Rodney A. Viereck

N2 triplet band systems and atomic oxygen in the dayglow

New spectrographic observations of the Earths dayglow have been acquired by the Arizona Airglow Experiment (GLO) flown on the space shuttle. GLO is an imaging spectrograph that records simultaneous vertical profiles of prominent Earth limb emissions occurring at wavelengths between 115 and 900 nm. This study addresses the measured emissions from the N2 triplet states (first positive, second positive, and Vegard-Kaplan band systems) and their excitation by the local photoelectron flux. The triplet state population distributions modeled for aurora by Cartwright [1978] are modified for dayglow conditions by changing to a photoelectron-flux energy distribution and including resonance scattering by the first positive system. Modeled and observed intensities are in excellent agreement, in contrast to the well-studied auroral case. This work concentrates on dayglow conditions at 200 km altitude near the subsolar point. Parameters to infer the local photoelectron flux from the emission band intensities are provided. Several atomic oxygen dayglow emission features were analyzed to complement the N2 analysis. The photoelectron-excited O I(135.6, 777.4 nm) lines were found to be 3 to 4 times weaker than predicted while the O I(630.0, 844.6 nm) lines were in close agreement with the model prediction.

Limb view spectrum of the Earth's airglow

Sometimes the Earths 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.

Spectrographic observation at wavelengths near 630 nm of the interaction between the atmosphere and the Space Shuttle exhaust

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 H2O. Calculations using the SOCRATES code show excellent agreement with the data.

O(1S) 557.7 nm and O(1D) 630 nm emissions in shuttle thruster plumes

Radiation resulting from interaction between the effluent cloud of a space shuttle thruster and the ambient atmosphere was observed with a spectrograph aboard the shuttle. The spectral measurements were made between 400 and 800 nm with a resolution of 3 nm. The primary emissions are identified as NO2, HNO, O(1D), and O(1S). These are the first observations of O(1S) emission in the shuttle plume. These data are compared with previous measurements, and possible excitation mechanisms are discussed. The results are also compared with a Monte Carlo simulation of thruster plume-atmosphere interaction radiation.

Mg+ and other metallic emissions observed in the thermosphere

Limb observations of UV dayglow emissions from 80 to 300 km tangent heights were made in December 1992 using the GLO instrument, which flew on STS-53 as a Hitchhiker-G experiment. STS-53 was at 330 km altitude and had an orbit inclination of 57 degree(s). The orbit placed the shuttle near the terminator for the entire mission, resulting in a unique set of observations. The GLO instrument consisted of 12 imagers and 9 spectrographs on an Az/El gimbal system. The data was obtained over 6 days of the mission. Emissions from Mg+ and Ca+ were observed, as were emissions from the neutral metallic species Mg and Na. The ultimate source of the metals is ablation of meteors; however, the spatial distribution of the emissions is controlled by upper mesospheric and thermospheric winds and, in the case of the ions, by the electromagnetic fields of the ionosphere. The observed Mg+ emission was the brightest of the metal emissions, and was observed near the poles and around the geomagnetic equator near sunset. The polar emissions were short-lived and intense, indicative of auroral activity. The equatorial emissions were more continuous, with several luminous patches propagating poleward over the period of several orbits. The instrumentation is described, as are spatial and temporal variations of the metal emissions with emphasis on the metal ions. These observations are compared to previous observations of thermospheric metallic species.

Thermospheric metal emissions: update on GLO Mg+ measurements

The GLO experiment includes a ground-controlled shuttle- based UV-vis-IR spectrograph and imager set, and has flown on four space shuttle flights, including three in 1995. Each flight returned limb-view on metal atom and ion emissions in the 80-350 km tangent height region. Improved optics provided 0.3 nm FWHM resolution in the ultraviolet, and simultaneous altitude profiles were routinely measured that spanned 150 km in tangent height with 10-15 km resolution. CLouds of metal ions, particularly Mg+, were observed in daytime above 120 km tangent height near the geomagnetic equator. The GLO project returned approximately 30 gigabytes of spectral data in 1995. The current high altitude metal ion emission measurements are reported here.

Panchromatic spectrograph with supporting monochromatic imagers

The Arizona Imager/Spectrograph is a set of imaging spectrographs and two-dimensional imagers for space flight. Nine nearly identical spectrographs record wavelengths from 114 to 1090 nm with resolution of 0.5 - 1.3 nm. The spatial resolution along the slit is electronically selectable and can reach 192 elements. Twelve passband imagers cover wavelengths in the 160 - 900 nm range and have fields of view from 2 degree(s) to 21 degree(s). The spectrographs and imagers rely on intensified CCD detectors to achieve substantial capability in an instrument of minimum mass and size. By use of innovative coupling techniques only two CCDs are required to record images from 12 imagers, and single CCDs record spectra from pairs of spectrographs. The fields of view of the spectrographs and imagers are coaligned, and all spectra and images can be exposed simultaneously. A scan platform can rotate the sensor head about two orthogonal axes. The Arizona Imager/Spectrograph is designed for investigations of the interaction between the Space Shuttle and its environment. It was flown on a sub-satellite deployed from and retrieved by STS-39.

Shuttle-based measurements: GLO ultraviolet earthlimb view

The GLO experiment is an on-going shuttle-based spectrograph/imager project that has returned ultraviolet (100 - 400 nm) limb views. High spectral (0.35 nm FWHM) and temporal (4 s) resolution spectra include simultaneous altitude profiles (in the range of 80 - 400 km tangent height with 10 km resolution) of dayglow and nightglow features. Measured emissions include the NO gamma, N2 Vegard-Kaplan and second positive, N2+ first negative, and O2 Herzberg I band systems and both atomic and cation lines of N, O, and Mg. This data represents a low solar activity benchmark for future observations. We report on the status of the GLO project, which included three space flights in 1995, and present spectral data on important ultraviolet band systems.

Optical Properties of Water Released in Low Earth Orbit

Results of an analysis of intensified video photographs of a twilight venting of excess water from Space Shuttle are presented. The particle sizes, densities, and temperatures derived from the visible data are applied in estimating UV and IR radiances of the ice/vapor-containing volumes near Shuttle Orbiter, using a recently developed gas-transport/excitation model. The mean radius of the fragmentation-product droplets is 0.13 +/- 0.02 cm. This radius decreases by less than 5 percent over a 2.5-km initial flight path, and these particles survive for several hr. In the UV, intensities of radiation from the fragmentation particles fall off with decreasing wavelength due to the decrease in spectral irradiance of sunlight. In the IR, the mm particles are optically thick, while ice particles not greater than 0.3 micron are inefficient scatterer-radiators, except near 2.7 microns. The large-droplet component thus dominates the radiances even in projections to distant sensors, suppressing the severe spectral structure characteristic of the small droplets.