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Publications (26)58.79 Total impact

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    ABSTRACT: The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European platform ENVISAT (ENVironment SATellite) was dedicated to the study of the of Earth's atmosphere using the stellar occultation technique. The spectral range of the GOMOS spectrometer extends from the UV (ultra violet) to the near infrared, allowing for the retrieval of species such as O3, NO2, NO3, H2O, O2, air density, aerosol extinction and OClO. Nevertheless, OClO cannot be retrieved using a single GOMOS measurement because of the weak signal-to-noise ratio and the small optical thickness associated with this molecule. We present here the method used to detect this molecule by using several GOMOS measurements. It is based on a two-step approach. First, several co-located measurements are combined in a statistical way to build an averaged measurement with a higher signal-to-noise ratio; then, a differential optical absorption spectroscopy (DOAS) method is applied to retrieve OClO slant column densities (SCD). The statistics of the sets of GOMOS measurements used to build the averaged measurement and the spectral window selection are analyzed. The obtained retrievals are compared to results from two balloon-borne instruments. It appears that the inter-comparisons of OClO are generally satisfying (relative differences are about 15-60%). Two nighttime climatologies of OClO based on GOMOS averaged measurements are presented. The first depicts annual global pictures of OClO from 2003 to 2011. From this climatology, the presence of an OClO SCD peak in the equatorial region at about 35 km is confirmed and strong OClO SCD in both polar regions are observed (more than 1016 cm-2 in the Antarctic region and slightly less in the Arctic region), a sign of chlorine activation. The second climatology is a monthly time series. It clearly shows the chlorine activation of the lower stratosphere during winter. Moreover the equatorial OClO SCD peak is observed during all years without any significant variations. This very promising method, applied on GOMOS measurements, allowed us to build the first nighttime climatology of OClO using limb-viewing instruments.
    Atmospheric Measurement Techniques 11/2013; 6(11):2953-2964. · 3.21 Impact Factor
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    ABSTRACT: The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European platform ENVISAT was dedicated to the study of the atmosphere of the Earth using the stellar occultation technique. The spectral range of the GOMOS spectrometer extends from the UV to the near infrared, allowing for the retrieval of species such as O3, NO2, NO3, H2O, O2, air density, aerosol extinction and OClO. Nevertheless, OClO can not be retrieved using a single GOMOS measurement because of the weak signal-to-noise ratio and the small optical thickness associated with this molecule. We present here the method used to detect this molecule by using several GOMOS measurements. It is based on a two-step approach. First, several co-located measurements are combined in a statistical way to build an averaged measurement with a higher signal-to-noise ratio. Then, a Differential Optical Absorption Spectroscopy (DOAS) method is applied to retrieve OClO slant column densities. The statistics of the sets of GOMOS measurements used to build the averaged measurement and the spectral window selection are analyzed. The obtained retrievals are compared to results from two balloon-borne instruments. It appears that the inter-comparisons of OClO are generally satisfying. Then, two nighttime climatologies of OClO slant column densities based on GOMOS averaged measurements are presented. The first depicts annual global pictures of OClO from 2003 to 2011. From this climatology, the presence of an OClO layer in the equatorial region at about 35 km is confirmed and strong concentrations of OClO in both polar regions are observed, a sign of chlorine activation. The second climatology is a monthly time series. It clearly shows the chlorine activation of the lower stratosphere during winter. Moreover the equatorial OClO layer is observed during all the years without any significant variations. Finally, the anti-correlation between OClO and NO2 is highlighted. This very promising method, applied on GOMOS measurements, allowed us to build the first nighttime climatology of OClO.
    Atmospheric Measurement Techniques Discussions. 04/2013; 6(2):3511-3543.
