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MIPAS Data Formats Products
2 MDSR per MDS 1 forward sweep 1 reverse sweep
2 MDSRs per MDS 1 forward sweep 1 reverse sweep
LOS calibration GADS
Spectral Lines MDS
P T Retrieval MW ADS
VMR Retrieval Parameters GADS
P t Retrieval GADS
Framework Parameters GADS
Processing Parameters GADS
Inverse LOS VCM matrices MDS
General GADS
Occupation matrices for vmr#1 retrieval MDS
MDS2 -- 1 mdsr forward sweep 1 mdsr reverse
Occupation matrices for p T retrieval MDS
General GADS
Priority of p T retrieval occupation matices
P T occupation matrices ADS
Summary Quality ADS
Instrument and Processing Parameters ADS
Microwindows occupation matrices for p T and trace gas retrievals
Scan information MDS
Level 2 product SPH
MDS1 -- 1 mdsr forward sweep 1 mdsr reverse sweep
H2O Target Species MDS
P T and Height Correction Profiles MDS
Continuum Contribution and Radiance Offset MDS
Structure ADS
Summary Quality ADS
Residual Spectra mean values and standard deviation data ADS
PCD Information of Individual Scans ADS
Instrument and Processing Parameters ADS
Microwindows Occupation Matrices ADS
Scan Information MDS
1 MDSR per MDS
Scan Geolocation ADS
Mipas Level 1B SPH
Calibrated Spectra MDS
Structure ADS
Summary Quality ADS
Offset Calibration ADS
Scan Information ADS
Geolocation ADS (LADS)
Gain Calibration ADS #2
Gain Calibration ADS #1
Level 0 SPH
DSD#1 for MDS containing VMR retrieval microwindows data
DSD for MDS containing p T retrieval microwindows data
VMR #1 retrieval microwindows ADS
P T retrieval microwindows ADS
1 MDSR per MDS
VMR profiles MDS (same format as for MIP_IG2_AX)
Temperature profiles MDS (same format as for MIP_IG2_AX)
Pressure profile MDS (same format as for MIP_IG2_AX)
P T continuum profiles MDS (same format as for MIP_IG2_AX)
GADS General (same format as for MIP_IG2_AX)
Level 0 MDSR
Values of unknown parameters MDS
Computed spectra MDS
Jacobian matrices MDS
General data
Data depending on occupation matrix location ADS
Microwindow grouping data ADS
LUTs for p T retrieval microwindows MDS
GADS General
P T retrieval microwindows ADS
ILS Calibration GADS
Auxilliary Products
MIP_MW1_AX: Level 1B Microwindow dictionary
MIP_IG2_AX: Initial Guess Profile data
MIP_FM2_AX: Forward Calculation Results
MIP_CS2_AX: Cross Sections Lookup Table
MIP_CS1_AX: MIPAS ILS and Spectral calibration
MIP_CO1_AX: MIPAS offset validation
MIP_CL1_AX: Line of sight calibration
MIP_CG1_AX: MIPAS Gain calibration
MIP_SP2_AX: Spectroscopic data
MIP_PS2_AX: Level 2 Processing Parameters
MIP_PS1_AX: Level 1B Processing Parameters
MIP_PI2_AX: A Priori Pointing Information
MIP_OM2_AX: Microwindow Occupation Matrix
MIP_MW2_AX: Level 2 Microwindows data
MIP_CA1_AX: Instrument characterization data
Level 0 Products
MIP_RW__0P: MIPAS Raw Data and SPE Self Test Mode
MIP_NL__0P: MIPAS Nominal Level 0
MIP_LS__0P: MIPAS Line of Sight (LOS) Level 0
Level 1 Products
MIP_NL__1P: MIPAS Geolocated and Calibrated Spectra
Level 2 Products
MIP_NLE_2P: MIPAS Extracted Temperature , Pressure and Atmospheric Constituents Profiles
MIP_NL__2P: MIPAS Temperature , Pressure and Atmospheric Constituents Profiles
Glossaries of technical terms
Level 2 processing
Miscellaneous hardware and optical terms
Spectrometry and radiometry
Data Processing
Alphabetical index of technical terms
Frequently Asked Questions
The MIPAS Instrument
Inflight performance verification
Instrument characteristics and performances
Preflight characteristics and expected performances
Subsystem description
Payload description and position on the platform
MIPAS Products and Algorithms
Data handling cookbook
Characterisation and calibration
Latency, throughput and data volume
Auxiliary products
Level 2
Instrument specific topics
Algorithms and products
Level 2 products and algorithms
The retrieval modules
Computation of cross-sections
Level 1b products and algorithms
Calculate ILS Retrieval function
Level 1a intermediary products and algorithms
Product evolution history
Definition and convention
MIPAS Products User Guide
Image gallery
Further reading
How to use MIPAS data?
Summary of applications and products
Peculiarities of MIPAS
Geophysical coverage
Principles of measurement
Scientific background
MIPAS Product Handbook
Site Map
Frequently asked questions
Terms of use
Contact us


