Optical Spectrograph And Infrared Imager System Research Articles

FeO emission in the mesosphere: Detectability, diurnal behavior, and modeling

[1] There continue to be surprises in the identification of the terrestrial nightglow components. Comparison of the nightglow as measured in astronomical sky spectra and Optical Spectrograph and Infrared Imager System (OSIRIS)/Odin orbiter data with that on FeO emission in meteor trains and laboratory experiments demonstrates that this emission is an important part of the nightglow, appearing as a quasi-continuum between 540 and 680 nm, with maximum intensity at 595 nm. In order to study the FeO emission, account must be taken of other nearby emissions, including the 589 nm sodium emission lines, and the underlying 8−2 OH Meinel band. The temporal behavior of the three emissions is normally closely related, but strong excursions can occur, which could provide valuable information on atmospheric processes. The temporal behavior of the FeO and OH emissions was compared to model results from the one-dimensional and time-resolved model FeMOD, which describes the iron chemistry in the mesosphere/lower thermosphere. Further observations and comparisons with satellite and lidar data as well as models will make it possible to improve our understanding of mesospheric chemistry, meteor ablation, and the role of iron in the atmosphere.

Fast NO<sub>2</sub> retrievals from Odin-OSIRIS limb scatter measurements

Abstract. The feasibility of retrieving vertical profiles of NO2 from space-based measurements of limb scattered sunlight has been demonstrated using several different data sets since the 1980's. The NO2 data product routinely retrieved from measurements made by the Optical Spectrograph and InfraRed Imaging System (OSIRIS) instrument onboard the Odin satellite uses a spectral fitting technique over the 437 to 451 nm range, over which there are 36 individual wavelength measurements. In this work we present a proof of concept technique for the retrieval of NO2 using only 4 of the 36 OSIRIS measurements in this wavelength range, which reduces the computational cost by almost an order of magnitude. The method is an adaptation of a triplet analysis technique that is currently used for the OSIRIS retrievals of ozone at Chappuis band wavelengths. The results obtained are shown to be in very good agreement with the spectral fit method, and provide an important alternative for applications where the computational burden is very high. Additionally this provides a baseline for future instrument design in terms of cost effectiveness and reducing spectral range requirements.

Transport and evolution of the 2009 Australian Black Saturday bushfire smoke in the lower stratosphere observed by OSIRIS on Odin

[1] We report the transport and evolution of the Victorian Black Saturday bushfire smoke plume in the lower stratosphere during the February–June 2009 period. Vertical profiles of limb-scattered spectral solar radiation measured by the Optical Spectrograph and Infrared Imager System (OSIRIS) instrument on the Odin satellite are used for this purpose. According to OSIRIS measurements, the main smoke material penetrated above the tropopause on 11 February 2009. It traveled westward and circled the globe in about 6 weeks from its first detection in the lower stratosphere, remaining in the tropical channel between 5°S and 25°S. The main plume gradually advected from ∼19 km in mid-February to ∼22 km in April, with a vertical extent varying from 2 to 6 km. The background radiances at corresponding altitudes and latitudes were subtracted from the plume-containing radiances to obtain net plume peak radiances. The latter parameter, governed by the number, size, and chemical composition of smoke particles, decreased by half every 19 days from 11 February until the end of April 2009. Pollution of the lower stratosphere at 18–22 km altitudes and 5°S–25°S latitudes, caused by the smoke plume dispersion, was up to 35% above the background. By mid-June 2009, OSIRIS radiances at these altitudes and latitudes had almost returned to their background levels.

The 10-day planetary wave examined by Odin/OSIRIS ozone profiles during late March 2002: comparison with UKMO and MF radar data

Ozone profile data have been retrieved from the Optical Spectrograph and InfraRed Imager System (OSIRIS) instrument onboard the Odin Satellite, which was launched on 20 February 2001. Considering the irregular sampling of the OSIRIS, a least-squares fit method is employed to provide the wavenumber and frequency spectra of the waves affecting the OSIRIS ozone profile distributions. Meanwhile, criteria are developed and applied for reducing spectral biases. Then spectral analysis of the global OSIRIS ozone profiles data is perfomred at altitudes of 38.5 km and 40.5 km within two latitude bins 35˚–45˚N and 45˚–55˚N during days 75–105 of the year 2002. Specifically, a westward-propagating 10-day planetary wave is detected and validated by medium frequency (MF) Radar (MFR) at Platteville (40˚N, 105˚W) and Saskatoon (52˚N, 107˚W), and UKMO global stratospheric data (UK Meteorological Office assimilated model). The spatial and temporal distributions of the global OSIRIS ozone profile data are shown to allow identification of major planetary wave features in the middle atmosphere.

