There’s much talk in the amateur community of the fact that parts of the ionosphere reflect (or is it, refract, or even scatter) radio waves. Often the assumption is quickly made that it’s ‘ions’ that do it, without any great thought about how those ‘ions’ manifest. And the discussion moves off to talk about what’s happening and who’s on the air and from where.
But what is ‘it’? What is up there and how does it reflect, refract, or scatter those waves? Are ions the big thing, or is there something more subtle going on? And why does the ionosphere behave as it does, sometimes supporting propagation for days or while only supporting an opening for minutes?
If we can understand what it is, we can then move to how it might come about. And if we know the mechanisms of how it comes about, we might be able to better forecast when to expect that ‘opening’.
The ionosphere is an extension of the lower atmosphere, the troposphere, the region closes to Earth. It’s just a lot less dense. Like the lower atmosphere, it comprises molecules of hydrogen, oxygen, nitrogen, and some other rarer gasses.
The Sun is all important. The Sun beats down on the upper atmosphere, from the D Region at about 70km to the upper regions of F1 and F2 at between 200km and 400km up. The Sun’s rays beat on the ionosphere and heat the gas molecules.
When heat is absorbed in this way, it causes electrons to be split off from their molecules. This happens most in the upper regions, the F1 and F2 layers. The highest, the F2 Region, is the highest and the most stable with the highest density of electrons (since it’s nearest to the Sun). The splitting happens more sporadically in the E Region. The diagram below shows what happens.
In this diagram, the non-ionised gas is shown as an atom. In practice it will exist as a molecule – for example, oxygen is stable existing as two atoms in a bond to give O2.
The heat from the Sun causes an electron to be spun off. That electron has a negative charge. Those free electrons can’t exist in isolation – similar charges repel, and so any cloud of electrons that might form, will instantly disperse. The same is true for the now-positive ions.
And of course, electrons and ions can get back together again to form a neutral atom.
Left to its own devices, a cooked gas cloud would never form something capable of supporting propagation.
The cloud comprising negative electrons and positive ions can, however, become stable or quasi-stable under two conditions – first, that neutral atoms/molecules are interspersed (as shown above) thereby weakening the repelling and re-combining forces, and second, that the Earth’s magnetic field, and the various tides and winds of the thermosphere causes particles to group. To a lesser extent, interplanetary geomagnetic forces, such as geomagnetic interaction between the Earth and the Sun, also have an effect.
Some call this quasi-stable amalgam of molecules, ions, and electrons a ‘soup’. More technically it’s a plasma.
The plasma is created and exists for a time if the conditions are right. So, to understand how this plasma supports propagation, we need to understand how it comes about, how the radio wave interacts with it, and hence how it supports reflection (or is it refraction, or even scattering)? More on that in another blog.