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    ABSTRACT: This paper presents a climatology of the mesospheric sodium layer built from the processing of 7 years of GOMOS data. With respect to preliminary results already published for the year 2003, a more careful analysis was applied to the averaging of occultations inside the climatological bins (10° in latitude-1 month). Also, the slant path absorption lines of the Na doublet around 589 nm shows evidence of partial saturation that was responsible for an underestimation of the Na concentration in our previous results. The sodium climatology has been validated with respect to the Fort Collins lidar measurements and, to a lesser extent, to the OSIRIS 2003-2004 data. Despite the important natural sodium variability, we have shown that the Na vertical column has a marked semi-annual oscillation at low latitudes that merges into an annual oscillation in the polar regions,a spatial distribution pattern that was unreported so far. The sodium layer seems to be clearly influenced by the mesospheric global circulation and the altitude of the layer shows clear signs of subsidence during polar winter. The climatology has been parameterized by time-latitude robust fits to allow for easy use. Taking into account the non-linearity of the transmittance due to partial saturation, an experimental approach is proposed to derive mesospheric temperatures from limb remote sounding measurements.
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    ATMOSPHERIC CHEMISTRY AND PHYSICS 09/2010; 10:8873-8879. · 5.30 Impact Factor
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    ABSTRACT: The stellar occultation spectrometer GOMOS (Global Ozone Monitoring by Occultation of Stars) on ESA's Envisat satellite measures vertical profiles O<sub>3</sub>, NO<sub>2</sub> and NO<sub>3</sub> with a high long-term stability due to the self-calibrating nature of the technique. More than 6 years of GOMOS data from August 2002 to end 2008 have been analysed to study the inter-annual variation of O<sub>3</sub>, NO<sub>2</sub> and NO<sub>3</sub> in the tropics. It is shown that the QBO of the equatorial wind induces variations in the local concentration larger than 10% for O<sub>3</sub> and larger than 25% for NO<sub>2</sub>. Quasi-Biennial Oscillation signals can be found in the evolution of the three constituents up to at least 45 km. We found that NO<sub>3</sub> is positively correlated with temperature up to 40 km in the region where it is in chemical equilibrium with O<sub>3</sub>. Above 40 km, NO<sub>3</sub> is no more in equilibrium during night and its concentration is correlated with both O<sub>3</sub> and NO<sub>2</sub>. For O<sub>3</sub> and NO<sub>2</sub>, our results confirm the existence of a transition from a dynamical control of O<sub>3</sub> below 28 km with O<sub>3</sub> correlated with NO<sub>2</sub> and temperature and a chemical/temperature control between 28 and 38 km with O<sub>3</sub> anti-correlated with NO<sub>2</sub> and temperature. Above 38 km and up to 50 km a regime never described before is found with both O<sub>3</sub> and NO<sub>2</sub> anti-correlated with temperature. For the NO<sub>2</sub>/temperature anti-correlation, our proposed explanation is the modulation of the N<sub>2</sub>O ascent in the upper stratosphere by the QBO and the modulation of the Brewer-Dobson circulation. The oxidation of N<sub>2</sub>O is the main source of NO<sub>y</sub> in this altitude region. An enhancement of the ascending motion will cool adiabatically the atmosphere and will increase the amount of N<sub>2</sub>O concentration available for NO<sub>y</sub> formation.
