At what altitude does the ionosphere begin?

The ionosphere begins at an altitude of approximately 25 miles (about 40 kilometers) above the Earth's surface. This layer is a region of the atmosphere that is ionized by solar and cosmic radiation, which means it is filled with charged particles (ions and free electrons). The ionosphere plays a crucial role in radio wave propagation and communication because it reflects certain radio frequencies back to Earth, enabling long-distance communication.

At lower altitudes, such as 10 miles, the atmosphere is denser and contains more oxygen, which does not contribute to the ionization effect that characterizes the ionosphere. As altitude increases and reaches approximately 25 miles, the conditions become suitable for the formation of the ionosphere.

Above this threshold, at 50 miles and beyond, the region continues to contain ionized particles, but the defining start point of the ionosphere is recognized at around 25 miles, making this option the accurate choice.