onosphere. This is called selective fading and it is often heard most distinctly when signals from the ground and sky waves are heard together. lonospheric disturbances At certain times ionospheric propagation can be disrupted and signals on the short wave bands can completely disappear. These result from disturbances on the sun called solar flares. These flares are more common at times of high sunspot activity, but they can occur at any time. When a flare occurs there is an increase in the amount of radiation that is emitted. The radiation reaches the earth in about eight minutes and causes what is termed a sudden ionospheric disturbance (SID). This is a fast increase in the level of absorption in the D layer lasting anywhere Figure 2.25 A flare appearing from the surface of the sun (courtesy NASA/Caltech/JPL) 44 Newnes Guide to Radio and Communications Technology from a few minutes to a few hours. This can affect all or part of the short wave spectrum, dependent upon the level of increase in radiation. The next stage of the process sees changes in the solar wind. Under normal conditions there is a flow of particles away from the sun. This is the solar wind, and the earth's magnetic field is able to give protection against this. However, after a flare there is a considerable increase in the solar wind. This normally occurs about 20 to 30 hours after the flare. When it arrives it starts a complicated chain of events. Large variations in the earth's magnetic field can be observed and a visible aurora may be seen in locations towards the poles. Generally it is necessary to be at latitude of greater than about 55 degrees to see this. Although the short wave bands may initially improve after the SID, the increase in solar wind causes a major degradation in communications over the HF portion of the spectrum. This mainly results from a drastic decrease in the level of ionization in the ionosphere including the D layer that absorbs signals. As a result radio signals are not reflected back to earth in the usual way, caus