D. Fonteyn

Four-dimensional variational chemical assimilation of CRISTA stratospheric measurements

This paper presents a chemical assimilation system based on the variational method. It has been applied to an off-line stratospheric three-dimensional chemical trans- port model. This technique determines the best model initial conditions that minimize the model errors when compared to a set of observations for a predefined time window. The system is applied to the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) measurements for the November 5-11, 1994, mission. Observed species are O3 ,C H4, HNO3 ,C lONO2 ,N 2O5 ,N 2O, and CFC-11. For an assimilation pe- riod of 12 hours, CRISTA observations are well estimated by the assimilation system. These results vary according to the species considered and depend on the CRISTAs ob- servational error. In particular, assimilated ozone differ from CRISTA by less than 5%. Analyses have been compared with independent observations in order to validate the as- similation system. It was found that observations by the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment and the Halogen Occultation Experiment (HALOE) agree well with the analyses. These comparisons suggested systematic differences between CRISTA and HALOE or ATMOS. Unconstrained constituents like NOx and HCl are in- fluenced through chemical coupling and show small discrepancies with ATMOS and HALOE data.

SPICAM IR acousto‐optic spectrometer experiment on Mars Express

SPICAV IR, a part of SPICAV/SOIR suite on Venus Express, is a compact single pixel spectrometer for the spectral range of 0.65–1.7 mm based on acousto-optical tunable filter (AOTF) technology. SPICAV IR is derived from SPICAM IR operating on Mars Express, the first AOTF spectrometer in the deep space, and adapted for Venus atmosphere measurements. The spectrometer sequentially measures spectra of reflected solar radiation from Venus on the dayside and the emitted Venus radiation in spectral ‘‘windows’’ on the nightside, and works also in solar occultation mode. The spectral range is 0.65– 1.1 mm with spectral resolution of 7.8 cm � 1 , and 1–1.7 mm with spectral resolution of 5.2 cm � 1 .A description of this near-IR instrument, its calibration, in-flight performances, and the modes of operations on Venus’ orbit are presented. A brief overview of the science measurements is given: water vapor measurements in the mesosphere on the day-side and near surface on the nightside, mapping of the O2(a 1 Dg) emission at 1.27 mm, aerosol studies via polarization and scattering solar radiation at the day-side, and measurements of aerosol properties at the tops of the clouds in solar occultations.

The study of the martian atmosphere from top to bottom with SPICAM light on mars express

Abstract SPICAM Light is a small UV-IR instrument selected for Mars Express to recover most of the science that was lost with the demise of Mars 96, where the SPICAM set of sensors was dedicated to the study of the atmosphere of Mars (Spectroscopy for the investigation of the characteristics of the atmosphere of mars). The new configuration of SPICAM Light includes optical sensors and an electronics block. A UV spectrometer (118–320 nm, resolution 0.8 nm) is dedicated to Nadir viewing, limb viewing and vertical profiling by stellar occultation (3.8 kg). It addresses key issues about ozone, its coupling with H2O, aerosols, atmospheric vertical temperature structure and ionospheric studies. An IR spectrometer (1.2– 4.8 μm , resolution 0.4–1 nm) is dedicated to vertical profiling during solar occultation of H2O, CO2, CO, aerosols and exploration of carbon compounds (3.5 kg). A nadir looking sensor for H2O abundances (1.0– 1.7 μm , resolution 0.8 nm) is recently included in the package (0.8 kg). A simple data processing unit (DPU, 0.9 kg) provides the interface of these sensors with the spacecraft. In nadir orientation, SPICAM UV is essentially an ozone detector, measuring the strongest O3 absorption band at 250 nm in the spectrum of the solar light scattered back from the ground. In the stellar occultation mode the UV Sensor will measure the vertical profiles of CO2, temperature, O3, clouds and aerosols. The density/temperature profiles obtained with SPICAM Light will constrain and aid in the development of the meteorological and dynamical atmospheric models, from the surface to 160 km in the atmosphere. This is essential for future missions that will rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow study of the ionosphere through the emissions of CO, CO+, and CO2+, and its direct interaction with the solar wind. Also, it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere. The SPICAM Light IR sensor is inherited from the IR solar part of the SPICAM solar occultation instrument of Mars 96. Its main scientific objective is the global mapping of the vertical structure of H2O, CO2, CO, HDO, aerosols, atmospheric density, and temperature by the solar occultation. The wide spectral range of the IR spectrometer and its high spectral resolution allow an exploratory investigation addressing fundamental question of the possible presence of carbon compounds in the Martian atmosphere. Because of severe mass constraints this channel is still optional. An additional nadir near IR channel that employs a pioneering technology acousto-optical tuneable filter (AOTF) is dedicated to the measurement of water vapour column abundance in the IR simultaneously with ozone measured in the UV. It will be done at much lower telemetry budget compared to the other instrument of the mission, planetary fourier spectrometer (PFS).

