat atoms in the ionosphere, creating a plasma. Ionization from heat shifts neutral atoms into positive ions and negative free electrons, as show above.
Now, you will find the neat thing about plasma it its index of refraction, which is less than unity. Typically, EM waves keep traveling straight in a vacuum or free space, which have refraction of 1.0.
But refraction in ionospheric plasma is typically 0.8 to 0.95. This means waves will bend over and return to earth when they travel through the ionosphere.
Only waves at frequencies below the natural plasma frequency can propagate. You may have heard of the critical frequency, fc of an ionospheric layer. This is based on the height at which the layer is most dense with free electrons. You can use the formula fc=9√N, where N is the maximum electron density per cubic meter for that layer.
This is also the frequency at which a signal sent vertically will reflect straight down, refraction index near zero. Any higher and it keeps going straight up, refraction index near 1.