Radio waves are reflected back to earth from the

Radio waves are reflected back to Earth by the ionosphere, a layer of the atmosphere containing electrically conducting layers of plasma [t2]. This layer extends from approximately 50 km to 400 km (and up to 950 km) above the Earth's surface [t2]. The ionosphere is formed by the ionization of atmospheric gases by solar ultraviolet and X-ray radiation [t2][t6]. This ionization allows the layer to refract and reflect high-frequency (HF) or short-wave radio signals back to the surface, a process known as sky-wave propagation [t4][t7]. While the troposphere is the lowest layer where weather occurs and the stratosphere contains the ozone layer, neither possesses the free electron density required for long-range radio reflection [t9][t10]. Consequently, the ionosphere is essential for long-distance radio communication, especially in the days prior to satellite technology [t2][t10].

Sources

1. [1] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 14: Climate > The Atmosphere > p. 130
2. [2] https://svs.gsfc.nasa.gov/5240