 the day. The D layer has a relatively low electron density and levels of ionization fall relatively quickly. As a result it is only present when Figure 2.14 Sunspots on the surface of the sun (courtesy NASA) Radio waves and propagation 33 F2 layer D layer F1 layer E layer The earth D layer disappears at night E layer becomes very much weaker at night F layer Figure 2.15 Variation of the ionized layers during the day radiation is being received from the sun. This means that it is much weaker in the evening and not present at night. When this happens it means that low frequency signals can be reflected by higher layers. This is why signals from much further afield can be heard on the medium wave band at night. Above the D layer the E layer is found. At a height of around 110 km, this layer has a higher level of ionization than the D layer. It reflects or more correctly refracts the signals that reach it, rather than absorbing them. However, there is a degree of attenuation with any signal reflected by the ionosphere. The atmosphere is still relatively dense at the altitude of the E layer. This means that the ions recombine quite quickly and levels of ionization sufficient to reflect radio waves are only present during the hours of daylight. After sunset the number of free ions falls relatively quickly to a level where they usually have little effect on radio waves. The F layer is found at heights between 200 and 400 km. Like the E layer it tends to reflect signals that reach it. It has the highest level of ionization, being the most exposed to the sun's radiation. During the 34 Newnes Guide to Radio and Communications Technology course of the day the level of ionization changes quite significantly. This results in it often splitting into two distinct layers during the day. The lower one called the F1 layer is found at a height of around 200 km, then at a height of between 300 and 400 km there is the F2 layer. At night when the F layer becomes a single layer its height is around 250 km. The levels of ionization fal