ved from the sun. At night when the layers are hidden from the sun, the level of ionization falls. Some layers disappear while others are greatly reduced in intensity. Other factors influence the level of ionization. One is the season of the year. In the same way that more heat is received from the sun in summer, SO too the amount of radiation received by the upper atmosphere is increased. Similarly the amount of radiation received in winter is less. The number of sunspots on the sun has a major affect on the ionosphere. These spots indicate areas of very high magnetic fields. It is 32 Newnes Guide to Radio and Communications Technology found that the number of spots varies very considerably. They have been monitored for over 200 years and it has been found that the number varies over a cycle of approximately 11 years. This figure is an average, and any particular cycle may vary in length from about nine to 13 years. At the peak of the cycle there may be as many as 200 spots while at the minimum the number may be in single figures, and on occasions none have been detected. Under no circumstances should the sun be viewed directly, or even through dark sunglasses. This is very dangerous and people have lost the sight of an eye trying it. Sunspots affect radio propagation because they emit vast amounts of radiation. In turn this increases the level of ionization in the ionosphere. Accordingly radio propagation varies in line with the sunspot cycle. Each of the bands or layers in the ionosphere acts in a slightly different way, affecting different frequencies. The lowest layer is the D layer at a height of around 75 km. Instead of reflecting signals, this layer tends to absorb any signals that it affects. The reason for this is that the air density is very much greater at its altitude and power is absorbed when the electrons are excited. However, this layer only affects signals up to about two megahertz or SO. It is for this reason that only local ground wave signals are heard on the medium wave broadcast band during