Charge density is electrons per cubic centimeter? Other than that I understood all you stated above
What actually matters is Total Electron Content - TEC. That's the total number of free electrons you encounter as you penetrate the plasma. In this case, the plasma is the earth's ionosphere.
There are actually multiple ionospheric layers created by multiple mechanisms, and some are more relevant than others at a given frequency. For example, the D layer is formed by solar UV at relatively low altitudes, 60-90 km. The recombination rate is high (electrons and ions easily find each other) so it disappears at night. It is significant mainly below 5 MHz or so and it absorbs more than it reflects, and this is why distant AM broadcast stations (0.54 - 1.7 MHz) disappear during the daytime. The 80 meter (3.5-4.0 MHz) and 160 meter (1.8-2.0 MHz) ham bands are also usually closed during the daytime, with local propagation only.
The F layer(s) at 150-500 km are the most important layers for long-distance "skywave" propagation. The air at those altitudes is so thin (the ISS flies in this region) that when ion/electron pairs form, they last a long time before recombining. They persist through the night, reflecting low frequency signals but letting high frequency signals escape to space.
Whether a signal reflects or goes through depends on both the critical frequency and the incidence angle. A signal might be reflected if it hits the ionosphere at a shallow angle but penetrate at a high angle. You can observe this phenomenon optically by going to the bottom of a swimming pool and looking up at the surface. At high angles you can see right through the surface, but at shallow angles (below the Brewster angle) the surface looks like a mirror.
This sets a Maximum Usable Frequency (MUF) for communication with some particular station. This explains the "skip" effect: you can only hear stations beyond a given distance because nearer signals hit the ionosphere at a high angle and go right through into space instead of being reflected. This is a very common (routine) phenomenon on the ham bands from 20 through 10 meters; very often you can only hear one side of a conversation. 20 meters is usually open to someplace on earth 24 hours/day, making it the most popular band for "DX" (working distant stations). The higher bands are often closed at night, with local propagation only, because the sun doesn't maintain sufficient ionization density in the F layers. Hams get very adept at picking frequency bands, usually a high frequency (short wavelength) during the daytime and a low one (long wavelength) at night.
Luke is quite right that at extremely high frequencies (above visible light) the atmosphere attenuates signals before they even get to the ionosphere. Most UV is absorbed by ozone and/or oxygen, and X- and gamma rays are absorbed entirely. It seems counter-intuitive that radiation famous for penetrating solid objects would be stopped by mere air, but it's true. Radio interacts only with free electrons, but ionizing radiation (anything shorter than mid-UV) will knock any electrons it finds loose even if they're bound to atoms. That takes energy from the radiation.
This is why lead is good shielding; it's not the mass per se but the high electron density that goes with it. Air has a sea level density of about a kilogram per cubic meter, and that can really add up. This is how nuclear weapons produce fireballs near the surface; most of their energy comes out as soft X-rays that are absorbed by the surrounding atmosphere, heating it to incandescence.
