New Observations of Hydroxyl from the Space Shuttle by NRL's SHIMMER

Introduction: Remote sensing from space uniquely enables global observation of hydroxyl (OH) by ultraviolet (UV) solar resonance fluorescence in the Earth's middle atmosphere. An understanding of the photochemistry of OH in the middle atmosphere (20-90 km altitude) is a primary focus of the Upper Atmospheric Physics Branch. Measuring the vertical distribution of OH is invaluable to understanding middle atmospheric chemistry including ozone destruction, the Polar Mesospheric Cloud phenomenon, and the water budget. Despite its importance, OH remains one of the least measured atmospheric constituents.

The observation of OH solar resonance fluorescence in the middle atmosphere requires high spectral resolution to separate the relatively bright and spectrally complex solar scattered background from the relatively weak OH emission features near 308 nm. NRL's MAHRSI instrument1 provided the first global measurements of mesospheric OH during two Shuttle missions in the 1990s. However, the size and weight of conventional grating spectrometers like MAHRSI renders them impractical for future long-duration space flight opportunities. As an alternative, the Spatial Heterodyne IMager for MEsospheric Radicals (SHIMMER),2 which is based on a breakthrough interferometric technique called spatial heterodyne spectroscopy (SHS), has been designed to make OH measurements similar to MAHRSI's while requiring far fewer spacecraft resources. SHIMMER flew on the Space Shuttle Atlantis mission STS-112 in October 2002. It has the advantages of high throughput, high spectral resolution, small size, and low mass, all in a rugged instrument with no moving optical components. The Shuttle flight successfully demonstrated, for the first time, the suitability of SHS for spaceflight applications.

The SHS Concept: SHS is similar to a Fourier transform spectrometer (FTS), with the mirrors in the Michelson interferometer arms replaced by fixed, tilted gratings equidistant from the beamsplitter. Diffraction at the gratings results in a wavenumber-dependent tilt of the wavefronts recombining at the beamsplitter, and interference of the tilted wavefronts creates Fizeau fringes at the instrument's fixed charge coupled device (CCD) detector (Fig. 8). The Fourier transform of the interferogram produced at the CCD yields the incident spectrum. The angle and groove density of the gratings is selected so that for a chosen wavelength, the beams retro-reflect along the optical axis, thus producing no interference fringes. Incident light at wavelengths close to this (heterodyne) wavelength produces fringes that are sampled at the CCD. When viewing the Earth's limb from space, the scene is imaged on the gratings and the gratings are imaged on the CCD, with the dispersion plane parallel to the horizon. Limb scanning is avoided since the detector simultaneously records interferograms in each horizontal row of the CCD corresponding to discrete altitudes.

The SHIMMER Instrument: Figure 8 includes a photograph of SHIMMER. The spectral resolution (0.0059 nm), passband (307-310 nm), field-of-view (2.3° × 2.3°), and sensitivity requirements of SHIMMER were determined using previous successful MAHRSI OH measurements. The telescope focuses the limb on the gratings, imaging altitudes along the columns of the CCD while producing interferograms along the rows. The interferometer uses a nonpolarizing beamsplitter, a pair of field-widening prisms, and a pair of 1200 l/mm gratings. Relay optics focus the plane of the gratings on the CCD. The interferometric elements are mounted in the triangular Vascomax steel structure, allowing very precise and stable positioning of the prisms and gratings relative to the beamsplitter.

FIGURE 8
(a) SHS conceptual diagram and (b) the SHIMMER instrument.

Measurement Results: Figure 9 shows an important measurement result for the SHS proof-of-concept. Since SHIMMER images the entire altitude range of interest simultaneously, and since the UV signal varies greatly over this range, minimization of scattered light in the instrument is critical. The triangles show the profile imaged by SHIMMER, and the dashed line shows the profile measured by MAHRSI (with its narrow field-of-view scanning up and down the limb). The figure demonstrates that SHIMMER succeeded in accurately imaging the profile. The highest spectral resolution mesospheric OH resonance fluorescence spectra ever measured were retrieved from the data (Fig. 10). Work in progress indicates that the radiance and noise evident in the spectrum are near expectations. Because of operational and limb viewing constraints, only 36 s of OH data were acquired. Nonetheless, the retrieval unambiguously demonstrates that SHIMMER accurately measured the UV emission from the limb at high spectral resolution.

FIGURE 9
Limb altitude intensity profile (peaks normalized to 41 km).

FIGURE 10
Hydroxyl spectrum (51-68 km).

Summary: Data from the STS-112 mission demonstrate that SHIMMER met its design goals. It produced high spectral resolution solar and OH spectra over a broad altitude range without the use of any moving optical components. The mesospheric OH measurements provide an important supplement to those made by the much larger and heavier MAHRSI instrument, and the mission has provided a successful and invaluable proof-of-concept of the innovative new SHS technology for space-based remote sensing.

Acknowledgments: The mission's success was made possible by the dedicated efforts of R. Feldman, J. Moser, L. Marlin, C. Brown, and the integration and operations team at the DOD Space Test Program Shuttle and ISS Payload Office. This research was supported by grants from the National Science Foundation, the Low Cost Access to Space Program of NASA's Office of Space Science, and the U.S. DOD Space Test Program.

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