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The earth's ionosphere possesses plasma properties at altitudes above 80 km where the collisions of charged particles with the neutral gas gradually lose their dominating influence. The ionosphere can be characterized as a weakly ionized low-β and low-temperature plasma of great homogeneity with respect to the spatial scales of most of the processes of interest. The basic properties of the ionospheric plasma are presented in Section 2.1. The role of thermal fluctuations and their use in a powerful diagnostic technique – the incoherent scatter technique – are reviewed in Section 2.2. Plasma instabilities of natural cause, occurring in different altitude regions, are briefly discussed in Sections 2.3 and 2.4. The major part of this article is devoted to active experiments in the ionospheric plasma, as initiated by powerful electromagnetic waves transmitted from the ground, and their theoretical understanding (Section 3). Nonlinear effects caused by strong electromagnetic waves are numerous, but they can be traced to either of two nonlinearity mechanisms, thermal or ponderomotive. Thermal nonlinearities in a homogeneous wave field are briefly sketched in Section 3.1. Thermal nonlinearities in an inhomogeneous wave field, having their origin in pressure gradient forces and resulting in two different types of density irregularities, both aligned with the magnetic field, are treated in Section 3.2. Ponderomotive nonlinearities have their source in the convective derivative in the electron momentum equation which gives rise to a force that pushes plasma from high to low wave intensity. Specific phenomena caused by ponderomotive action are Langmuir turbulence and stimulated electromagnetic emissions (SEE). Both these phenomena are presently the subjects of intensive experimental and theoretical investigations, and they are described at some length in Section 3.3.