The ionosphere is the name given to the region of the thermosphere that has a particularly useful effect on HF and lower-VHF radio waves.
As I discuss elsewhere, the Sun causes the gases of the ionosphere to ionise. The degree of ionisation and the resulting electric charge density differ with height. Molecules higher up and closer to the Sun get cooked more, and hence produce more ions.
Since it’s all about charge density, it will come as no surprise also that the influence on the radio waves also changes with frequency. Conceptually, short wavelengths (in the UHF bands and above) don’t interact with the electrons and ions of the plasma or soup. They fly straight through ‘without touching the sides’. Longer wavelengths (from around 1MHz to 200MHz) suffer interaction because, relatively, there are more charges to interact with.
The ionosphere is not homogenous. As a result of the mix of molecules, ions and electrons in the soup or plasma, and the geomagnetic forces, winds and tides in the thermosphere, the charges cluster in clouds or regions of differing charge densities. There are two main regions (E and F) that split into four (D, E, F1 and F2) on the sunny side of the Earth. The regions span heights from about 80km to about 400km.
The diagram below shows the differing charge densities and the regions. The D Region is omitted.
Generally, since the Sun is absent, recombination occurs and charge densities reduce at night.
Importantly, the E Region is thin, from around 100km to about 130km. Its charge density changes hugely between night and day. During the day, it comes close to the charge density of the F1 Region. Note the logarithmic scale – at night the charge density of the E Region is about a hundredth of that of day.
Those regions are shown below in the often illustrated and stylised image of the ionosphere. The creation of the four regions from two is evident. I’ve called them regions (in line with academics, rather than other commentators) because that’s what they are. They vary dynamically in thickness and density and hence it would be wrong to describe them as layers.
The heights and characteristics of the four layers are key in understanding how they perform and hence the nature of the propagation supported. The key player in determining the characteristics of each region is the Sun. The regions of plasma are held in place by two primary forces – the Earth’s geomagnetic field and the various winds and tides of the thermosphere and other regions of the upper atmosphere.
As I note elsewhere, the time and location varies for which a useful area or patch in each region. Understanding how the time and location changes is key to exploiting the regions.
As I note above, the image is stylised. It’s not to scale. It’s more useful if we were to draw the regions and the Earth to scale. That way we can understand the all-critical launch of the radio wave from the antenna and its receipt at the receiving station.
The following shows this scenario roughly to scale.
One must remember that these are regions. They change dynamically. And they surround the Earth in the fashion suggested in the stylised image with four regions in the day reducing to two at night.
The important point here is that a wave launched at a sensible angle of say 10 degrees from the Earth will be returned to Earth from a low height by the E region. The maximum path length from Station A to some other point on the Earth’s surface is, as I argue elsewhere, about 1,200km. This is easily seen by ray tracing on the above image.
Conversely, ray tracing from Station A via the F2 layer gives a maximum path length of more like 4,000km.
So there’s the first basic set of laws of HF and lower VHF propagation.
 Derived from Goodman, JM (1992) HF Communications: Science and Technology, p96, New York, Van Nostrand Reinhold.