Which region of the ionosphere is ionized at all hours of day and night?

The F layer of the ionosphere is the region that remains ionized at all hours of the day and night. This layer is situated at higher altitudes, generally ranging from about 150 km to 1,000 km. During daytime, it receives significant ionization from solar radiation, leading to a higher density of electrons. At night, even though solar radiation diminishes, the F layer continues to retain a certain level of ionization due to its altitude and the presence of residual particles.

The continuous ionization of the F layer makes it a critical component for long-distance radio communication, as it can reflect radio waves back to Earth, allowing them to travel further than would be possible without this layer. This consistent ionization contrasts with the other layers, such as the E and D layers, which experience variations in ionization based on the time of day, primarily due to their lower altitudes and susceptibility to diminishing solar radiation at night.