n density on the changes in solar radiation which accom- because of the increased ionisation at the higher latitudes. Note that pany the long-term fluctuations in the state of the sun. Maximum for this ionosphere and frequency there are two ground zones which E-region electron density is approximately proportional to sunspot cannot be illuminated. number, which varies over a cycle of roughly 11 years. The ionosphere is of considerable importance in the engineering In the daytime the F-region splits into two, with the lower part or radio communication systems because: known as the F1-region and the upper part as the F2-region. This splitting arises because the principal loss mechanism is an ion-atom 1. It provides the means of establishing various communication interchange process followed by dissociative recombination, the paths, calling for system-design criteria based on a knowledge former process controlling the loss rates in the F2-region and the of ionospheric morphology. latter in the F1-region. Although maximum production is in the 9/4 The lonosphere and the Troposphere for its formation principally involve nitric oxide and other minor atmospheric constituents. The D-region is mainly responsible for the absorption of radio Night waves because of the high electron-collision frequencies at such altitudes (see below). While the electron densities in the upper part of the D-region appear linked to those in the E-region, leading to systematic latitudinal, temporal and solar-cycle variations in ab- sorption, there are also appreciable irregular day-to-day absorption changes. At middle latitudes anomalously high absorption is experi- enced on some days in the winter. This is related to warmings of the stratosphere and is probably associated with changes in D-region F2-Region composition. In the lower D-region at heights below about 70km the ionisation is produced principally by energetic cosmic rays, uni- formly incident at all times of day. Since the free electrons thereby generated tend to collide and become at