The useful model: The D region is the lowest daytime ionospheric region important to HF, roughly 60–90 km high, where frequent particle collisions convert radio energy into heat and cause absorption.
What controls the result
Solar X-rays and ultraviolet strengthen daytime ionization; after sunset it weakens quickly. Absorption is generally greater at lower HF frequencies and during solar flares.
The misleading shortcut
The D region is not mainly the useful “reflecting layer” for ordinary HF contacts, and darkness does not instantly remove every absorption effect on every path.
Make the better decision
For difficult daylight paths, inspect D-region absorption, try a higher usable band or a path with less sunlit distance, and revisit low bands after local or path sunset.
Evidence boundary
Propagation and antenna metrics are meaningful only with their frequency, path, geometry, reference plane, loss assumptions, and measurement method. Use current observations and the exact installed system.
