Joel G. Cardon

SHIMMER: A Spatial Heterodyne Spectrometer for Remote Sensing of Earth's Middle Atmosphere

It is well known and demonstrated that interference spectroscopy offers capabilities to obtain passive remote optical sensing spectra of high precision and also achieves economies in size, cost, and ease of deployment compared with more conventional systems. We describe the development of a near-ultraviolet spatial heterodyne spectrometer designed for remote sensing of the global distribution of the hydroxyl radical OH in the Earths middle atmosphere. The instrument, known as SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals), is expected to obtain its first OH measurement from space in early 2002 from the Space Shuttle.

Satellite observations of upper stratospheric and mesospheric OH: The HOxdilemma

We report the first observations of the vertical distribution of hydroxyl (OH) from the upper stratosphere to the mesopause. The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) made these measurements in August 1997. The data confirm the results from the earlier November 1994 MAHRSI mission that were confined to altitudes above 50 km, namely that mesospheric OH densities are 25 to 35% lower than predicted by standard photochemical theory. However, the new observations show that below 50 km the OH density increases rapidly and at 43 km altitude it is larger than that expected from standard theory. This represents a serious dilemma for our understanding of odd-hydrogen chemistry because the same key reactions are thought to dominate OH/HO2 partitioning in both regions. We show that neither standard photochemical theory nor any previously proposed changes are adequate to explain the OH observations in both the upper stratosphere and mesosphere.

Robust monolithic ultraviolet interferometer for the SHIMMER instrument on STPSat-1

We describe the design, fabrication, and testing of a monolithic interferometer consisting entirely of optically contacted fused-silica optical elements that are assembled, adjusted, and permanently bonded in place. The interferometer is part of a spatial heterodyne spectrometer (SHS) [SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals)] that will be used for near-ultraviolet high-spectral-resolution limb imaging of OH solar resonance fluorescence from low Earth orbit aboard the satellite STPSat-1 scheduled for launch in 2006. The stability of the monolith coupled with the relaxed tolerances on optical quality and alignment inherent to SHS make this new instrument extremely robust and especially attractive for applications in harsh environments.

Middle Atmosphere High Resolution Spectrograph Investigation

The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) was developed specifically to measure the vertical density profiles of hydroxyl (OH) and nitric oxide (NO) in the middle atmosphere from space. MAHRSI was launched on its first flight in November 1994 on the CRISTA-SPAS satellite that was deployed and retrieved by the space shuttle. The instrument measured the radiance profiles of ultraviolet solar resonance fluorescence on the Earths limb with a spectral resolving power of 15,600 at a wavelength of 308 nm and 7200 at 215 nm. The instantaneous height of the field of view projected to the tangent point was about 300 m. OH limb radiance measurements were made between altitudes of 90 and 30 km, and each limb scan extended over a horizontal distance of 1200 km. For NO a limb scan extended between altitudes of 140 and 76 km and over a horizontal distance 700 km. Observations were made from 52°S latitude to 62°N latitude. The OH measurements have been inverted to provide the first global maps of the vertical distribution of OH between 90 and 50 km. The data show a detailed history of the morning formation of a strongly peaked layer of OH at an altitude of 68 km. This layer was produced by solar photodissociation of a thin layer of water vapor peaked at 65 km extending between 30°S and 35°N observed contemporaneously by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite. MAHRSI was successfully flown for a second time in August 1997 under conditions that extended the geographical coverage to 72°N latitude and local solar time coverage through the afternoon hours. This paper provides a detailed description of the experiment and instrumentation, of the algorithms used to reduce the spectral data and perform the inversions, and presents examples of key results from the 1994 flight.

Correction of phase distortion in spatial heterodyne spectroscopy

The detailed analysis of measured interferograms generally requires phase correction. Phase-shift correction methods are commonly used and well documented for conventional Fourier-transform spectroscopy. However, measured interferograms can show additional phase errors, depending on the optical path difference and signal frequency, which we call phase distortion. In spatial heterodyne spectroscopy they can be caused, for instance, by optical defects or image distortions, making them a characteristic of the individual spectrometer. They can generally be corrected without significant loss of the signal-to-noise ratio. We present a technique to measure phase distortion by using a measured example interferogram. We also describe a technique to correct for phase distortion and test its performance by using a simulation with a near-UV solar spectrum. We find that for our measured example interferogram the phase distortion is small and nearly frequency independent. Furthermore, we show that the presented phase-correction technique is especially effective for apodized interferograms.

Satellite measurements of hydroxyl in the mesosphere

The global distribution of hydroxyl (OH) in the middle atmosphere was recently measured by the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) on a satellite deployed and retrieved by the space shuttle. During 75 orbits, MAHRSI acquired 1800 daytime limb scans of the OH ultraviolet solar resonance fluorescence intensity. Each limb scan extends over the altitude region from 30 to 90 km and across 10° of latitude between 53°S and 63°N. OH number densities were retrieved using a Twomey regularization scheme constrained by the smoothness of the retrieved profile. Results provide a detailed description of the diurnal variation of mesospheric OH. Midmorning OH densities had a well defined peak of about 6 ×106 cm³ near 70 km, a broad minimum centered near 64 km, and rose to about 1 × 107 cm³ at 50 km. This profile is in substantial disagreement with photochemical model predictions [Summers et al., this issue]. The observations are compared with the two previous measurements.

Spatial Heterodyne Spectroscopy For High Spectral Resolution Space-Based Remote Sensing

Spatial heterodyne spectroscopy (SHS) offers a number of advantages compared to conventional Fourier transform spectroscopy. An extremely robust, monolithic SHS interferometer will fly aboard a small satellite scheduled for launch in 2006.

MAHRSI observations of nitric oxide in the mesosphere and lower thermosphere

In November, 1994, the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) observed the distribution of NO between the altitudes of 76 and 140 km by measuring limb intensity profiles of solar resonance fluorescence in the NO A ²Σ → X ²II (1,0) γ band near 215 nm. The observations were made from the ASTRO-SPAS (Shuttle Pallet Satellite) spacecraft which was deployed and retrieved by the space shuttle. The data provided a seven hour snapshot of lower thermospheric and mesospheric NO from sunrise near 48°S to sunset near 61° N latitude following a period of low solar and high geomagnetic activity. Inferred peak lower thermospheric NO densities ranged from 3 × 107 cm−3 near the equator to 2 × 108 cm−3 at high northern latitudes, roughly consistent with previous observations for the same conditions. Individual vertical density profiles showed substantial structure and large orbit to orbit variations, suggesting that the distribution of mesospheric and lower thermospheric NO is dynamically influenced.

Shimmer on STS-112: Development and Proof-of-Concept Flight

Abstract : The Spatial Heterodyne Imager for Mesospheric Radicals (SHIMMER), which is based on a new interferometric technique called Spatial Heterodyne Spectroscopy (SHS), flew on the Space Shuttle Atlantis mission STS-112 in October 2002. SHS has the advantages of high throughput, high spectral resolution, small size, low mass, all in a rugged instrument with no moving optical components. The SHS proof-of-principal flight successfully demonstrated the suitability of SHS for spaceflight applications where high spectral resolution measurements over a relatively narrow spectral band are required. In addition, the highest spectral resolution measurement of middle atmospheric hydroxyl (OH) solar resonance fluorescence ever achieved was made by SHIMMER during this mission.

The SHIMMER Instruments: Spatial Heterodyne Imagers for Space Based Remote Sensing of the Atmosphere

SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals) denotes a family of SHS instruments for space based UV remote sensing of the atmosphere. Two future satellite missions, a shuttle mission and their objectives are presented here.