Next consider the fraction of these trapped ions which are precipitated into the earth's atmosphere. As noted earlier it is assumed that the dominant loss mechanism is charge exchange between the argon ions and thermospheric hydrogen. (Actually near LEO the charge exchanges are between the argon ions and oxygen, the major constituent in the upper atmosphere.) The average hydrogen density3 encountered by a trapped ion with mirror latitude Xm along the field lines through re can be approximated by where o is the charge exchange cross section. After an ion charge exchanges it will follow a straight line path away from the neutralization point since its speed is much higher than the escape velocity. The situation is illustrated in Figure 3 for a special case. For each latitude, X, the charge exchanged particles come out at a specific pitch angle, a, so that the escaping particles describe the surface of a cone of half angle a about the field direction. Only the edge of the cone nearest the earth's surface is depicted in Figure 3 for clarity. For small X essentially none of the argon atoms are precipitated. At the other latitudes the fraction of atoms precipitated is proportional to the portion of the conical surface subtended by the earth's surface. The number of atoms precipitated in a given AX is also proportional to the average time spent in AX(i.e., At/Tu), and the hydrogen density in AX. Although this general procedure will allow total argon precipitation in the earth's atmosphere to be calculated for all SPS operations, detailed calculations have not yet been carried out. One hand calculation has been made for an OTV at 1.82 earth radii. It was found that only 16% of the argon precipitated in these orbits. At higher altitudes and smaller inclinations much less argon would intercept the atmosphere. REFERENCES 1) Liemohn, H. B., D. H. Holze, W. M. Leavens, and R. L. Copeland, "Ion Thruster Plasma Dynamics Near High Voltage Surfaces on Spacecraft", paper to be published in AIAA book, Space Systems and Their Interaction with the Earth's Space Environment, 1980. 2) Holze, D. H., "Deposition of Solar Power Satellite Thruster Exhaust in Earth's Magnetosphere", Boeing Document D180-25574-1 , October, 1979. 3) Liemohn, H. B., "Proton Lifetimes in the Van Allen Radiation Belts", Boeing (BSRL) Document Dl-82-0117, June, 1961. 4) Johnson, F. S. and R. A. Fish, "The Telluric Hydrogen Corona", Astrophysics J. 131, 502-515, 1960.
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