    Atmospheric Chemistry and Physics 01/2010; · 5.51 Impact Factor
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    ABSTRACT: Although the retrieval of aerosol extinction coefficients from satellite remote measurements is notoriously difficult (in comparison with gaseous species) due to the lack of typical spectral signatures, important information can be obtained. In this paper we present an overview of the current operational nighttime UV/Vis aerosol extinction profile results for the GOMOS star occultation instrument, spanning the period from August 2002 to May 2008. Some problems still remain, such as the ones associated with incomplete scintillation correction and the aerosol spectral law implementation, but good quality extinction values are obtained at a wavelength of 500 nm. Typical phenomena associated with atmospheric particulate matter in the Upper Troposphere/Lower Stratosphere (UTLS) are easily identified: Polar Stratospheric Clouds, tropical subvisual cirrus clouds, background stratospheric aerosols, and post-eruption volcanic aerosols (with their subsequent dispersion around the globe). For the first time, we show comparisons of GOMOS 500 nm particle extinction profiles with the ones of other satellite occultation instruments (SAGE II, SAGE III and POAM III), of which the good agreement lends credibility to the GOMOS data set. Yearly zonal statistics are presented for the entire period considered. Time series furthermore convincingly show an important new finding: the sensitivity of GOMOS to the sulfate input by moderate volcanic eruptions such as Manam (2005) and Soufrière Hills (2006). Finally, PSCs are well observed by GOMOS and a first qualitative analysis of the data agrees well with the theoretical PSC formation temperature. Therefore, the importance of the GOMOS aerosol/cloud extinction profile data set is clear: a long-term data record of PSCs, subvisual cirrus, and background and volcanic aerosols in the UTLS region, consisting of hundreds of thousands of altitude profiles with near-global coverage, with the potential to fill the aerosol/cloud extinction data gap left behind after the discontinuation of occultation instruments such as SAGE II, SAGE III and POAM III.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2010; · 5.30 Impact Factor
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    ABSTRACT: The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument onboard the European Space Agency's ENVISAT satellite measures ozone, NO2, NO3, H2O, O2, and aerosols using the stellar occultation method. Global coverage, good vertical resolution and the self-calibrating measurement method make GOMOS observations a promising data set for building various climatologies and time series. In this paper we present GOMOS nighttime measurements of ozone, NO2, and NO3 during six years 2002-2008. Using zonal averages we show the time evolution of the vertical profiles as a function of latitude. In order to get continuous coverage in time we restrict the latitudinal region to 50° S-50° N. Time development is analysed by fitting constant, annual and semi-annual terms as well as solar and QBO proxies to the daily time series. Ozone data cover the stratosphere, mesosphere and lower thermosphere (MLT). NO2 and NO3 data cover the stratosphere. In addition to detailed analysis of profiles we derive total column distributions using the fitted time series. The time-independent constant term is determined with a good accuracy (better than 1%) for all the three gases. The median retrieval accuracy for the annual and semi-annual term varies in the range 5-20%. For ozone the annual terms dominate in the stratosphere giving early winter ozone at mid-latitudes. Above the ozone layer the annual terms change the phase which results to ozone summer maximum up to 80 km. In the MLT the annual terms dominate up to 80 km where the semiannual terms start to grow. In the equatorial MLT the semi-annual terms dominate the temporal evolution whereas in the mid-latitude MLT annual and semi-annual terms compete evenly. In the equatorial stratosphere the QBO dominates the time development but the solar term is too weak to be determined. In the MLT above 85 km the solar term grows significant and ozone has 15-20% dependence on the solar cycle. For NO2 below the maximum at 30 km the annual summer maxima dominates at mid-latitudes whereas in the equatorial region a strong QBO prevails. For NO3 the annual variation dominates giving rise to summer maxima. The NO3 distribution is controlled by temperature.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2010; 10(2):2169-2220. · 5.30 Impact Factor
