Larmor radius as the outer spatial scale (see Table 1). Plasma density striations formed by the zoo of electrostatic modes cause signal scintillation effects which will be discussed below. The anticipated signal scintillation effects, or equivalently the formation of plasma iregularities connected with ion engine beam-exhaust in the plasmasphere, can be attributed to various classes of electrostatic modes: a) the current-driven plasma instability of the exhaust beam such as entries 1 and 2 in Table III, and b) plasma drift instabilities associated with the density and velocity gradients of the exhaust plasma such as entries 3, 4 and 5 in Table III. The former is very intense because the exhaust beam has a great deal of available free energy, but the turbulent region will probably be confined to the vicinity of the space transport. The latter includes a number of individual modes (typified by Kelvin-Helmholtz and drift modes) which may occur over an extensive region and at a range of plasmaspheric parameters since the instabilities are associated with the diffusion of the injected plasma cloud. The cross-field current-driven ion acoustic instability has been observed in beam-plasma devices with operating conditions similar to those projected for ion engines (Barrett et al., 1972), except that the ion engines will have much higher beam current. This instability draws its energy from the free energy of the streaming plasma beam and generates ion acoustic waves (density irregularities) propagating at large angles to the magnetic field. These waves have frequencies well below the electron cyclotron frequency and have wavelengths of the order of the electron gyroradius, which is ~ 30 cm in the ionosphere and is ~ 20 m at 4 RE. Since signal scintillation effects are most severe when the signal wavelength closely matches the irregularity size, we expect that the 30 cm irregularities of the beam-plasma instability will cause 60
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