Atomic oxygen densities retrieved from Optical Spectrograph and Infrared Imaging System observations of O2A-band airglow emission in the mesosphere and lower thermosphere

[1] Atomic oxygen densities, [O], have been retrieved from Optical Spectrograph and Infrared Imaging System (OSIRIS) observations of the O2 (b1Σg+, v′ = 0 − X3Σg+, v″ = 0) A-band airglow emission in the mesosphere and lower thermosphere. Daytime atomic oxygen densities, [O], have been inferred from ozone densities, [O3], that were retrieved from OSIRIS O2 A-band dayglow observations, and nighttime [O] have been retrieved directly from A-band nightglow observations employing an iterative-Newtonian optimal estimation inversion technique. Low-latitude results from 2007–2009 are presented and discussed. Variations in [O] between daytime (early morning, ∼0700 LT) and nighttime (late evening, ∼1900 LT) are shown to be strongly altitude dependent, with average nighttime densities in the tropics larger than daytime densities by ∼10% at 95 km and by more than a factor of 4 at 85 km. Seasonal variations in [O] are also examined and indicate that the seasonal variations of [O] are strongly influenced by the local photochemistry and are less dependent on dynamical variations. OSIRIS measurements are also compared to [O] predicted by the NRL-MSISE-00 model. The comparisons show that the derived OSIRIS densities are up to an order of magnitude larger than the MSIS values and that the seasonal oscillations of MSIS [O] are 180° out of phase with the OSIRIS measurements.

Research Spotlight: Iron oxide observed in Earth's airglow

The Earth's night airglow, a weak emission of light caused by chemical reactions in the planet's atmosphere, has been studied for more than a century. Because iron is common in meteorites that deposit debris in Earth's atmosphere, it was expected that iron oxide (FeO) would be found in the airglow, but it had not been detected until now. Evans et al. report observations made using the Optical Spectrograph and Infrared Imager System (OSIRIS) on the Odin spacecraft that show orange bands at about 600 nanometers in the night airglow spectrum. Comparison with published laboratory measurements of FeO emission spectra indicate that the observed orange bands in the airglow are probably due to FeO. The researchers suggest that the orange‐band FeO emission is probably produced through interactions of iron with ozone in the atmosphere. (Geophysical Research Letters, doi:10.1029/2010GL045310, 2010)

Temperatures in the upper mesosphere and lower thermosphere from OSIRIS observations of O2 A-band emission spectra

Temperatures in the mesosphere – lower thermosphere region (MLT) have been derived from the Optical Spectrograph and InfraRed Imaging System (OSIRIS) observations of O2 A-band (b1Σ g+ –X3Σ g– )O–O emission spectra. The observed OSIRIS spectra are inverted pixel by pixel, producing inverted volume emission rate spectra at altitudes between 90 and 110 km, which are compared to modelled temperature dependent O2 A-band spectra. The estimated accuracy of the retrieved temperatures is approximately ±2 K near 90 km and up to ±6 K at higher altitudes. The developed temperature retrieval technique is presented, and some initial retrieval results are briefly discussed.

Observations of the eruption of the Sarychev volcano and simulations using the HadGEM2 climate model

In June 2009 the Sarychev volcano located in the Kuril Islands to the northeast of Japan erupted explosively, injecting ash and an estimated 1.2 ± 0.2 Tg of sulfur dioxide into the upper troposphere and lower stratosphere, making it arguably one of the 10 largest stratospheric injections in the last 50 years. During the period immediately after the eruption, we show that the sulfur dioxide (SO2) cloud was clearly detected by retrievals developed for the Infrared Atmospheric Sounding Interferometer (IASI) satellite instrument and that the resultant stratospheric sulfate aerosol was detected by the Optical Spectrograph and Infrared Imaging System (OSIRIS) limb sounder and CALIPSO lidar. Additional surface‐based instrumentation allows assessment of the impact of the eruption on the stratospheric aerosol optical depth. We use a nudged version of the HadGEM2 climate model to investigate how well this state‐of‐the‐science climate model can replicate the distributions of SO2 and sulfate aerosol. The model simulations and OSIRIS measurements suggest that in the Northern Hemisphere the stratospheric aerosol optical depth was enhanced by around a factor of 3 (0.01 at 550 nm), with resultant impacts upon the radiation budget. The simulations indicate that, in the Northern Hemisphere for July 2009, the magnitude of the mean radiative impact from the volcanic aerosols is more than 60% of the direct radiative forcing of all anthropogenic aerosols put together. While the cooling induced by the eruption will likely not be detectable in the observational record, the combination of modeling and measurements would provide an ideal framework for simulating future larger volcanic eruptions.

