eric the past been regarded as anomalous by comparison with the E-re- storms. These storms, which result principally from movements in gion, namely, diurnal changes in the maximum of electron density ionisation, take the form of depressions in the maximum electron in polar regions in the seasons of complete darkness, maximum density of the F2-region (Matsushita, 1967). They can last for electron densities at some middle-latitude locations at times dis- several days at a time with effects which are progressively different placed a few hours from local noon with greater electron density in in detail at different latitudes. Since the sun rotates with a period of the winter than the summer, and at low latitudes longitude variations about 28 days, sweeping out a column of particles into space when linked more to the magnetic equator than to the geographic equator, it is disturbed, there is a tendency for ionospheric storms to recur with a minimum of electron density at the magnetic equator and after this time interval. maxima to the north and south where the magnetic dip is about 30°. Additional ionisation is sometimes found in thin layers, 2km or At all latitudes electron densities in the F2- region, like those in the less thick, embedded in the E-region at heights between 90 and E- and Fl-regions, increase with increase of sunspot number. The 120km. This has an irregular and patchy structure, a maximum electron densities at heights above the maximum of the F2-region electron density which is much greater than that of the normal E- are controlled mainly by diffusion processes. region, and is known as sporadic-E (or Es) ionisation because of its The D-region shows great variability and fine structure, and is the intermittent occurrence. It consists of patches up to 2000km in least well understood part of the ionosphere. The only ionising extent, composed of large numbers of individual irregularities each radiations that can penetrate the upper regions and contribute to its less than 1km in size. Sporadic-E tends to be