outer radiation belt is likely to be made inoperative by SPS ion engine injection of Ar+, resulting in enhancement of relativistic electron flux. Discussion of radiation belt relativistic electron effects is found in the next subsection. There is no danger that the population of ring-current protons would be similarly enhanced, since these are subject to rapid charge exchange with ambient neutral hydrogen, as are ring-current helium and oxygen ions on a substantially longer time scale (Tinsley, 1976; Lyons and Evans, 1976). Moreover, the addition of argon plasma (or any other type of plasma) to the natural plasmasphere would tend to enhance the loss rate of ring-current electrons (Brice, 1970; 1971). One should not worry about the development of a charge imbalance in the magnetosphere on account of modified precipitation rates. There is ample cold plasma in the plasmasphere to balance charges through minor modifications of the already weak ambipolar electric field, which has been included in the model calculations in the previous section. Suppression of the electromagnetic ion-cyclotron wave mode is illustrated in Figure 16. The normalized growth rate [[spi:math]] is plotted as a function of normalized wave frequency [[spi:math]] for selected argon/hydrogen plasmaconcentration ratios. For the parameters chosen it is clear that a ratio NAr/NH [[spi:math]] 1 sufficient to suppress the proton-cyclotron branch altogether. The lower-frequency argon-cyclotron branch achieves a growth rate of at most 3% of the maximum proton-cyclotron growth rate attained in the absence of argon. Thus, the electromagnetic ion-cyclotron mode is very much stabilized overall by the addition of cold argon plasma to the medium. This conclusion is consistent with the results obtained by similar methods for the addition of even lighter ions such as Li+ and He+ (Mark, 1974). 50
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