Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours?

The D region is the lowest layer of the ionosphere, forming only during daylight hours when solar radiation is present. Because it is located lower in the atmosphere (around 30 to 60 miles above the Earth), it has a much higher density of neutral gas molecules compared to the higher ionospheric layers. When lower-frequency RF signals, particularly those below 10 MHz (like the 160m, 80m, and 40m bands), enter the D region, they cause free electrons to vibrate. Due to the high gas density, these vibrating electrons frequently collide with neutral molecules, dissipating the radio wave's energy as heat. This collision process heavily absorbs the signals, effectively blocking daytime skywave propagation on these lower bands. Conversely, the E, F1, and F2 regions are much higher up where the atmosphere is significantly thinner. In these upper layers, electron collisions are rare, allowing them to efficiently refract (bend) radio signals back to Earth for long-distance communication rather than absorbing them.