It's not quite as simple as that, because there are multiple ionospheric layers with different physical properties that affect RF propagation. An over-simplifi

It's not quite as simple as that, because there are multiple ionospheric layers with different physical properties that affect RF propagation. An over-simplified explanation is that the outer layers of the atmosphere persist continuously, and can be used for very long distance communication, but only reflect radio waves at relatively low frequencies (<10MHz). The lower layers that become ionized during the day absorb some of the radio waves, reducing overall efficiency and range, but also allow operation at higher frequencies which would be unusable at night. And you can get very efficient long-distance propagation along the terminator at dawn and dusk: https://www.qsl.net/w2vtm/grayline.html https://www.qsl.net/w2vtm/grayline.html