ing a radio blackout. During some stages of the aurora very high levels of ionization are seen towards the poles. As a result signals may be reflected back to earth in these regions at frequencies up to about 0MHz, although HF communications will be absorbed. When signals are reflected in this way they generally have a distinctive buzz superimposed upon them. This results from the constantly changing nature of the ionosphere under these conditions. The blackout in HF radio communications may last anywhere from a few hours to a few days after which the bands slowly recover. The first signs of the end of the blackout are normally seen at the low end of the spectrum first. It is also found that further disruption may occur after 28 days, the period of the sun's rotation. Very low frequency propagation Propagation of long radio waves is of importance for some long-distance communications and also for some long-distance navigation. In recent years considerable progress has been made in the understanding of the way in which the earth and the ionosphere act as a waveguide at these frequencies. However, for a more simplified approach the way in which propaga- tion occurs can be considered in a number of ways. For short distances the signal is received mainly as a result of ground wave propagation and it is found that the intensity is virtually inversely proportional to the distance between the transmitter and the receiver. However, beyond a certain point the signal falls at a greater rate because of the earth's Radio waves and propagation 45 curvature and losses in the ground. At large distances the received signal is chiefly due to reflected signals from the ionosphere. As might be expected at intermediate distances the received signal results from a combination of both modes and this results in an interference pattern. At very low frequencies the D layer reflects rather than absorbs signals. VHF and above At frequencies above the limit of ionospheric propagation but below about 3000 MHz communication can be established