A. Argon Ion Precipitation Near LEO By examination of (A4), it is well-established that 3.5 keV Ar+ emitted at LEO with nonvanishing [[spi:math]] is likely to charge-exchange with neutral thermospheric constituents without first losing its initial energy by other interactions. A more quantitative consideration requires determination of the charge-exchange altitude zce [or [[spi:math]] in (A4)]. Since major thermospheric neutral constituents are O N2 and O2, the following charge-exchange processes must be considered: [[spi:math]] (5) [[spi:math]] (6) [[spi:math]]. (7) These processes at 3.5 keV incident energy are needed to determine zce for a given distribution of neutrals. The cross-sections associated with (6) and (7) were measured by Hedrick et al. [1977] but there is no known measurement of total cross-section for (5). If the incident energy far exceeds the O2 binding energy, we would expect that [[spi:math]] . (8) With the use of the CIRA/COSPAR International Reference Atmosphere (1972) at an average 1000° K exospheric temperature, it is a simple numerical procedure to determine zce from the functional relation (2) for each of the processes (5) - (7). We obtain: zce [(5)] ~ 280 km, zce [(6)] ~ 200 km, and zce 14
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