The global mesospheric sodium layer observed by Odin/OSIRIS in 2004–2009

The source of the mesospheric sodium layer is the daily ablation of 10–100tons of meteoric material in Earth’s atmosphere. Global studies of this layer yield important information about the chemistry and dynamics of Earth’s mesosphere and lower thermosphere (MLT). For nine years the Optical Spectrograph and Infra-Red Imager System (OSIRIS) on-board the Odin satellite has observed Earth’s middle atmosphere by limb measurements of scattered sunlight from the ultraviolet to the infrared. In its aeronomy mode, Odin performs limb scans during 15 near-polar sun-synchronous orbits each day. The current measurement programme provides scans up to 110km on about 300 days per year. Above 70km, Na D resonance scattering at 589nm results in a strong limb signal. Retrievals from this dayglow feature have provided a global database of the mesospheric sodium layer. We present an updated sodium climatology from the Odin mission, including latitudinal and seasonal dependence, and interannual variability. We find a weak seasonal variation at low latitudes and an annual variation at mid- and high-latitudes with a clear summer minimum. An interesting feature is an interhemispheric asymmetry in the global dataset with larger sodium abundances during fall in the northern hemisphere and during spring in the southern hemisphere.

Odin/OSIRIS observations of stratospheric BrO: Retrieval methodology, climatology, and inferred Bry

A 7+ year (2001–2008) data set of stratospheric BrO profiles measured by the Optical Spectrograph and Infra‐Red Imager System (OSIRIS) instrument, a UV‐visible spectrometer measuring limb‐scattered sunlight from the Odin satellite, is presented. Zonal mean radiance spectra are computed for each day and inverted to yield effective daily zonal mean BrO profiles from 16 to 36 km. A detailed description of the retrieval methodology and error analysis is presented. Single‐profile precision and effective resolution are found to be about 30% and 3–5 km, respectively, throughout much of the retrieval range. Individual profile and monthly mean comparisons with ground‐based, balloon, and satellite instruments are found to agree to about 30%. A BrO climatology is presented, and its morphology and correlation with NO2 is consistent with our current understanding of bromine chemistry. Monthly mean Bry maps are derived. Two methods of calculating total Bry in the stratosphere are used and suggest (21.0 ± 5.0) pptv with a contribution from very short lived substances of (5.0 ± 5.0) pptv, consistent with other recent estimates.

NO2 air afterglow and O and NO densities from Odin‐OSIRIS night and ACE‐FTS sunset observations in the Antarctic MLT region

The continuum spectrum produced by the NO + O(+M) → NO2(+M) + hv chemiluminescent reaction has been detected in the upper mesospheric dark polar regions by Optical Spectrograph and Infra‐Red Imager System (OSIRIS) on the Odin spacecraft. For the sample period of 8–9 May 2005, Southern Hemisphere, limb radiance profiles of continuum spectra, resolved from OH airglow and auroral contamination, are inverted to obtain volume emission rate altitude profiles. The maximum observed differential brightness referred to zenith viewing is 1.2 × 107 photons cm−2 s−1 nm−1 at 580 nm with a measurement uncertainty 5 × 105 photons cm−2 s−1 nm−1, for an analysis range 80–101 km. Atomic oxygen densities [O] required by the analysis to derive NO densities [NO] are determined from OSIRIS O2(b1Σg+ − X3Σg−) 0‐0 band night airglow observations and from Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer (ACE‐FTS) sunset ozone observations. The derived Southern Hemisphere [O] map that shows pronounced longitudinal variation, maximum of 7 × 1011 cm−3, considerably exceeding MSIS model values, and suggests significant dynamical influence. Combining the continuum observations and the derived [O], a hemispheric map of derived [NO] is assembled that also shows considerable spatial variation, maximum of 1.1 × 109 cm−3, measurement uncertainty of 3 × 107 cm−3. Comparing the two maps, the geographical distribution of [NO] differs considerably from that of [O]. From a qualitative comparison, the distribution of derived [NO] is similar to that in coordinated GUVI LBH1 auroral precipitation images. The OSIRIS‐derived [NO] agrees with the measured ACE‐FTS [NO] to within 1 × 108 cm−3. The estimated systematic uncertainty of the NO densities derived from OSIRIS observations is approximately 30%.

