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Atmospheric Limb Remote Sounding by Star Occultation

This talk explores the remote sounding techniques used at BIRA-IASB to illustrate the coexistence of technical aspects and scientific questions. It includes an overview of occultation technique, vertical and spectral inversions, and the use of GOMOS for global ozone monitoring. The talk also discusses the importance of mesospheric metallic layers and potential objectives for LYRA occultation data.

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Atmospheric Limb Remote Sounding by Star Occultation

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  1. 29-Nov-2006 @ ISSIAtmospheric limb remote sounding by star occultationDr. Didier FUSSENBelgian Institute for Space Aeronomy3, avenue CirculaireB-1180 Brussels Didier.Fussen@oma.be

  2. Outline • CONTEXT & GENERALITIES • ORA • GOMOS • FUTURE • CONCLUSIONS Advertisement: this talk is not aimed at presenting the present atmospheric state or trend. Rather, it is a short walk around some remote sounding techniques used @ BIRA-IASB to illustrate that technical aspects and scientific questions may cohabit...

  3. GENERALITIES • The simplest example of remote sounding: a sunset! Do the same from space, above and through the atmosphere: this is the OCCULTATION technique... • A relative measurement produces an absolute quantity: the slant optical thickness. The occultation technique is SELF-CALIBRATING. • The occultation technique leads, as in many fields, to several INVERSE PROBLEMS: vertical inversion, spectral inversion, optical inversion...

  4. Vertical inversion ONION PEELING...

  5. Spectral inversion For most constituents: For aerosols: O3 Aerosol O3 NO2 Rayleigh

  6. M(λ)=aN2τN2(λ)+ a03τ03(λ)+ aNO2τNO2(λ)+... Solve a linear system : C a=b • random error -> • bias error ->

  7. ORA onboard EURECA(European Retrievable Carrier) • 1992/1993 In memoriam E. Arijs

  8. Orbital sunsets and sunrises observed from a low inclination orbit.... • orbit: • circular • h: 508 km • inclination: 28° • spacecraft: • speed: 7.6 km/s • period: 95 minutes • 30 occultationsper day latitudes: 40°S - 40°N

  9. A solar occultation experiment developed at IASB : ORA • 8 channels (259 nm -> 1013 nm): O3, NO2, Aerosols • Aug. 1992 - May 1993 / Coverage 40°S-40°N / 7000 occultations • see Fussen et al., Applied Optics, 37, 3121 [1998]

  10. The Sun has a large apparent size (25 km) but produces a very intense signal …The final obtained vertical resolution was about 2-5 km The vertical resolution (related to the « averaging kernel ») is a meaningless concept if dissociated from the measurement error (related to the signal-to-noise ratio)

  11. Envisat, Gomos Global OzoneMonitoring by Occultation of Stars Look ! It’s Gomos GOMOS 8140 kg 10 m Orbital motion 15 m 70 m2 of solar arrays Envisat was launched on the first of March 2002

  12. Occultations may be vertical, oblique or « tangent » orbit pole Altair Arcturus Envisat orbit Seginus Vega

  13. GOMOS principle (1) Below 10 km Up to 120 km One star spectrum every 0.5 s Pointing information 100 Hz Scintillation information 1000 Hz

  14. The Orion bell is setting

  15. Retrieved species with altitude ranges 100 90 80 70 SPE 60 50 x 40 30 ? 20 10

  16. GOMOS instrument: optics and detectors 2photometers: 1 kHz: HRTP 650-700 nm 625-959 nm star tracker 100 Hz 470-520 nm grating grating 1.2 nm 0.2 nm 0.2 nm 2 Hz 250 nm 675 nm 926-952 nm 756-773 nm O3, NO2, NO3, air, aerosols H2O O2

  17. Some stars… Hot star Cold star

  18. Transmission spectra

  19. Dilution & Scintillation Twinkle, twinkle dilution atmosphere observation direction  of  the  light  scintillation fluctuations effect of refraction only no absorption Atmospheric lens  Intensity spike

  20. Photometers data dilution, scintillation, & absorption absorptionscatteringRayleighMie

  21. The chromatic refraction does allow for the retrieval of a temperature profile with a high vertical resolution R B  The refraction angle  is proportional to the refractivity  which is proportional to the density gradient tR time delay t   tB B =   (B/)

  22. Sample high resolution temperature profile and comparison with lidar Example

  23. Ozone mixing ratio(20 days median) in 20S-20N

  24. NO2 50N - 90N 90S - 50S

  25. PSC descent rates (*)Von Savigny et al, Atmos. Chem.Phys., 5, 3017-3079, 2005 PSCs descend faster at the edge of the vortex…

  26. Intercomparison with ACE data...

  27. PSC’s with mono- or bimodal structure can be distinguished...

  28. Equatorial clouds (h ~ 16 km)

  29. GOMOS serendipitous data products .... for each measurement: inversion + regularization (higher resolution/low SNR) -> individual profiles -> climatologies CHANGE RETRIEVAL STRATEGY TO transmittance binning (lower resolution/high SNR) -> unregularized inversion

  30. MESOSPHERIC SODIUM LAYER Meteoric metal layers (Fe, Na, K, Ca,..) formed by ablation of shooting stars. Destruction by recombination with O, O2, O3.

  31. Geophysical importance of mesospheric metallic layers • wind/waves dynamical signatures • Metallic layers could be the source of nucleation seed of polar mesospheric clouds[« Removal of Meteoritic Iron on Mesospheric Clouds » by Plane et al., Science, vol 304, pp 426-428, [16 April 2004] • adaptive optics …

  32. The importance of the mesospheric layer for astronomy: let’s use sodium beacons!

  33. Possible objectives for LYRA occultation data • mesospheric-thermospheric O2 , mesospheric O3 (Herzberg and tail of Hartley band) • water vapor (?) and Polar Mesospheric Clouds • Ly alpha: thermospheric O2 (Schumann-Runge) • ionic and atomic species below 100 nm

  34. Conclusions • The remote sounding of the atmosphere (Earth and planets) by the occultation technique is elegant and efficient. • Main advantages: self-calibration, vertical resolution, altitude registration, diurnal cycles • Main drawbacks: weak SNR (stars), low coverage (Sun), scintillation (stars) • It is important to monitor the upper atmosphere BUT it is essential to continue its exploration. This is just science. All « Hot topics » have been « cold » in the past! • Waiting for LYRA/SWAP on PROBA 2 ...

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