The F layer or region, also known as the Appleton–Barnett layer, extends from about to more than above the surface of Earth. It is the layer with the highest electron density, which implies signals penetrating this layer will escape into space. Electron production is dominated by extreme ultraviolet (UV, 10–100 nm) radiation ionizing atomic oxygen. The F layer consists of one layer (F) at night, but during the day, a secondary peak (labelled F) often forms in the electron density profile. Because the F layer remains by day and night, it is responsible for most skywave propagation of radio waves and long distance high frequency (HF, or shortwave) radio communications.