Negligible climatic effects from the 2008 Okmok and Kasatochi volcanic eruptions

We used a general circulation model of Earth's climate to conduct simulations of climate response to the July 12, 2008, eruption of Okmok volcano and the August 8, 2008, eruption of Kasatochi volcano, which injected a total of 1.6 Tg of SO2 into the Arctic upper troposphere and lower stratosphere (0.1 Tg from Okmok and 1.5 Tg from Kasatochi). For the climate model simulations, we placed all the SO2 into the lower stratosphere. The temporal and spatial distribution of model predictions of sulfate aerosol optical depth agrees with measurements made by the Optical Spectrograph and InfraRed Imaging System (OSIRIS), a Canadian satellite instrument. After accounting for differences due to different wavelengths, different sampling volumes, and an overestimate of stratospheric SO2 loading in the model, the optical depths measured by OSIRIS are consistent with the modeled values. Although the shortwave radiative effects of the eruption are detectable in model output, perturbations in surface air temperature and precipitation were negligible, since the Okmok injection was quite small and the Kasatochi eruption was too late in the year for there to have been large radiative forcing in 2008 and was of insufficient magnitude for the sulfate aerosols to persist in the stratosphere into the following spring. We conducted further experiments with lower stratospheric injections of 3 and 5 Tg of SO2 into the Arctic on August 8, 2008. Although the sulfate aerosol optical depth and resulting shortwave radiative forcing increase linearly with atmospheric loading of SO2, the radiative forcing was still small due to the timing of the eruption, with little insolation by the time the sulfate aerosol cloud would form. High latitude eruptions of this size occurring in August or later in the calendar year would still be of insufficient magnitude for the sulfate aerosols to persist in the stratosphere into the following spring, and climate effects would be negligible.

Odin stratospheric proxy NO<sub>y</sub> measurements and climatology

Abstract. Five years of OSIRIS (Optical Spectrograph and InfraRed Imager System) NO2 and SMR (Sub-millimetre and Millimetre Radiometer) HNO3 observations from the Odin satellite, combined with data from a photochemical box model, have been used to construct a stratospheric proxy NOy data set including the gases: NO, NO2, HNO3, 2×N2O5 and ClONO2. This Odin NOy climatology is based on all daytime measurements and contains monthly mean and standard deviation, expressed as mixing ratio or number density, as function of latitude or equivalent latitude (5° bins) on 17 vertical layers (altitude, pressure or potential temperature) between 14 and 46 km. Comparisons with coincident NOy profiles from the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) instrument were used to evaluate several methods to combine Odin observations with model data. This comparison indicates that the most appropriate merging technique uses OSIRIS measurements of NO2, scaled with model NO/NO2 ratios, to estimate NO. The sum of 2×N2O5 and ClONO2 is estimated from uncertainty-based weighted averages of scaled observations of SMR HNO3 and OSIRIS NO2. Comparisons with ACE-FTS suggest the precision (random error) and accuracy (systematic error) of Odin NOy profiles are about 15% and 20%, respectively. Further comparisons between Odin and the Canadian Middle Atmosphere Model (CMAM) show agreement to within 20% and 2 ppb throughout most of the stratosphere except in the polar vortices. The combination of good temporal and spatial coverage, a relatively long data record, and good accuracy and precision make this a valuable NOy product for various atmospheric studies and model assessments.

Odin/OSIRIS observations of stratospheric NO<sub>3</sub> through sunrise and sunset

Abstract. The nitrate radical (NO3) has been detected in visible limb-scattered spectra measured by the Optical Spectrograph and InfraRed Imager System (OSIRIS) on-board the Odin satellite when observing at large solar zenith angles (91–97°). Apparent slant column densities of NO3 at tangent heights between 10 and 45 km are derived via spectral fitting in the 610–680 nm window. Using observations from multiple scans spanning solar zenith angles of 91–97°, the rapid evolution of NO3 through sunrise and sunset can be traced. Slant column densities are found to be generally consistent with those simulated using a radiative transfer model with coupled photochemistry. In addition, a strong dependence of NO3 with temperature is observed. These results indicate that our current knowledge of NO3 photochemistry is generally consistent with OSIRIS observations to within the limitations of the radiative transfer modeling. Furthermore, they reveal that OSIRIS possesses signal-to-noise sufficient to make useful measurements of scattered sunlight out to solar zenith angles of 91–97° and suggest the possibility of retrieving profile information for NO3 and other species at large solar zenith angles.

