Middle Atmosphere High Resolution Spectrograph Investigation Research Articles

Spatial Heterodyne Imager for Mesospheric Radicals on STPSat‐1

The Spatial Heterodyne Imager for Mesospheric Radicals (SHIMMER) was a high‐resolution, near ultraviolet spectrometer that imaged the Earth's limb for 2.5 years between March 2007 and October 2009. The instrument used the Spatial Heterodyne Spectroscopy technique for the first time on a satellite and successfully demonstrated its capabilities. SHIMMER measured the solar resonance fluorescence of the OH A2Σ+‐X2Π (0, 0) band around 309 nm, which allows the retrieval of mesospheric OH density profiles. It also measured the Rayleigh scattered background from the clear atmosphere and solar scattering from polar mesospheric cloud particles. We present details on the SHIMMER mission, the payload design, and the data analysis. A comparison between SHIMMER and concurrent Microwave Limb Sounder OH data shows good agreement between 60 and 90 km altitude for several latitudes and seasons. We also find good agreement of the SHIMMER OH densities and standard photochemical model calculations between 60 and 80 km. We find no evidence of a 25%–35% mesospheric OH deficit, previously reported using Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) OH data. However, independent analysis of Rayleigh scattered background signals observed by SHIMMER and MAHRSI under similar lighting conditions revealed that MAHRSI radiances are systematically smaller than SHIMMER radiances by 24%. Although this difference is well outside of the combined uncertainties for both experiments, the agreement of SHIMMER OH with Microwave Limb Sounder OH and standard photochemistry results, together with our Rayleigh scattering comparison, suggests an unidentified MAHRSI calibration problem that effectively eliminates the mesospheric OH deficit reported using MAHRSI observations.

First UV satellite observations of mesospheric water vapor

We report the first UV satellite observations of mesospheric water vapor. The measurements are of nonthermal OH prompt emission between 300–330 nm produced directly from the photodissociation of water vapor by H Lyman‐α. This technique is most sensitive to water vapor concentrations between 70–90 km altitude. We present OH data from two limb scanning experiments: the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) and the Optical Spectrograph and Infra‐Red Imager System (OSIRIS). Interpretation of the lower resolution (∼1 nm) OSIRIS spectra requires the rotational emission rate factors for OH(1,1) solar fluorescence between 313–318 nm, which we present for the first time herein. Comparison of water vapor concentration profiles with the most coincident profiles from the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite shows agreement to within 30% between 75–80 km for both MAHRSI and OSIRIS. We discuss the benefits of this promising new approach to measuring upper mesospheric water vapor and the need for new laboratory measurements to improve the analysis.

Validation of Aura Microwave Limb Sounder OH and HO2 measurements

The Microwave Limb Sounder (MLS) instrument on the Aura satellite obtains global measurements of both OH and HO2 radicals. This paper describes the precision and systematic errors of the MLS version v2.2 of the retrieval software. Estimated systematic errors are less than 8% for OH over 32–0.003 hPa and HO2 over 6.8–0.21 hPa. Comparison of measurements from MLS OH and HO2 profiles and three balloon‐based instruments show good agreement among themselves and with a photochemical model using standard chemistry (i.e., recommended rate constants). Similarly, good agreement is seen between column OH found by integrating satellite profiles and ground‐based measurements of column OH. The agreement between measured and modeled OH and HO2 is improved following perturbations to the rate constants O + OH and OH + HO2 that are within the recommended uncertainties. Measurements of OH obtained a decade ago by the Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) are smaller than MLS measurements by 20% at 70 km, are similar to MLS data near 50 km, and are 50% larger than MLS observations near 42 km. The MLS and MAHRSI measurements of OH overlap at the limit of their respective 2‐σ uncertainties. Most importantly, we find the shape of the OH profile measured by MLS is simulated well using standard chemistry.

Optical Spectrograph and Infra‐Red Imaging System (OSIRIS) observations of mesospheric OH A2Σ+‐X2Π 0‐0 and 1‐1 band resonance emissions

Although only a minor species, the OH molecule plays an important role in the photochemical control of mesospheric ozone density and has been the target of a number of observational programs, principally through the OH A2Σ+‐X2Π 0‐0 band emission at 308 nm. This emission band arises from solar resonance fluorescence excitation of OH X2Π ground state molecules, and its observation is complicated by the presence of an underlying atmospheric Rayleigh scattering spectrum. We show that the OH A2Σ+‐X2Π 0‐0 band emission has been reliably and routinely detected with the moderately low, 0.9 nm, spectral resolution Optical Spectrograph and Infra‐Red Imaging System (OSIRIS) limb scanning spectrograph. Changes in upper mesospheric water vapor observed by the Halogen Occultation Experiment (HALOE) are readily detected as changes in the OH density profiles seen by OSIRIS. Altitude profiles of OH density in the middle and upper mesosphere are in good agreement with model results that incorporate coordinated HALOE water vapor measurements. The agreement is within the HALOE and OSIRIS error limits when the recommended standard reaction rates are assumed. Conversely, model calculations of OH density using the revised reaction rates proposed to explain the Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) OH profiles typically fall outside the observed OSIRIS error limits. The OSIRIS results suggest that the probable difference between the observed and modeled OH densities is less than 15% from 55 to 80 km. Diurnal and seasonal OH variations observed by OSIRIS are in good agreement with model predictions. These successful comparisons suggest that the OSIRIS OH database, spanning more than 4 years of operation and broad ranges of latitude and local time, can contribute significantly to studies of OH photochemistry and upper mesospheric water vapor.

