in the solar far ultraviolet and soft X-ray radiation. Solar flares are most common at times of high sunspot number. The excess radiation leads to sudden ionospheric disturbances (SIDs), which are rapid F1-region, maximum electron density results in the F2- region, and large increases in ionospheric absorption occurring simulta- where the loss rates are lower. The maximum electron density of the neously over the whole sunlit hemisphere. These persist for from a F1-region closely follows that of the E-region, but there are signifi- few minutes to several hours giving the phenomena of short-wave cant and less predictable changes in its height. The maximum fadeouts (SWFs), first explained by Dellinger Accompanied by electron density and height of the F2-region are subject to large solar flares are eruptions from the Sun of energetic protons and changes which have important consequences to radio wave propa- electrons. These travel as a column of plasma, and depending on the gation. Some of these changes are systematic but there are also position of the flare on the Sun's disc and on the trajectory of the major day-to-day variations. The F2-region is controlled mainly by Earth, they sometimes impinge on the ionosphere. Then, the proton- ionisation transport to different heights along the lines of force of S, which are delayed in transit from fifteen minutes to several hours, the Earth's magnetic field under the influence of thermospheric produce a major enhancement of the D-region ionisation in polar winds at high and middle latitudes (Rishbeth, 1972) and by electric regions that can persist for several days. This gives the phenomenon fields at low latitudes (Duncan, 1960). These effects, taken in of polar cap absorption (PCA) with complete suppression of h.f. conjunction with the known variations in atmospheric composition, signals over the whole of both polar regions (Bailey, 1964). Slower can largely explain characteristics of the F2-region which have in particles, with transit times of 20-40 hours, produce ionosph