An AOTF-based spectrometer for the studies of Mars atmosphere for Mars Express ESA mission

The SPICAM Light optical package on the ESA Mars Express mission is dedicated to the nadir and limb observations in the UV between 118 nm and 320 nm, and has originally included an IR solar occultation channel, an inheritance of the IR part of the SPICAM solar occultation instrument for Mars 96. Because of severe mass constrains of the mission this channel has been replaced by a lightweight (0.7 kg) near infrared instrument that employs a new technology acousto-optical tuneable filter (AOTF). This channel is dedicated to the nadir measurements of water vapour column abundance in the near infrared between 1 and 1.7 μm simultaneously with ozone measured in the UV. In addition to the measurements of water vapour column abundance in the band of 1.38 μm, the NIR nadir spectrometer will measure the CO2 quantity in the bands of 1.43, 1.57-1.6 μm, and, consequently, the surface pressure (with known topography); and will contribute to the studies of atmospheric aerosols and the surface, by spectro-polarimetry measurements. Fully functional model of the instrument has been assembled, has been undergone a number of tests; the spectra of terrestrial atmospheric transmittance have been recorded. The scientific context of the experiment will be discussed along with the instruments description; current development status and the calibration results will be presented.

Determination of the aerosol size distribution by analytic inversion of the extinction spectrum in the complex anomalous diffraction approximation

A new derivation is presented for the analytical inversion of aerosol spectral extinction data to size distributions. It is based on the complex analytic extension of the anomalous diffraction approximation (ADA). We derive inverse formulas that are applicable to homogeneous nonabsorbing and absorbing spherical particles. Our method simplifies, generalizes, and unifies a number of results obtained previously in the literature. In particular, we clarify the connection between the ADA transform and the Fourier and Laplace transforms. Also, the effect of the particle refractive-index dispersion on the inversion is examined. It is shown that, when Lorentzs model is used for this dispersion, the continuous ADA inverse transform is mathematically well posed, whereas with a constant refractive index it is ill posed. Further, a condition is given, in terms of Lorentz parameters, for which the continuous inverse operator does not amplify the error.

A global OClO stratospheric layer discovered in GOMOS stellar occultation measurements

The stratospheric ozone depletion observed in polar regions is caused by several catalytic cycles induced by reactive chlorine and bromine species. By reacting with BrO, ClO causes the formation of OClO which is considered as a proxy of the halogen activation. We present the first global determination of the stratospheric OClO distribution measured during the year 2003 by the stellar occultation spectrometer GOMOS. Besides its expected polar abundance, we discovered the presence of a worldwide OClO layer in the upper stratosphere. At lower altitudes, OClO seems also to be present beyond the limit of the polar vortices, an unreported feature.

4D-var assimilation of stratospheric aerosol satellite data

Abstract A 4D Variational assimilation system of stratospheric aerosols has been developed. The aerosol assimilation system is applied to the 1.02 μm optical extinction data as measured by SAGE—II (Stratospheric Aerosol and Gas Experiment). A very simple model, a minimal standard aerosol model, was used for both the evolution of aerosols and for the inverse observational operator to map observations into model values. Since this assimilation was set up to provide aerosol analyses for isolated cases, the implementation of a straightforward choice of the first-guess field and corresponding background error covariances is discussed. The results of the aerosol analysis for January 1989 are discussed focussing on the added value of aerosol data assimilation. With a minimal aerosol model, the results show that the assimilation analyses offer the possibility to obtain direct global dynamically consistent information of the 1.02—μm aerosol extinction and aerosol surface area densities. The aerosol analyses can also be used to study dynamical features in greater detail.