1.1.2 Scientific background

Early in 1987 the European Space Agency asked a group of experts, the LISA (Limb Sounder of the Atmosphere) Consultancy Group, to assess the scientific utility of a limb sounder flown on a polar platform, to review and contrast the available technologies for limb sounding in the various parts of the electromagnetic spectrum, and to prepare outline specifications of a suitable instrument for use in a concept study.

The Group highlighted three major areas on which to focus remote sensing of the middle atmosphere in the late 1990s ( ESA 1992 Ref. [1.34 ] ), reaching the following conclusions:

Atmospheric Chemistry there was a clear need to increase understanding of the processes which control the distribution of trace species in the middle atmosphere.

Climatology it was necessary to monitor the concentrations of trace species whose temporal changes affect the Earth's climate by modifying radiative transfer.

Operational Meteorology it was important to recognise the influence of the dynamic and radiative state of the lower stratosphere and upper troposphere on the atmosphere as a whole and hence its importance to operational meteorology.

To satisfy the scientific requirements defined above it is necessary to measure simultaneously a considerable number of species. To achieve this a set of instruments, operating in different parts of the spectrum, is essential. Light molecules with permanent dipole moments (such as OH, HO2, ClO) have rotational features in the far infrared or microwave regions. Heavier molecules, such as NO2 and HNO3, can be identified and measured by analysing spectral features in the mid-infrared.

The Group concluded that the most appropriate type of instrument for such a mission would be a limb sounder capable of observing emitted radiances from the atmosphere. To cover the requisite wide spectral range this needed to be an interferometer. However, at the same time, it was clear that no one single instrument could be expected to adequately cover the complete spectral range required. The Group therefore recommended the provision of a complementary package of three instruments (mid-infrared, far-infrared and microwave) as the core of a middle atmosphere chemistry mission flown on a polar orbiting satellite. In so doing it highlighted the need for the measurements to be simultaneous (all related species), to be global in extent (pole to pole) and to cover day and night (diurnal variations). Heritage of MIPAS

In deciding on the mission objectives for MIPAS and to focus on the mid-infrared, IMK not only took the views of the LISA Group into account but also the following:

¢ Within this part of the spectrum there is a wide variety of important molecules which have vibration-rotation spectra with absorption lines well suited for detection, so a large group of trace gases (e.g. the whole NOy trace gas family, including the source gas N2O) should be accessible to an instrument operating in this part of the spectrum.

¢ Atmospheric signals are generally higher here than in other parts of the spectrum because the location of the maximum of the Planck function (at 250 K) is at about 11 µm.

¢ Generally instruments working in the mid-infrared can be significantly smaller than those operating in the far-infrared. This is dictated by diffraction limits and the high spectral resolution needed to observe the chemical species of interest.

Another advantage of the mid-infrared is that instruments operating in this region can, in principle, be calibrated by observing cold space and black bodies. This gives them a significant advantage over those operating in the ultraviolet/visible region where reference calibration targets are not readily accessible.

The concept underlying the space version of MIPAS draws on the experience gained from several experiments exploiting Fourier transform spectrometers. In particular, the MIPAS-B (balloon) experiment can be regarded as a precursor of the MIPAS satellite experiment even if the type of interferometer is not exactly the same (see Fischer 1992 Ref. [1.38 ] ). Since 1989 MIPAS-B has been successfully operated during several field experiments held in southern France as well as in Kiruna, northern Sweden (figure1.1 ). The corresponding measurements have established the feasibility of detecting high quality emission spectra in the mid-infrared with the aid of a moderately cooled interferometer, i.e. sufficient sensitivity can be achieved by cooling the optical system to 200 K and the detectors to liquid Helium temperatures ( Friedl-Vallon et. al. 1992 Ref. [1.40 ] , Fischer and Oelhaf 1996 Ref. [1.36 ] ).