OH A2Σ+ –X2Π band ratios observed in the mesosphere by OSIRIS

In this study, we present spectra of the mesospheric OH A2Σ+ –X2Π band system, including the 0–0, 1–1, and 1–0 bands, as observed by OSIRIS (Optical Spectrograph and Infrared Imaging System). Spectral components due to Rayleigh-scattered sunlight, lower thermospheric dayglow emission features, and baffle scatter have been removed to isolate the OH emission signature. The observed spectra are compared with model spectra assembled using rotational emission rate factors for solar resonance fluorescence (g-factors) plus prompt emission of the OH A2Σ+ –X2Π band system from solar Lyman-α photodissociation of water. The observed band ratios are in good agreement with the model values. The altitude variation of the 0–0 band, relative to the 1–1 band, is in agreement with model predictions based on vibrational energy transfer from OH A2Σ+ ν′ = 1 to OH A2Σ+ ν′ = 0. This detailed understanding of the OH A2Σ+ –X2Π system is critical for the successful application of OH observations to the determination of mesospheric OH densities and water vapor concentrations.PACS Nos.: 33.20.Lg, 33.20.Tp, 33.70.Fd, 92.60.hc, 92.60.hw

Technical Note: Validation of Odin/SMR limb observations of ozone, comparisons with OSIRIS, POAM III, ground-based and balloon-borne instruments

Abstract. The Odin satellite carries two instruments capable of determining stratospheric ozone profiles by limb sounding: the Sub-Millimetre Radiometer (SMR) and the UV-visible spectrograph of the OSIRIS (Optical Spectrograph and InfraRed Imager System) instrument. A large number of ozone profiles measurements were performed during six years from November 2001 to present. This ozone dataset is here used to make quantitative comparisons with satellite measurements in order to assess the quality of the Odin/SMR ozone measurements. In a first step, we compare Swedish SMR retrievals version 2.1, French SMR ozone retrievals version 222 (both from the 501.8 GHz band), and the OSIRIS retrievals version 3.0, with the operational version 4.0 ozone product from POAM III (Polar Ozone Atmospheric Measurement). In a second step, we refine the Odin/SMR validation by comparisons with ground-based instruments and balloon-borne observations. We use observations carried out within the framework of the Network for Detection of Atmospheric Composition Change (NDACC) and balloon flight missions conducted by the Canadian Space Agency (CSA), the Laboratoire de Physique et de Chimie de l\\'{}Environnement (LPCE, Orléans, France), and the Service d'Aéronomie (SA, Paris, France). Coincidence criteria were 5° in latitude×10° in longitude, and 5 h in time in Odin/POAM III comparisons, 12 h in Odin/NDACC comparisons, and 72 h in Odin/balloons comparisons. An agreement is found with the POAM III experiment (10–60 km) within −0.3±0.2 ppmv (bias±standard deviation) for SMR (v222, v2.1) and within −0.5±0.2 ppmv for OSIRIS (v3.0). Odin ozone mixing ratio products are systematically slightly lower than the POAM III data and show an ozone maximum lower by 1–5 km in altitude. The comparisons with the NDACC data (10–34 km for ozonesonde, 10–50 km for lidar, 10–60 for microwave instruments) yield a good agreement within −0.15±0.3 ppmv for the SMR data and −0.3±0.3 ppmv for the OSIRIS data. Finally the comparisons with instruments on large balloons (10–31 km) show a good agreement, within −0.7±1 ppmv. The official SMR v2.1 dataset is consistent in all altitude ranges with POAM III, NDACC and large balloon-borne instruments measurements. In the SMR v2.1 data, no different systematic error has been found in the 0–35km range in comparison with the 35–60 km range. The same feature has been highlighted in both hemispheres in SMR v2.1/POAM III intercomparisons, and no latitudinal dependence has been revealed in SMR v2.1/NDACC intercomparisons.

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.

