charge-separation electric field E and transfer to the magnetosphere the major portion of the beam energy in a distance of ~ 1000 km. The basic physics of beam plasma cloud interaction with the magnetosphere and ionosphere is well-known, and was worked out by Scholer (1970) and Pilipp (1971) in connection with the HEOS release of an ionized barium cloud at L = 12 (Haerendel and Lust, 1970). This high-altitude release had, as will the argon engines, a high initial [[spi:math]], where [[spi:math]] is the pressure perpendicular to the field lines of the injected plasma). The beam expands rapidly, in a direction perpendicular to [[spi:math]] and to [[spi:math]], to the point where [[spi:math]] <= 1. Of course, the beam also spreads without constraint (except for mirroring forces) along B as in Figure 1 and as discussed above. One could calculate the final beam spread Ay in the y-direction (or [[spi:math]] direction; see Figure 1) using zero-Larmor-radius magnetohydrodynamics, that is, by equating [[spi:math]] (including thermal pressure nAkT plus dynamic pressure (l/2)nAmA v2 tan2 [[spi:math]] to the asymptotic plasmaspheric pressure, which is essentially B2/8[[spi:math]]. Flux conservation requires [[spi:math]], where n0A and A are the initial beam density and area (see Table I) and Az is the beam spread along the field. Assuming [[spi:math]], one finds that Ay is less than ~ 1 km for L < 4, much smaller than the argon Larmor radius RA. In effect, this calculation of the confinement of gyration centers tells us that [[spi:math]] is of the order of the argon Larmor radius (40-80 km at L = 4) because the gyration centers are confined to a [[spi:math]] much smaller than the argon Larmor radius, at least for the first ten or so Larmor radii downstream from the nozzle; past this, the ~ 10° angular divergence of the beam could produce substantially larger [[spi:math]] [as long as (9) continues to hold]. In first approximation, then, we have a beam of [[spi:math]] propagating across the earth's field as shown in Figure 8. In this figure, the dotted 17
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