... Because of the Sun 's UV radiation, Earth 's upper atmosphere is partly (0.1% or less) ionized plasma at altitudes of 70-1500 km. ... Around the plasma density maximum (F-layer, see below) a so-called ionospheric waveguide is formed for magnetosonic waves . ... It is interesting to note that this works also the other way round and, for example, the auroral kilometric radiation created by the precipitating particles high above the ionosphere does not reach the ground because of the ionospheric E...