A gap in the midday sector of the auroral oval is examined on the basis of Defense Meteorological Satellite Program (DMSP) satellite auroral photographs. All DMSP photographs recorded in June 1975 over the southern hemisphere were examined, and 38 photographs were found suitable to specify details of auroral features in the midday sector. There is not a single example showing the discrete auroral arcs extending continuously across the noon sector, bridging the morning and afternoon sectors of the auroral oval. Hence a gap, manifested by the absence of extended, discrete auroral arcs, exists in the midday sector of the auroral oval. The gap appears typically to extend over more than 15 ø of longitude or, correspondingly, more than 1 hour of local time and is centered about the 1000 corrected geomagnetic local time meridian. During an early phase of substorms the gap appears to fill with rays and fragmented arcs and to empty after substorm recovery. Similarly, polar cap arcs appear and disappear at the same times that the gap fills and empties of rays and fragmented arcs. The gap therefore appears to be a semipermanent feature of the midday auroral distribution.

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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.

Sometimes the Earth's airglow layer, when observed in the limb view, appears to be a double layer. During one of these occasions we were able to obtain absolute spectral intensity and altitude profiles for most well‐known airglow features from the space shuttle on the STS‐37 mission. The lower of the two airglow layers consisted of the OH Meinel bands, the ground state of the O2 atmospheric band at 762 nm and an intense nighttime sodium layer at 598 nm peaking near 92 km. The upper airglow layer consists of the atomic oxygen lines at 557.7, 630, and 636.4 nm, and a blue green continuum. Significant 557.7, 630, and 636.4 nm OI emissions were observed to extend above 110 km into the lower F region. The volume emission rate distribution as a function of altitude was modeled, and the model limb intensity distribution was compared to the data. From the spectral profile of the modeled volume emission distribution, the green continuum located at 103 km ± 2 km altitude was identified as recombination emission of NO2 produced by a dense NO layer. Another layer which was present at each of the modeled wavelength regions had a volume emission peak at 90 km. Our observations did not show the presence of continuum emission with increasing intensity toward the near infrared. The observed regions were quite near the equator.

Observations of the optical emissions from the space shuttle's thrusters have been examined. Particular attention has been paid to the interaction of the thruster plume with the atmosphere. Emissions from CN, CH, C2, HNO, and NO 2 have been observed near the nozzle of the thruster in the vacuum core region of the plume, but these emissions are the direct result of the combustion process. Other emissions including OI and NH have been observed in the downstream region of the plume, where the plume effluents interact with the atmosphere. The NH emission is one of the most dominant UV/ visible wavelength emissions observed in the plumes. This emission was observed to extend several thousand meters from the shuttle, and detailed analysis shows that the total intensity of the emission depends on the ram angle (angle in the shuttle reference frame between the plume effluents and the ramming atmosphere) and altitude, indicating an interaction process with the atmosphere. Data from two observational experiments are presented. The Air Force Maui Optical Site (AMOS) experiment includes ground-based spectral and spatial measurements of the shuttle plumes as the thrusters were fired over the AMOS site on top of Haliakala Volcano on the island of Maui in the mid-Pacific. The GLO experiment was flown in the payload bay of the space shuttle and also includes spectral and spatial measurements of the shuttle plumes. During both of these experiments, the primary reaction control system (PRCS) engines (870 lb (394 kgf) thrust) and Vernier reaction control system (VRCS) engines (25 lb (11 kgf) thrust) were fired at various angles relative to the ram, thus providing a range of collision velocities (4.5-11 km/s) between the thruster plume and the atmosphere. In this report the dependence of the NH emission on ram angle, thruster size, and distance from the shuttle is presented and analyzed using a three-dimensional Monte Carlo simulation of the plume-atmosphere interactions called spacecraft/orbiter contamination representation accounting for transiently emitted species (SOCRATES). The chemical reactions deemed most likely involve collisions of the plume products HNC, HNCO, and CH2NH with atmospheric O, and all of these processes are examined. The ram-angle dependence is used to determine a threshold energy required for the reaction that leads to the NH emission and to conclude that the most likely reaction involves CH2NH collisions with O. orbit (-200-500 km) undergo complex interactions with the atmosphere because of the rarefied and reactive nature of the atmosphere at these altitudes. These interactions lead to the generation of local atmospheres that are quite different from the ambient atmosphere. The rarefied atmosphere and the relatively high orbit velocity also produce a unique opportunity to study high-velocity one-step and twostep chemical processes. The large scale lengths provide an environment where it is possible to study extremely long lived metastable species. The state of the local spacecraft environ-Paper number 95JA...