OH observations of space shuttle exhaust

We report the unexpected observation of a large hydroxyl (OH) cloud north and east of the United States a day after a space shuttle launch in November, 1994. The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) observed OH(0,0) solar fluorescence near 309 nm while staring toward a tangent altitude of 87 km, where OH can be produced from water vapor photodissociation. The OH(0,0) band has a rotational temperature of 252 ± 23 K corresponding to an altitude of 110 ± 3 km, where nearly half of the shuttle's main engine water vapor exhaust is released on ascent. The location of the cloud one day after injection into the atmosphere implies that its average velocity is between 26–40 m/s northward. We also report strong evidence of water ice measured simultaneously along the same line of sight, suggesting that water vapor exhaust is redistributed by condensation and sedimentation.

PMCs and the water frost point in the Arctic summer mesosphere

In August, 1997 the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) obtained vertical profiles of OH number density and polar mesospheric cloud (PMC) brightness by scanning the limb up to 71° N while the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) obtained co‐located vertical profiles of temperature. MAHRSI OH densities are converted to water vapor using a one‐dimensional model that assumes photochemical equilibrium. By combining water vapor profiles with CRISTA temperatures we map the frost point both vertically and horizontally in the Arctic summer mesosphere. Our data show that supersaturation can exist between 80‐87 km suggesting that growth of ice particles is limited to these altitudes. Additionally, we find that not only is supersaturation an insufficient condition for a PMC but also that PMCs can exist in apparently unsaturated air.

Discovery of a water vapor layer in the Arctic summer mesosphere: Implications for polar mesospheric clouds

We report the discovery of a layer of enhanced water vapor in the Arctic summer mesosphere that was made utilizing two new techniques for remotely determining water vapor abundances. The first utilizes Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) OH measurements as a proxy for water vapor. The second is a reanalysis of Halogen Occultation Experiment (HALOE) water vapor data with a technique to simultaneously determine polar mesospheric cloud (PMC) ice particle extinction along with the water vapor abundance. These results reveal a narrow layer of enhanced water vapor centered between 82–84 km altitude and coincident with PMCs, that exhibits water vapor mixing ratios of 10–15 ppmv. This indicates that a higher degree of supersaturation is present in the PMC region, and that PMCs are thus more efficient at sequestering total water (both ice particles and vapor) within the layer, than previously believed.

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.

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 Earth's 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.

Calculated OH A2Σ+−X2Π (0,0) band rotational emission rate factors for solar resonance fluorescence

The Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) measured vertical density profiles of hydroxyl (OH) by observing A2Σ+−X2Π (0,0) solar resonance fluorescence in the mesosphere near 309 nm [Conway et al., this issue]. Interpretation of the MAHRSI results requires calculation of the OH(0,0) rotational emission rate factors (g factors) which describe the molecule's fluorescent scattering efficiency. This calculation relies heavily on a solar irradiance measurement with high enough spectral resolution to identify the amount of excitation for each rotational transition (resolving power >3.1×105). Recent high‐resolution ground‐based measurements of the solar irradiance and more accurate absolute solar irradiance observations allow for much more accurate g factors. We find that the relative intensities of calculated OH (0,0;) rotational g factors are in excellent agreement with MAHRSI spectra. The calculated OH (0,0) band g factor is found to be 3.51×10−4 s−1 at 200 K, in good agreement with the most recent work on the subject.

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.

Implications of Satellite OH Observations for Middle Atmospheric H 2 O and Ozone

Satellite observations by the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) have produced global measurements of hydroxyl (OH) in the atmosphere. These observations reveal a sharp peak in OH density near an altitude of 65 to 70 km and are thus consistent with observations from the Halogen Occultation Experiment (HALOE) on the NASA Upper Atmosphere Research Satellite (UARS), which showed an unexplained H 2 O layer at the same level. Analysis of stratopause (about 50 kilometers) OH measurements and coincident ozone observations from the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment reveals that the catalytic loss of ozone attributable to odd-hydrogen chemistry is less than that predicted with standard chemistry. Thus, the dominant portion of the ozone deficit problem in standard models is a consequence of overestimation of the OH density in the upper stratosphere and lower mesosphere.

Mesospheric HOx photochemistry: Constraints from recent satellite measurements of OH and H2O

We use recent measurements of OH made with the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) along with satellite observations of water vapor abundances, to study odd‐hydrogen photochemistry in the mesosphere. The MAHRSI data sampled primarily the morning part of the diurnal variation of OH, and here we focus on one orbit of data that is representative of the MAHRSI observations during the mission. Our approach is to use a photochemical model, with input H2O abundances fixed to observed values, to simulate the diurnal variation of mesospheric HOx species. Models of OH using standard recommended HOx chemical rate coefficients are found to substantially overpredict OH between 5065 km. Proposed modifications to HOx chemistry that produce lower OH and higher HO2 and O3, yield better agreement in the lower mesophere but worsen the agreement at the observed OH peak near∼70 km. We find that neither standard HOx chemistry, nor models incorporating proposed HOx modifications, adequately reproduce the observed OH density profile over the entire 50–75 km altitude range for any of the observed early to mid morning local times.

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.