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Earth‐limb observations by the GLO instrument aboard space shuttle mission STS‐53 include the first simultaneous resonant‐scattering detection of Mg+ (279.6–280.3 nm) and neutral Mg (285.2 nm) in the thermosphere. Although Mg+ emissions are observed in approximately 45% of GLO scans over a wide geographic and local‐time range, neutral Mg is observed in fewer than 1% of the measurements, and most of these sightings occur on two consecutive orbits in a near‐equatorial patch at local dusk. Neutral Mg is observed at tangent heights between 150 and 230 km, with an instrument‐view‐column density of 0.4–1.4 × 108 cm−2, compared with simultaneous Mg+ values of 1.4–5.8 × 109 cm−2. Calculated in‐cloud densities are [Mg+] ∼250 cm−3 and [Mg] ∼5 cm−3, and the average ratio of [Mg+] to [Mg] in the individual measurements is 34:1.

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The collision between the exhaust from the Primary Reaction Control System (PRCS) engines (870 pounds thrust) of the space shuttle and the ambient atmosphere has been observed from the Air Force Maui Optical Station (AMOS). Spectra have been obtained in the wavelength region near 630 nm. The temporal, spatial, and spectral distribution of the emission in this region has been recorded. The results reported here indicate that when the exhaust of the space shuttle interacts with the atmosphere in the ram direction, an intense, long-lasting emission at 630 nm due to O(1D --> 3p) is generated. A substantial amount of O(1D) is swept back onto the orbiter. Two processes are proposed for the formation of O(1D): (1) excitation of atmospheric O(3p) by collisions with the exhaust of the space shuttle engines; and (2) charge exchange between ambient O + and exhaust H20. Calculations using the SOCRATES code show excellent agreement with the data. 19,501

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