Which layer of the atmosphere reflects radio waves?

The ionosphere is the layer of the atmosphere that reflects radio waves. This effect is caused by the ionization, or electrical charging, of the gas particles present in the upper atmosphere by X-rays and ultraviolet rays emitted by the sun.

The ionization of gas particles in the ionosphere aids in the transmission of radio waves by reflecting waves back to Earth, rather than allowing them to escape the atmosphere. Solar flares, the spontaneous eruption of high-energy radiation from the surface of the sun, can cause a sharp increase in the number of ionized particles in the upper atmosphere and can interfere with radio wave transmissions.