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SPACE PLASMAS 107 craft observations have characterized Earth's magnetosphere as a collection of discrete regions with distinctive physical properties separated by well-defined boundaries. Strong plasma waves have been observed within the boundary regions, particularly the magnetopause and plasma sheet boundary layers. For the magnetopause boundary layer, this has important consequences for the entry of solar wind plasma into Earth's magnetosphere. Spacecraft have mapped out the locale and statistics of occurrence of most important classes of plasma waves in Earth's magnetosphere. Besides being important for boundary region dynamics, plasma waves provide the dominant loss mechanism for energetic electrons in the inner magnetosphere via pitch-angle diffusion and are drivers for the precipitation of ions and electrons into the lower atmospheric regions. Plasma waves are thought to play a principal role in heating ionospheric ions in the topside ionosphere. Upwelling of these heated ions along magnetic field lines and their subsequent trapping in the equatorial plane due to interactions with regions of plasma turbulence provide an important source of magnetospheric plasma. A wide variety of plasma waves participates in the complex energy transfer processes on auroral field lines. Plasma waves have been used as a diagnostic tool to obtain properties of the plasma from both ground-based and space-based systems. Plasma irregularities in the ionosphere occur with scale-size distributions covering tens of kilometers to fractions of a meter. These irregularities, which result from poorly understood instability mechanisms, are a major source for the disruption of high-frequency (HF) and extremely-high-frequency (EHF) communication systems. Charged-Particle and Plasma Energization Charged particles in plasma can be accelerated to high energies through a variety of mechanisms, some of which occur in nature or can be induced in suitably arranged space experiments. These include particle acceleration through resonance with quasi-monochromatic waves; stochastic acceleration resulting from resonance overlap due to large wave amplitudes or the presence of a finite spectrum of waves; acceleration by parametric processes, such as beat waves, Brillouin, and Raman scattering; and acceleration by electric fields that result from changes in macroscopic plasma morphology as encountered, for example, on auroral field lines. These phenomena are fundamentally nonlinear and extremely complicated, from both theoretical and observational points of view. Although measurements of electric and magnetic fields can be made with very high time (spectral) resolution, particle measurements are comparatively crude. For many purposes, the particle distribution function must be known to an accuracy that cannot be obtained with present-day technology. As an example, only two of the three velocity components of a distribution are generally known (perpendicular and parallel to Earth's magnetic field). Yet in many resonance inter