Comparison of OSIRIS stratospheric NO2 and O3 measurements with ground-based Fourier transform spectrometer measurements at the Toronto Atmospheric Observatory

Stratospheric NO2 and O3 retrieved from measurements of limb-scattered sunlight made by the Optical Spectrograph and InfraRed Imager System (OSIRIS) are compared with like observations made by a ground-based infrared Fourier Transform Spectrometer at the Toronto Atmospheric Observatory (TAO-FTS). Two different versions of OSIRIS NO2 are compared (DOAS version 3.0 and MART version 2.0) with partial column concentrations retrieved from the TAO-FTS. Two OSIRIS O3 versions are also compared (Triplet version 3.0 and MART version 2.0) with O3 retrieved from the TAO-FTS. To accommodate the most coincidences, comparisons are based on monthly mean stratospheric partial columns covering 16–50 km. All coincident monthly means display high correlations: 0.82–0.97. The monthly mean NO2 at TAO compared with the monthly mean NO2 from OSIRIS shows an average difference of less than ~3% with standard deviations up to 6%. The OSIRIS NO2 observations show a multiplicative bias of ~0.8–0.9 and a systematic difference of 5–10% greater then those of the TAO-FTS. O3 differences are less than 5%, on average, with standard deviations ranging from 2% to 2.8%. There is a pronounced multiplicative bias of OSIRIS compared with the TAO-FTS ranging from 0.55 to 0.73. The systematic O3 differences are less than 5% larger for OSIRIS. These small differences meet the standards outlined in the Integrated Global Observing Strategy and confirm the quality of the OSIRIS data for studying stratospheric ozone and nitrogen chemistry.PACS{ 92.60.hd, 92.60.Ry, 92.70.Cp, 93.30.Hf

Internal consistency in the Odin stratospheric ozone products

The two independent instruments on the Odin satellite, the Optical Spectrograph and Infrared Imaging System (OSIRIS) and the Sub-Millimetre Radiometer (SMR) produce atmospheric profiles of various atmospheric species including stratospheric ozone. Comparisons are made between OSIRIS version 3.0 and SMR version 2.1 ozone data to evaluate the consistency of the Odin ozone data sets. Results show good agreement between OSIRIS and SMR in the range 25–40 km, where systematic differences are less than 15% for all latitudes and seasons. Larger systematic differences are seen below 25 km, which can be explained by the increase of various error sources and lower signals. The random differences are between 20–30% in the middle stratosphere. Differences between Odin up-scans and down-scans or AM and PM are insignificant in the middle stratosphere. Furthermore, there is little variation from year to year, but a slight positive trend in the differences (OSIRIS minus SMR) of 0.045 ppmv/year at 30 km over validation period (2002–2006). The fact that the two fundamentally different measurement techniques, (absorption spectroscopy of scattering sunlight and emission measurements in the sub-millimetre region) agree so well, provides confidence in the robustness of both techniques.PACS Nos.: 92.60.Hd, 92.75.Rs, 95.55.Fw, 95.55.Jz

Properties of polar mesospheric clouds measured by Odin/OSIRIS in the northern hemisphere in 2002–2005

Interseasonal variations in the properties of Polar Mesospheric Clouds (PMC) measured by the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite during the northern hemisphere (NH) summers of 2002–2005 are described in this work. The lowest PMC latitudes were about 50°N for every season with the number of detections smallest in 2002 and largest in 2004. In 2004 and 2005, the detection of PMCs at lower latitudes was asymmetric with the larger number of clouds observed during the first half and fewer at during the second half of the season. PMC occurrence frequency in 2002 was 25–30% lower than in 2003–2005, and the season duration was shortest in 2002 and longest in 2004. For all NH seasons except 2002, PMC occurrence frequency was systematically 20–50% higher than the Solar Mesosphere Explorer climatology. Similar to PMC occurrence frequency, cloud brightness was lowest in 2002 and highest in 2004 at all latitudes. The daily mean brightness maximum at 50°–60°N was less than 8% of that at highest latitudes. This contrasts with the maximum PMC occurrence frequency that reached nearly 30% at these latitudes in 2004 and 2005. PMC brightness showed no apparent seasonal asymmetry at lower latitudes in 2004 and 2005 that was seen in the occurrence frequency. Significant, by about a factor of 2, oscillations observed in the daily mean cloud brightness at high latitudes were also not seen in the corresponding occurrence frequency. These results suggest that the occurrence frequency alone does not provide detailed information on the cloud population and ice mass in the mesosphere. There is no significant interannual variability in the seasonal mean OSIRIS PMC altitude. Its value was very close to 8350 km for all seasons except 2004 when it was 83.42 km. The mean PMC altitudes for each season increased by 0.3–0.6 when the minimum altitude in the database was increased from 80 to 82 km. PACS Nos.: 92.05.Fg, 92.60.hc, 92.60.Jq, 92.60.Mt, 92.60.Nv, 92.60.Vb