What you are about to read should be taken more along the lines of semi-informed interpretation. The truth of the matter is no one knows for a fact everything about propagation and all the factors in creating it. These are all things that I've learned and hopefully interpreted correctly. I may be wrong and if I am please let me know. I do not claim to be an expert in these things, but like most people, I want to learn.
The Chemistry of Propagation
Ahhhh chemistry, words that can either excite or depress a person. If you are like me and think "oooo fake volcano time" then it can be a good thing. Or, likewise, you can be like me and have nightmares many years later of college chemistry classes talking about things you just can't grasp, it can be a bad thing. In any event, whether you like it or not, it exists. And even more oddly, if it wasn't for chemistry .... we wouldn't have any sky-wave propagation.
Let me back-track a bit and explain. Several months ago I had prepared (and subsequently gave) a presentation for our local club about the sun, the earth, and propagation. Fairly standard stuff, the different ionospheric layers, sunspots, coronal mass ejections, flares, etc. While I was researching some of these topics I did a rather large amount of reading of the subject and discovered a few interesting things that I shall now share with you.
The IonosphereEver since we were all young hamlings, we were ingrained with the following picture. (The one on the left)
Well, as I researched the answer became clear, Chemistry.
Or more appropriately the air that we breathe. After all what is air than our atmosphere, and the ionosphere is part of our atmosphere. Back in our younger days, we learned that our air is composed of many things, nitrogen, oxygen, argon, carbon dioxide, neon, helium, methane, krypton, hydrogen, nitrous oxide, carbon monoxide, xenon, ozone, etc etc etc... Then we tend to forget about it. Then we start assuming that our air is some mostly consistent homogenized mass of this mixture. We rarely ever stop to consider what the mix of compounds are at different points in the atmosphere.
But the truth of the matter is, as the altitude rises, things change. Sure we all understand that the pressure drops, temperature drops, and molecules of the different things become farther and father apart. What we need to also understand is that the mix changes as well. A sample of the atmosphere in the "D-Layer" will contain vastly different compounds than what you would find in the "F-Layer".
So stop and think about that for a minute, these layers really are layers. Layers of a specific "mix" of elements, atoms, and molecules. And each layer reacts to different things. Specifically different types of solar radiation.
After all, most people are familiar with the "Ozone Layer", where the concentration of ozone absorbs "soft" UV radiation and prevents us all from getting more skin cancer than we already do. These other layers absorb different types of radiation.
The D LayerAt the very bottom of the pile we consider as the ionosphere we find the D-Layer. The D-Layer is known for having a larger concentration of nitric oxide (NO) and serves to absorb a very specific type of radiation known as "Lyman Series-alpha hydrogen radiation", which isn't important really. The point to be remembered is this.
1. As the radiation is absorbed, free electrons pop out of the NO. This is good!
2. However they don't stay free for long. This is bad.
3. They very frequently collide with each other. This is worse.
Because the electrons don't stay free very long, there isn't much net ionization going on for signals to reflect off of. In fact because the ones that are free are colliding with each other, they tend to attenuate waves traveling through them.
The E LayerAnything happening in the normal E-Layer is usually due to the ionization of regular Oxygen (O2) from soft X-Rays and Far UV radiation, which tends to be slightly reflective under 10 MHz and slightly absorptive over 10 MHz.
Es events are a different matter. And while many have conjectured various theories (many having to due with Magnesium), this is still mostly a mystery.
The F LayerThe Ham's happy layer. It is here that things exist a bit differently. It contains a mix of molecular and atomic ions (O2+, O+, and NO+) and creates free electrons when absorbing extreme UV radiation. These electrons tend to float free for quite awhile and don't collide with each other all that often. This allows them reflect signals back down for us all to enjoy.
So how does it all come together?Well truth be told, physics steps in here. I know the post was about chemistry, but chemistry provides the resources for what we behold as the ionosphere, while physics provides the mechanism. What we all really want is a bunch of free electrons buzzing around as actively as possible without colliding with each other. The more active these electrons are, the more reflective they can be. These electrons act like tiny dipoles, and if you put enough of them together they can reflect signals back down to the ground.
So when the sun gets active, the more active each layer gets. The D Layer starts absorbing higher and higher frequencies while the F layer starts reflecting higher and higher frequencies.
But that's about all for now. 73