onized layer, the D layer, occurs at 70 km. Above the D layer, temperature and incident radiation increase and the E, F1, and F2 layers are formed between 120 km and 450 km. The ionosphere, which is the region in which the ionized layers are formed, spans the region from 50 to 600 km. Ionized trails from meteors occur at around 100 km, and the lowest satellite orbit is at about 150 km. F2 Layer F1 Layer lonosphere E Layer Meteor Scatter Region D Layer Stratosphere Aircraft Mount Everest Cloud Formations Tropospheric Troposphere Scatter Region 10 X 1011 Free Electron Density (m³ Fig. 8.3 Earth's atmosphere: major features and ionization. Chapter 8 Radio Wave Propagation The primary influence on the atmosphere is radiation from the Sun, which causes the ionized layers to form and creates global climatic and regional weather patterns. Solar radia- tion follows an 11-year sunspot cycle, which has a direct correlation with ionization and is recorded on a scale from 0 to about 200. This is a smoothed average of the number of sun- spots, which are observed disturbances on the Sun's surface. A sunspot number is issued by the Sunspot Data Center in Brussels and by the Telecommunications Services Center in Boulder, Colorado, and is a smoothed 12-month average of solar activity indicating the degree of ionization that can be expected. High sunspot numbers are an indication of better conditions for long-range HF communication, and this might also lead to unwanted long- distance propagation in the VHF bands, creating interference in mobile communications and FM and TV broadcasting services. Because the sunspot number is a running average, it might not be an accurate indication of daily or hourly conditions. Solar flux is a measure of solar activity taken at 2.8 GHz and is a better indication of real-time ionospheric conditions. It is measured continuously and quoted on a scale usually in the range from 60 to 260. The U.S. National Bureau of Standards radio station WWV transmits ionospheric data information hourly. The signal-