The ionosphere is generally recognized to have three, sometimes four layers. The D layer is the innermost layer, and mostly absorbs radio waves. The E layer is the middle layer. The F layer combines into one layer at night, but in sunlight divides into two layers, the F1 and F2. The F layers are responsible for most skywave propagation of radio, and are thickest and most reflective of radio on the side of the Earth facing the sun.
Scientists explore the structure of the ionosphere by bouncing radio waves of different frequencies from it, and using special receivers to detect how the reflected waves have changed from the transmitted waves. They also examine the changes to radio waves from satellites and stars transmitted through the ionosphere.