the polarization electric field E seen by a co-moving observer above is that, in the coordinate system of the stationary magnetic field outside of the cloud, the plasma cloud, under the force of E, appears to be drifting with a velocity [[spi:math]]. But [[spi:math]] above is also the drift velocity of magnetic field lines in the cloud induced by the electric field [[spi:math]]; hence, the field lines in the cloud are said to be "frozen" into the plasma, drifting with velocity [[spi:math]] relative to the field lines outside of the cloud. For this condition to apply, the beam density nA must satisfy [[spi:math]] (9) where mA is the argon-ion mass (Curtis and Grebowsky, 1980). Numerically, (9) yields (500-30000) nA (cm-3) [[spi:math]] 1 for 2 <= L <= 4, which would seem to be well- satisfied for the beam parameters of Table I; if so, the beam simply moves out of the magnetosphere to be dissipated in space. Unfortunately, this conclusion is conditionally false. The magnetosphere cannot be considered in terms of a vacuum magnetic field because disturbances of the magnetospheric plasma must necessarily involve the dissipative ionospheric plasma. In their latest consideration, Curtis and Grebowski (1980) invoke a non-propagating sheath to shield the beam from interaction with the rest of the magnetosphere and ionosphere. Such a situation depends critically on how and if a sheath, and especially a non-propagating one, can be formed. Since there is no present knowledge of this mechanism, we shall, without prejudice, assess the beam-magnetosphere interaction according to the "estab- lishmentarian" picture of plasma interactions in the magnetosphere, whose origin can be traced to considerations of Alfven. In this picture, plasmas in the magnetosphere, and especially in the ionosphere, act to short out the 16
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