tribution NA, which depends in turn on the dynamics of the ion engine beam at LEO. Considerations of pitch-angle distributions such as in Figure 7 indicate that the magnetic field exerts little influence on the beam density distribution between LEO (~ 400 km) and the charge exchange altitude (~ 300 km); therefore, to study the mapping of beam density from the nozzle at LEO to the airglow-emission altitude, we need only solve the collisionless Boltzmann equation as in Section II.A. The magnitude of [[spi:math]] in (4) is somewhat uncertain. Here we consider two limiting cases. For Case 1, a rather unrealistic case, all particles make a 10° angle with the nozzle axis as they emerge (including those from the middle of the nozzle); we have [[spi:math]] tan 10°. From (4), it is seen that Case 1 yields a beam which spreads very fast. For Case 2, the perpendicular spread of the beam is due entirely to thermal mo- tion, a more realistic case for an exhaust jet; then, [[spi:math]] where T is the Ar+ temperature (~ 1000°K). This yields a dense beam. The truth probably lies between these two limits. We have computed the column emission intensity for these two cases (I1 and I2) using (4) and (44) for propellant flow rate given in Table I and for a CIRA (1972) atmosphere with 1000°K exospheric temperature. The artificial [[spi:math]] aurora brightness stimulated by 3.5 keV Ar+ in the spectral range 2500- 5500 [[spi:math]] range is given in Table II. For comparison, the brightness of the natural aurora of moderate strength emitted by the same [[spi:math]] band in the 3500- 5000 [[spi:math]] range (Baker and Pendleton, 1976) is also listed. It is seen that the artificial aurora may be an order of magnitude brighter in intensity than the natural aurora in these spectral bands. It must be noted, however, that this result applies only to Ar+ beams emitted at LEO. Since these spectral regions may be of importance to optical earth-sensors, our considerations suggest that such interference can be mitigated by a thrusting schedule which directs the 37
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