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    ABSTRACT: The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument onboard the European Space Agency's ENVISAT satellite measures ozone, NO<sub>2</sub>, NO<sub>3</sub>, H<sub>2</sub>O, O<sub>2</sub>, and aerosols using the stellar occultation method. Global coverage, good vertical resolution and the self-calibrating measurement method make GOMOS observations a promising data set for building various climatologies and time series. In this paper we present GOMOS nighttime measurements of ozone, NO<sub>2</sub>, and NO<sub>3</sub> during six years 2002–2008. Using zonal averages we show the time evolution of the vertical profiles as a function of latitude. In order to get continuous coverage in time we restrict the latitudinal region to 50° S–50° N. Time development is analysed by fitting constant, annual and semi-annual terms as well as solar and QBO proxies to the daily time series. Ozone data cover the stratosphere, mesosphere and lower thermosphere (MLT). NO<sub>2</sub> and NO<sub>3</sub> data cover the stratosphere. In addition to detailed analysis of profiles we derive total column distributions using the fitted time series. The time-independent constant term is determined with a good accuracy (better than 1%) for all the three gases. The median retrieval accuracy for the annual and semi-annual term varies in the range 5–20%. For ozone the annual terms dominate in the stratosphere giving early winter ozone maxima at mid-latitudes. Above the ozone layer the annual terms change the phase which results in ozone summer maximum up to 80 km. In the MLT the annual terms dominate up to 80 km where the semiannual terms start to grow. In the equatorial MLT the semi-annual terms dominate the temporal evolution whereas in the mid-latitude MLT annual and semi-annual terms compete evenly. In the equatorial stratosphere the QBO dominates the time development but the solar term is too weak to be determined. In the MLT above 85 km the solar term grows significantly and ozone has 15–20% dependence on the solar cycle. For NO<sub>2</sub> below 32 km the annual summer maxima dominates at mid-latitudes whereas in the equatorial region a strong QBO prevails. In northern mid-latitudes a strong solar term appears in the upper stratosphere. For NO<sub>3</sub> the annual variation dominates giving rise to summer maxima. The NO<sub>3</sub> distribution is controlled by temperature and ozone.
    Atmospheric Chemistry and Physics 01/2010; · 4.88 Impact Factor
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    ABSTRACT: This paper presents a climatology of the mesospheric sodium layer built from the processing of 7 years of GOMOS data. With respect to preliminary results already published for the year 2003, a more careful analysis was applied to the averaging of occultations inside the climatological bins (10° in latitude-1 month). Also, the slant path absorption lines of the Na doublet around 589 nm shows evidence of partial saturation that was responsible for an underestimation of the Na concentration in our previous results. The sodium climatology has been validated with respect to the Fort Collins lidar measurements and, to a lesser extent, to the OSIRIS 2003–2004 data. Despite the important natural sodium variability, we have shown that the Na vertical column has a marked semi-annual oscillation at low latitudes that merges into an annual oscillation in the polar regions, a spatial distribution pattern that was unreported so far. The sodium layer seems to be clearly influenced by the mesospheric global circulation and the altitude of the layer shows clear signs of subsidence during polar winter. The climatology has been parameterized by time-latitude robust fits to allow for easy use. Taking into account the non-linearity of the transmittance due to partial saturation, an experimental approach is proposed to derive mesospheric temperatures from limb remote sounding measurements.
    Atmospheric Chemistry and Physics 01/2010; 10(19). · 4.88 Impact Factor
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    ABSTRACT: In this paper, we discuss the development of the inversion algorithm for the GOMOS (Global Ozone Monitoring by Occultation of Star) instrument on board the Envisat satellite. The proposed algorithm takes accurately into account the wavelength-dependent modeling errors, which are mainly due to the incomplete scintillation correction in the stratosphere. The special attention is paid to numerical efficiency of the algorithm. The developed method is tested on a large data set and its advantages are demonstrated. Its main advantage is a proper characterization of the uncertainties of the retrieved profiles of atmospheric constituents, which is of high importance for data assimilation, trend analyses and validation.
