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SPACE PLASMAS 108 actions with waves, it is the unknown component (the phase coherence) that is the key to the interaction. Dust-Plasma Interactions Dusty plasmas are the most common type of plasmas in space. It is now believed that the long-term evolution of the dust and plasma environments is strongly coupled. The dust grains collect electrostatic charges from the plasma, and the evolution of their spatial distribution, size distribution, and lifetime can be determined by electrostatic forces and plasma drag. On the other hand, the dust can alter the plasma composition, density, momentum, and energy distribution, as well as the dispersion relations of the waves propagating in a dusty plasma medium. In the past decade, a growing effort (laboratory experiments and theory) has focused on problems related to dusty plasmas. We now understand the important processes that determine the charge of the dust grains and have learned the transport processes that shape the fine dust components in planetary rings embedded in magnetospheric plasmas. Magnetospheric perturbations were clearly shown to be responsible for the observed spatial distribution of small dust grains in the Jovian and Saturnian rings. Collective dusty plasma effects were suggested to explain the spokes (transient radial dust features on Saturn's main ring system) observed on Voyager images. The large scattering cross section of charged ice grains in noctilucent clouds is thought to be responsible for the observed anomalous radar echoes. The differential settling of bigger and smaller grains toward the midplane in the early solar system was suggested to cause spatial charge separation that might have resulted in large-scale electrostatic discharges. These lightning bolts could explain the existence of chondrules (small molten beads of rocks found in meteorites). The Critical Ionization Velocity Effect Investigations of the critical ionization velocity effect are an important part of space plasma science. The phenomenon involves the nonclassical ionization of energetic neutral atoms and molecules as they move through a background magnetized plasma. From laboratory studies and some space measurements, it is thought that when the center of mass energy of the neutrals rises above their ionization threshold, there is rapid ionization of the neutrals. This process apparently involves energization of the ambient electron gas by plasma waves associated initially with the transformation of a few energetic neutrals to ions. The newly born ions have considerable kinetic energy and heat the electrons through collective plasma processes. When sufficient neutrals are converted to ions, as might happen through charge exchange, for example, the energy density of the
