. The diurnal variation is shown in Chapter 8 Radio Wave Propagation E Layer D Layer Earth Night Fig. 8.7 Diurnal variation in the Earth's ionized layers. Figure 8.7, and it will be seen that at night, when incident radiation is at a minimum as the ionosphere is in the Earth's shadow, the ionosphere comprises two comparatively thin layers at 110 km and 210 km, called the E and F layers, respectively. During daylight, these two layers increase in thickness and intensity, and the F layer divides into the separate F1 and F2 layers at 210 km and 300 km. In addition, the D layer forms at 80 km during daylight but disappears at night. Similar variations occur with seasons, SO there is a greater level of ionization in northern latitudes during July (summer) than in January (winter), but the reverse applies in the southern hemisphere. The D layer actually spans an altitude from 50 to 90 km and absorbs frequencies in the MF and lower HF bands. Higher frequencies pass through the D layer suffering some attenu- ation and can be reflected back to Earth by the E layer at distances up to 2000 km. Still higher frequencies within the HF band will pass through the E layer and can be reflected by one of the F layers, providing very-long-distance communications. Frequencies in the VHF bands and above (i.e., above 30 MHz) generally pass through all the layers and into space, except in unusual conditions of strong ionospheric activity. Although a radio wave can be visualized as being reflected by the ionosphere, the process is in fact one of refraction, and the angle of refraction is proportional to both the angle of incidence and the frequency. When the wavefront enters an ionized layer, it excites free electrons into oscillation, which reradiates electromagnetic energy, and this reradiation modifies the direction of the wavefront, tending to bend it back to Earth. For a given fre- quency, this tendency increases as the angle of incidence is reduced, SO at the critical angle the wave will be refracted back to Earth, but at greater a