    Atmospheric Measurement Techniques 01/2010; · 3.21 Impact Factor
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    ABSTRACT: We present the first nighttime measurements of OClO from a limb-viewing satellite instrument in the Arctic polar vortex. The relationship between OClO, NO<sub>2</sub> and O<sub>3</sub> slant column densities in the Arctic polar vortex are analyzed from the GOMOS measurements. The retrieval process is based on a differential optical absorption spectroscopy (DOAS) method applied on the weighted median GOMOS transmittances. A study of the longitudinal distributions of OClO, NO<sub>2</sub> and O<sub>3</sub> above 65° north in January 2008 is presented. It shows a strong halogen activation in the lower stratosphere and a strong denoxification in the entire stratosphere inside the Arctic polar vortex. Time series of temperatures and OClO, NO<sub>2</sub> and O<sub>3</sub> slant column densities for the winters 2002/2003 to 2007/2008 are also presented. They highlight the correlation between temperature, OClO and NO<sub>2</sub>. The GOMOS instrument appears to be a very suitable instrument for the monitoring of OClO, NO<sub>2</sub> and O<sub>3</sub> in the stratosphere during nighttime.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 10/2009; 9:7857-7866. · 5.30 Impact Factor
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    ABSTRACT: GOMOS (Global Ozone Monitoring by Occultation of Stars), on ESA's Envisat-satellite launched in March 2002,measures transmission of light through the Earth's atmosphere in the limb direction using the stellar occultation method. Vertical density profiles of O3, NO2, NO3, H2O, O2 and aerosols in the stratosphere and mesosphere are retrieved from the transmission measurements. High-resolution temperature profiles are retrieved using data from the two fast photometers. In this presentation we show the main scientific results from GOMOS during the six years in operation. They include new global climatologies of stratospheric and mesospheric ozone and other stratospheric species (NO2, NO3, aerosols) and exciting new results on OClO, mesopause sodium layer, polar mesospheric clouds, gravity waves and turbulence and impact of energetic particles on polar NOy and ozone.
    AGU Fall Meeting Abstracts. 12/2008;
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    ABSTRACT: [1] Stellar scintillations observed through the Earth atmosphere are caused by air density irregularities generated mainly by internal gravity waves and turbulence. We present global analysis of scintillation variance in two seasons of year 2003 based on GOMOS/Envisat fast photometer measurements. Scintillation variance can serve as a qualitative indicator of intensity of small-scale processes in the stratosphere. Strong increase of scintillation variance at high latitudes in winter is observed. The maximum of scintillation variance can be associated with the polar night jet. The simplified spectral analysis has shown the transition of scintillation spectra toward small scales with altitude, which is probably related with turbulence appearing as a result of wave breaking. The breaking of gravity waves in the polar night jet seems to start in the upper stratosphere, a predicted, but not confirmed by observations before, feature. Weaker enhancements in tropics are also observed; they might be related to tropical convection.
    Geophysical Research Letters 02/2007; 34:3812. · 4.46 Impact Factor
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    ABSTRACT: GOMOS (Global Ozone Monitoring by Occultation of Stars) on ESA's Envisat-satellite measures transmission spectra of light through the Earth's limb using the stellar occultation method. From spectra it is possible to retrieve profiles of O3, NO2, NO3, H2O, O2, neutral density, and aerosols in the stratosphere and mesosphere. GOMOS has an UVIS spectrometer at 248-690 nm and two IR spectrometers at 750-776 nm and 916-956 nm. The spectrometers use 0.5 sec integration time, which provides 1.6 km or better vertical sampling resolution. Two photometers (1 KHz) record fast fluctuations of light (scintillations) caused by small-scale turbulence in the stratosphere. During 24 h GOMOS measures 300-500 occultations leading to a good global coverage. During the first three years of operation GOMOS has measured about 300 000 occultations. An extensive validation program has shown that ozone profiles agree well with various validating data. Using available GOMOS data it is now possible to build global nighttime profile distributions of some key constituents. GOMOS provides a unique view to the development of the ozone layer during the polar night. During the winter months in 2003 ozone was strongly depleted (compared to climatological values) over the southern and northern polar areas. The depletions were associated with strong increases of NO2. The development in the Arctic stratosphere was found to be linked with strong solar proton events in October-November 2003 (Seppälä et al., GRL, 2004 ) but no such an explanation was evident for the Antarctic case.