Figure 1.1 The MIPAS balloon gondola in Kiruna, North Sweden,1995

An instrument similar to MIPAS-B was flown on a Transall aircraft (MIPAS-FT) during the first half of the '90s ( Gulde et al. 1994 Ref. [1.43 ] ). These experiments showed quite clearly that even strong vibrations caused by an aircraft cannot seriously disrupt the operation of this type of interferometer. Strongly disturbed Phase-And magnitude spectra were corrected using the so-called double differencing method ( Blom et al. 1996 Ref. [1.3 ] ). Both types of experiment (i.e. balloon and aircraft) have helped establish the feasibility of MIPAS. Basic knowledge about interferometers gained from Fourier spectrometers measuring the attenuation of solar radiation in the atmosphere has also been taken into account during the development of the MIPAS space experiment. Here specific mention must be made of the ATMOS instrument which has yielded simultaneous measurements of a large number of trace constituents in the middle atmosphere ( Farmer et al. 1987 Ref. [1.35 ] ). A similar type of instrument has flown several times on a balloon platform and been used to investigate dynamical and chemical processes in the lower stratosphere ( Camy-Peyret et al. 1993 Ref. [1.10 ] ).

Analyses of MIPAS-B data have confirmed that cooled Michelson interferometers, operating in the mid-infrared, can observe many trace species simultaneously ( Fischer and Oelhaf 1996 Ref. [1.36 ] ). Vertical profiles of a large number of trace species have been derived, notably O3, H2O, HDO, CH4, N2O, CFCl3, CF2Cl2, CHF2Cl, CCl4, CF4, NO2, HNO3, HNO4, N2O5 and ClONO2; this despite the fact that the balloon instrument did not cover the whole of the mid-infrared ( Fischer 1992 Ref. [1.38 ] , von Clarmann et al. 1994 Ref. [1.21 ] , Oelhaf et al. 1994 Ref. [1.55 ] , von Clarmann et al. 1995 Ref. [1.20 ] , Wetzel et al. 1995 Ref. [1.66 ] ). In addition, ClO and HOCl concentrations have been estimated under disturbed chemistry conditions. Data from the MIPAS-FT has provided new information on the horizontal distributions of various trace gases of relevance to stratospheric ozone research ( Blom et al. 1994 Ref. [1.5 ] , Blom et al. 1995 Ref. [1.4 ] , Hopfner et al. 1996 Ref. [1.44 ] ). Atmospheric Chemistry

In many ways, the study of the stratosphere is the study of ozone and ozone-related chemistry. Infrared absorption and emission by ozone is a significant component in the radiation budget of the stratosphere, and is part of the Greenhouse Effect. Thus the absorption of shortwave radiation by ozone in the stratosphere is responsible for the temperature inversion which defines the height of the troposphere, the lowest part of the atmosphere where most biological activity takes place and where weather resides.

This inversion acts as a cap on vertical motion, limiting (but not stopping) the movement of water vapour and trace species into the stratosphere. The warming of the stratosphere, resulting from the absorption of solar radiation by ozone, controls air motion over a range of spatial scales. The restriction on vertical motion in the lower atmosphere, imposed by the stability of the stratosphere, has fundamental and wide-ranging effects on the global-scale circulation in the lower atmosphere. The limits on the vertical extent of convective activity, coupled with the influence of the Coriolis acceleration imposed by the Earth's rotation, determine the global pattern of zonal winds.

Ozone was first discovered to be present in the atmosphere in the mid-nineteenth century because the absorption of ultraviolet radiation by ozone in the stratosphere causes a sharp cut-off in levels of solar radiation reaching the ground. This occurs in the near-ultraviolet toward shorter wavelengths (i.e.325 nm). This absorption was measured and used to estimate the total amount of ozone in the atmosphere. It led to the discovery that nearly all the ozone in the atmosphere is to be found well above the Earth's surface.

With the publication of the Chapman theory for the photochemical production of ozone in the upper atmosphere ( Chapman 1930 Ref. [1.16 ] ), the primary processes involved in the production of ozone and the establishment of its equilibrium vertical profile were enunciated. As first proposed by Chapman ( Chapman 1930 Ref. [1.16 ] ), ozone is created in the stratosphere as a result of the dissociation of molecular oxygen by ultraviolet radiation according to the reaction equation:

    O2 + hv “> O + O eq 1.1

where the reaction of "O2 + hυ"represents the absorption of a photon of light. This reaction is followed by:

    O2 + O + M “> O3 + M         eq 1.2

where M is another molecule (probably O2 or N2) which allows the reaction to occur by absorbing excess energy and momentum.

