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(the ionosphere IIRC)

You recall correctly! You would not believe how exciting it was to have a frequency go crashing down, the orderwires chattering away with noise, then to quickly guesstimate the new frequency that would propagate to get the station back on the air! Thirty years ago and I still miss it.

The E layer can be found between 90km and 120km above earth. Reflection of RF waves by this layer is responsible for most of the long distance propagation valued by hams. The E layer both thins out at night, and rises, again resulting in increased propagation distances. E layer propagation is most useful for frequencies below 10MHz.
Sporadic E-Skip is caused by "clouds" of especially intense ionisation. These are highly reflective and allow DX communication from 25MHz to 225MHZ. The cluods are usually relatively short lived, and occur mostly during summer months.
Th F-layer (between 120km and 400km above earth) is contains the highest densities of ionised particles, is the most reflective of RF energy, and hence is responsible for most DX propagation of HF communications. During the day, energy from the sun splits the F layer into two distinct parts known as F1 and F2.
The F region is the very thickest region of the ionosphere, which makes the F region special. It is the only layer of the ionosphere that is subdivided into two parts, the F1 layer and the F2 layer. During the day the F region ionizes at different rates, due to the thickness.
The F2 layer is on top of the F1 layer (making it situated closer to the sun). As a result of being closer to the sun, it comes into contact with more of the UV and x ray energy. Because of this, the F2 layer becomes more ionized than the F1 layer. When night settles in, the F1 layer quickly loses its energy while the F2 layer loses its energy much more slowly--usually at the lowest point right before sun-up. It is the F2 layer of the ionosphere that provides the capacity of the ionosphere to reflect radio energy (this is why the F2 layer is more important to hams).

Alas, I was never fortunate enough to be assigned to a FTPS Troposcatter site. They bounced massive power off the much lower troposphere, some amped up through huge walk-in Klystrons.