AM radio signals can often be received from extremely distant transmitters at night due to changes that occur in the earth’s ionosphere after dark.
The ionosphere is an atmospheric layer of electrically-charged atoms, molecules and free electrons that mainly arise due the ionizing effect of the sun’s ultraviolet radiation. The ionosphere extends closer to the earth’s surface during daylight, and retreats at night when the ionizing UV light from the sun fades.
Lower energy radio waves, such as AM signals, are less able to penetrate the ionosphere, and during the daytime, the denser, low levels of the ionosphere absorb much of the medium wave AM signals. At night, as those lower ionosphere levels disappear, AM radio waves can reach the upper layers of the ionosphere where rather than being absorbed, they tend to get reflected back towards the ground.
This process, called skywave propagation, results in AM radio waves being bounced back to earth over a much wider geographical area at night.