Ozone is destroyed when it absorbs radiation shorter than 1.18 µm:

    O3 + hv + M “> O2 + O + M         eq 1.3

The last two of these three reactions comprise a fast reaction cycle that neither destroys nor produces ozone, but which injects a large amount of energy into the stratosphere. Because of the speed of these two reactions, O and O3 are, to a certain extent, 'equivalent' and their total concentration, [O] + [O3], is often referred to as "odd oxygen". It is this absorption of ultraviolet energy by ozone, as represented by these equations, which is responsible for the temperature structure and consequent vertical stability of the stratosphere.

The primary production process (equation eq. 1.1 ) is balanced by reactions in which ozone is destroyed, such as:

    O + O3 “> 2 O2         eq 1.4

As levels of oxygen decrease with height the absorption of photons in reaction 1 also decreases with height. Also, reaction 2 decreases with increasing height as the atmospheric density decreases, producing a level of maximum ozone in the stratosphere.

However, to achieve quantitative agreement with observed ozone profiles, many more ozone destroying reactions, plus some other minor source terms, must be included in the chemical scheme. These include reactions with water-related radicals (OH and HO2), nitrogen compounds (NO and NO2), chlorine compounds (Cl and ClO), bromine compounds (Br and BrO) and others. It is now known that more than 100 reactions and dozens of chemical species must be included for a chemical model of the stratosphere to calculate ozone amounts with reasonable accuracy over the whole globe. Furthermore, there are relatively few regions in the atmosphere, particularly those with the highest ozone levels, where the local concentration of ozone is determined by local photochemical equilibrium alone.

Despite of the fact that much of the basic knowledge of the stratosphere was developed several decades ago, the science of ozone is still far from being completely understood. Indeed, the recent (since 1985) development of the Antarctic ozone hole (figure1.2 ) and contemporary observations of very low ozone amounts in the Arctic are stimulating active research in the field. There are a number of outstanding scientific questions which can be addressed with the aid of MIPAS data.

imagefull size
Figure 1.2 Arctic March total ozone (monthly means) observed in 1980 and 1982 by TOMS/Nimbus (top) and in 1996 and 1997 by GOME / ERS-2 (bottom) document the decline in polar total ozone in spring (source: WMO 1998)

Although, as indicated above, the general features of the stratospheric ozone layer and the global ozone budget now appear to be relatively well understood, there are a number of observations which are not properly explained by current scientific theory. Chemical and temperature data, particularly chemical tracer data, from MIPAS will contribute to further research in this area.

It is believed that the chemistry and dynamics of the stratosphere are qualitatively quite well understood. However, there are significant quantitative differences between the predictions of current models and observations of the distribution of stratospheric constituents which may have significant implications on the accuracy of long-term predictions of ozone change. In the lower stratosphere, the decline in ozone amounts at mid-latitudes has been roughly double the amount predicted by models, though otherwise there is in good agreement throughout most of the atmosphere. These differences are probably a result of insufficient (or inaccurate) knowledge of the chemical, microphysical and dynamical state of the region. In particular, stratosphere-troposphere exchange processes are not well understood on the global scale. MIPAS is expected to provide additional insight in the mechanisms involved.

Changes are taking place in the ozone layer in the Arctic which seem to lie outside the predictions of current models. Since MIPAS will provide observations of many species, as well as of temperature, in the polar night, its data set will contribute significantly to work in this area. The relatively high vertical resolution of the MIPAS data set will permit smaller scale chemical effects to be studied over the whole globe. These observations may be crucial to advancing understanding of the effects of heterogeneous chemistry (i.e. chemistry on surfaces or in the liquid phase in droplets) which is responsible for much of the downward trend in total ozone levels.

Also, the MIPAS experiment should help to provide a baseline for the (future) monitoring of constituents involved in climate change. For example, a number of radiatively active gases, such as the CFCs, ozone and water vapour, will be measured with high accuracy over the whole globe by MIPAS.

Finally, the sensitivity of the MIPAS instrument will allow the observation of important atmospheric parameters in the mesosphere and lower thermosphere, namely the temperature, H2O, CH4, CO, CO2, O3 and NO. Based on these measurements several research aspects such as the ozone deficit in the lower mesosphere, the energy balance in this region, dynamic processes in the mesosphere can be investigated in some detail. Taking into account the good spectral resolution of MIPAS manifold studies of the non-Local Thermodynamic Equilibrium (LTE) in the middle and upper atmosphere can be performed with the measured spectra.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry