Nonvertical propagation and delayed-echo generation observed by the topside sounders

Knowledge of the topside ionosphere resulting from studies of nonvertical propagation, such as scatter from ionization irregularities and minitroughs, hemispherically conjugate echoes, combination modes, multiple-hop propagation, unusual Z-wave propagation, and whistler-mode propagation, is reviewed. From these studies there appear to be two major types of magnetic field-aligned ionization irregularities: a thick type and a thin type. The thick type has a thickness of tens or hundreds of kilometers and occurs both near the auroral zone and at latitudes corresponding to the equatorial anomaly. The thin type has a thickness of a few hundred meters and may extend thousands of kilometers along a field line. At high latitudes the thin type has a tubular cross section, may have an electron density either greater or less than the ambient, and may result from fluxes of energetic particles; at low latitudes the thin type has an electron density deviation of the order of 1 percent or less. The thin type of ionization irregularity supports propagation which is responsible, at some distance from the satellite, for two different delayed-echo phenomena. In the vicinity of the satellite electrostatic waves with near-zero group velocity are probably responsible for the fH, fT, and nfHresonance spikes and also for the newly discovered fQnresonance spikes which occur at nonzero and noninfinite values of the wavenumber. A novel spike, called the "floating spike," is believed to result from propagating electrostatic waves.