    AGU Fall Meeting Abstracts. 12/2005;
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    ABSTRACT: Stratospheric aerosols play an important role in a number of atmospheric issues such as midlatitude ozone depletion, atmospheric dynamics and the Earth radiative budget. Polar stratospheric clouds on the other hand are a crucial factor in the yearly Arctic and Antarctic ozone depletion. It is therefore important to quantify the stratospheric aerosol/PSC abundance. In orbit since March 2002, the GOMOS instrument onboard the European Envisat satellite has provided a vast aerosol extinction data set. In this paper we present aerosol/PSC zonal median values that were constructed from this data set, together with a discussion of the results.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2005; · 5.30 Impact Factor
  • 01/2005;
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    ABSTRACT: An assimilation system allowing combining the forecasts of a CTM with observations is used to assess the quality of GOMOS ozone measurements. Only a sub-sample of all GOMOS observations performed so far were available in the validation dataset that was available to the ACVT community. This allowed us to find that the average precision of GOMOS local O3 density retrievals is of the order of 5% in the low stratosphere. It has not been possible to draw any significant conclusions on the bias using this dataset and further investigations are required
    01/2004;
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    ABSTRACT: The GOMOS instrument on board the Envisat satellite is equipped with two fast photometers operating at 1 kHz sampling frequency in blue and red wavelengths. The bichromatic scintillations recorded by the photometers allow the determination of refractive angle, which is proportional to the time delay between the photometer signals. The high resolution density and temperature profiles (HRTP) can be reconstructed from these data with a high accuracy and with a vertical resolution of about 200 m. The HRTP products have been extensively validated since the Envisat launch in 2002. The validation includes statistical comparison with ECMWF data, validation with sounding and lidar measurements, and spectral analysis. In our presentation, we show the results of the validation. The GOMOS HRTP products are in good agreement with ECMWF data. The shape of spatial spectra of relative temperature fluctuations shows the theoretical slope -3, predicted by the model of saturated internal waves. These results can be considered as encouraging, but the algorithm can be further improved in order to obtain products useful for scientific studies and meteorological applications.
    01/2004;
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    ABSTRACT: The GOMOS (Global Ozone Monitoring by Occultations of Stars) instrument on board the Envisat satellite is the first operational instrument that uses stellar occultation technique to study the atmospheric composition between 15 and 100 km. The GOMOS products from the UV-Visible spectrometer are vertical profiles of ozone, neutral density, aerosols, NO2 and NO3. A global coverage, self-calibration, and a good vertical resolution are the strong features associated with the GOMOS measurements. In this work, the quality of constituent profiles retrieved from GOMOS measurements is investigated by careful error analysis and the characterization formalism. The specific property of stellar occultation measurements is the dependency of the signal-to-noise ratio on the stellar brightness and spectral class. In addition, occultations suffer from scintillation caused by the atmospheric turbulence below 45 km. The error characterization is therefore a challenging task. We will discuss the various modelling errors contributing to the total error budget: stratospheric turbulence, uncertainties in cross-sections, and the uncertainty in the atmospheric temperature. In this study, theoretical estimations are combined with the analysis of real measurement data. The vertical resolution of the retrieved profiles depends, in addition to the sampling resolution, also on the smoothing properties of the retrieval algorithm. We study the actual resolution of the GOMOS profiles via analysis of the averaging kernel. The sampling resolution of the GOMOS measurements varies strongly depending on occultation geometry: for significantly oblique occultations it can be almost twice better than for vertical ones. This allows the additional smoothing for oblique occultations, which suffer more from scintillation. The Tikhonov-type regularization according to the target resolution provides profiles having approximately the same actual resolution regardless the varying sampling resolution. The error analysis has shown that ozone profiles can be reconstructed with accuracy of better than 1-3% and resolution 1-3 km in the stratosphere and in the lower mesosphere. The best/worst/typical performances of NO2, NO3, aerosols and air density profiles are also shown.
    01/2004;
  • A. Mangin, O. Fanton D'Andon, J.-L. Bertaux, A. Hauchecorne
    01/2003;