n density at incidence, give the direction and velocity of the horizontal compo- high latitudes (Whalen, et. al., 1971). Over 100 ground-based iono- nent of drift (Mitra, 1949). The heights, patch sizes and incidence sondes throughout the world make regular soundings each hour of of F-region irregularities responsible for oblique-path forward-scat- each day; data are published at monthly intervals (Rutherford, ter propagation at frequencies around 50MHz may be investigated 1991). by means of highly-directional antennas and from signal transit Over several years beginning in 1962 swept-frequency iono- times (Cohen and Bowles, 1961). Measurements of the Doppler sondes have been operated in satellites orbiting the earth at altitudes frequency variations of signals from stable C.W. transmitters may be of around 1000km. These are known as topside sounders and they used to study: give the distributions of electron density from the satellite height down to the peak of the F2-region. They also yield other plasma-res- 1. lonisation enhancements in the E- and F-regions associated onance information, together with data on electron and ion tempera- with solar flares. tures lionic composition. Fixed-frequency topside sounders have 2. Travelling ionospheric disturbances (Munro, 1950). been used to study the spatial characteristics of spread-F irregu- 3. The frequency-dispersion component of the ionospheric chan- larities and other features with fine structure. el-scattering function (Bello, 1965). Many different monitoring probes have been mounted in satellites orbiting the earth at altitudes above 100km, to give direct measure- Sporadic-E and F-region irregularities associated with visual au- ments of a range of ionospheric characteristics. These include r.f. rorae may be examined by pulsed-radar techniques over a wide impedance, capacitance and upper-hybrid resonance probes for range of frequencies from about 6MHz to 3000MHz. They may also local electron density, modified Langmuir probes for electron tem- be investigated usi