Second flight of the Spacelab Grille Spectrometer during the ATLAS‐1 mission

The SPACELAB grille spectrometer on its second space flight during the ATLAS-1 mission (March 24-April 2, 1992) took advantage of the favorable timeline and of the extra day to perform more than 65 successful solar occultation runs. It succeeded in obtaining spectra pertinent to its ten target molecules in the full range of altitudes available to the solar infrared occultation technique. These ten molecules are H[sub 2]O, CO, CO[sub 2], CH[sub 4], NO, NO[sub 2] N[sub 2]O, HCl, HF and O[sub 3]. The preliminary analysis of the sunset observation presented here adds new information to the available database on HCl vertical profiles, for assessing long-term trends of this important stratospheric species. 11 refs., 4 figs., 1 tab.

SPICAM-light on Mars-Express as a monitor of surface UV radiation and atmospheric oxidants

Abstract The SPICAM-light optical package on the ESA Mars-Express mission includes a UV radiation channel to perform nadir and limb observation between 118 and 320 nm as well as an infrared channel extending between 1 and 1.7 μm . The UV resolution will be 0.5 nm per pixel. This spectral range was chosen for the observation of the total column and vertical distribution of ozone, aerosols and other gases; it is also aimed at the Martian upper atmosphere where the obtained data will help to constrain the escape problem. The associated infrared channel specialises in water vapour. The UV range corresponds exactly to the UV-B and UV-C spectral domains and will be used to determine the solar mid-UV radiation received at the surface and to establish its climatology. Signal simulations are performed using the results of a 2-D Martian model as input (Moreau, Thesis, Universite Libre de Bruxelles, Brussels, Belgium 1995; Moreau, Fonteyn, Two-dimensional study of the atmospheric and surface oxidants on Mars, Proceedings of the Fifth International Conference on Mars, Lunar and Planetary Institute Contribution, Vol. 972, 1999, p. 6093) in order to show the influence of ozone and aerosol distributions on this radiation. Possibility of a UV shield exists during extreme polar ozone maxima as well as during dust storms, due to the UV absorption of Martian dust. In the other more normal cases, the daytime mid-UV radiation between 200 and 320 nm penetrates in sufficient amount to prevent the development of surface micro-organisms. Other life science conclusions could be drawn from the SPICAM-light observations: the direct mapping of water vapour will constrain the possible occurrences of subsurface liquid water that could be a favourable environment for the survival of life. SPICAM-light by its mapping of ozone and water vapour will also allow determining the amount of oxidants and especially OH. These could also affect the survival of Martian or contaminating bacteria. It is also intended to achieve a sufficient signal quality to measure an upper limit of lower atmospheric H2O2 and NO which are also related in diverse ways to living processes.

Image compression based on a multipoint Taylor series representation

An image compression/decompression method is proposed, based on a new interpolation formula of Hermite-Birkhoff type. It is based on a function representation that uses a truncated multipoint Taylor (MT) series. Given a regularly tabulated image, first spatial derivatives (up to a chosen order) are computed by a FFT algorithm. Secondly a knot placing strategy selects sample point positions. The image values and spatial derivatives at these sample points are stored as the compressed data. Decompression is achieved by applying the MT interpolant to the compressed data. The quality of compression is user controlled by specifying the maximum order of spatial derivatives and the number of knots to be used. The method is found to be superior to JPEG in both reduction factor and quality, when reasonable smooth data is to be compressed with high accuracy. In this case, data reduction ratios in the range 10-100 can be obtained with a root mean square reconstruction error of the order of 1-2%. For compression of real-life color images it is inferior to JPEG and yields reduction ratios of about 4, at the threshold of notable picture degradation to the eye. A number of compression/decompression examples are shown.