C. Further Fate of Argon Ions The further fate of the injected Ar+ after the major part of the initial 3.5 keV energy has been imparted to the ambient magnetosphere may be very complex because it depends on the plasma dynamics of the Ar+ cloud. We would expect a whole zoo of plasma instabilities to be operative; some of these may indeed hasten and spread the uniform beam stopping process discussed above. Some discussion of plasma instabilities are given in Section IV. G. The longer-range evolution of Ar+ is characterized by the competition between loss of energy by Coulomb collisions and the loss of Ar+ itself by chargeexchange. Long range Coulomb collisions with ambient electrons degrade the kinetic energy into ambient thermal energy without causing substantial pitchangle change, because these Coulomb collisions are primarily forward scatterings. For energetic Ar+ to be physically lost from the magnetosphere, they will have to suffer charge-exchange collisions whereupon the neutral argon atom would escape magnetic trapping. Figure 11 shows the thermalization due to charge-exchange of Ar+ in the magnetosphere (L >= 2) compared with the thermalization time due to Coulomb collisions. In this Figure, the lifetimes are computed for lower energies because substantial fractions of the initial 3.5 keV have been lost in the beam stopping process. Also, these lifetimes are computed for Ar+ mirroring in the equatorial plane primarily because of the previous consideration that Ar+ are unlikely to reach the loss-cone if they are injected at >= 2000 km from LEO. The charge-exchange process in the magnetosphere is quite different from the processes (5), (6) and (7) because the magnetospheric neutral constituent is primarily hydrogen, i.e., [[spi:math]] (38) 31
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