1. ISSI Atmospheric limb remote sounding by star occultation Dr. Didier FUSSEN Belgian Institute for Space Aeronomy 3, avenue Circulaire B-1180 Brussels

2. CONTEXT & GENERALITIES
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 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....
circular
h: 508 km
inclination: 28°
spacecraft:
speed: 7.6 km/s
period: 95 minutes
30 occultations per 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 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)

12. Orbital motion Envisat, Gomos GOMOS
Global Ozone Monitoring by Occultation of Stars
8140 kg
Look ! It’s
Gomos
10 m
Envisat, Gomos
Orbital motion
Side of Envisat always looking the Earth
Gomos looks star occultations at the limb
15 m
70 m2 of solar arrays
GOMOS
Envisat was
launched on the
first of March 2002

13. Occultations may be vertical, oblique or « tangent »
orbit
pole
Altair
Arcturus
Envisat orbit
Seginus
Vega
Arcturus (alpha Bootis); Seginus (gamma Bootis); Vega (alpha Lyrae); Altair (alpha Aquilae)
Antarctic peninsula; Weddell sea (atmosphere and clouds removed); wide view angle (120°)
Apparent motion of stars : vertical and oblique occultations
Large choice of targets

14. Scintillation information
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

15. The Orion bell is setting

16. Retrieved species with altitude ranges
100 O3
NO2
NO3
Aero
Air
HRTP
H2O O2 90 80 70
SPE 60 50 x 40 30 ? 20 10

17. GOMOS instrument: optics and detectors
2 photometers: 1 kHz: HRTP nm nm
star tracker
100 Hz nm
grating
grating
1.2 nm
0.2 nm
0.2 nm
2 Hz
250 nm
675 nm nm nm
O3, NO2, NO3, air, aerosols O2
H2O

18. Some stars…
Hot star
Cold star

19. Transmission spectra

20. Dilution & Scintillation
atmosphere
observation
direction  of  the  light 
scintillation
Twinkle, twinkle
fluctuations
Regular change of refraction with altitude : dilution (increase with distance) light curve
Added fluctuations : dilution + scintillation (increase with distance) light curve : peaks larger than 1
Focus and defocus : No absorption included here
effect of refraction only
no absorption
Atmospheric lens  Intensity spike

21. Photometers data dilution, scintillation, & absorption
absorption scattering Rayleigh Mie
Smooth amplitude sliding average 0.5 s; correction of photometers
non-linear altitude scale
Impossible to recognize individual spikes : use of cross correlation for series of spikes

22. The chromatic refraction does allow for the retrieval of a temperature profile with a high vertical resolution
The refraction angle  is proportional to the refractivity  which is proportional to the density gradient B  R
 tR
time delay t  
 tB
Two rays with the same minimal altitude : cross the same lens, produce the same spike
Monochromatic -> bichromatic -> finite bandwidth
Color spectra are (approximately) realistic ; Refractivity and dispersion (strongly) exaggerated
B =   (B/)

23. Sample high resolution temperature profile and comparison with lidar
Possibility to reprocess lidar data in order to increase the resolution
Small scale fluctuations are NOT errors
Resolution smaller than 200m are meaningless because of spherical symmetry hypothesis
Example

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

25. NO2
50N -
90N
90S -
50S

26. Stratospheric cloud detection

28. Aerosol ext. 500 nm vs. altitude and time
PSC descent
Cirrus up-down oscillation
Temperature at the considered altitude above tropical regions can fall below 195K. No significantly stronger extinction (NAT) has been found.

29. PSC descent rates -1.2 -1.3 -1.0 -2.0 -1.8 -1.5 -2.5 -2.4 -2.7
SCIAMACHY
2003 (*)
[km/month]
GOMOS
2003
2004
70°S - 80°S
-1.2
-1.3
-1.0
60°S – 70°S
-2.0
-1.8
-1.5
50°S – 60°S
-2.5
-2.4
-2.7
(*)Von Savigny et al, Atmos. Chem.Phys., 5, , 2005
PSCs descend faster at the edge of the vortex…

30. Intercomparison with ACE data...

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

32. Equatorial clouds (h ~ 16 km)

33. 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

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

37. 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 , [16 April 2004]
adaptive optics …

38. Na slant path optical thickness can be extracted by simple DOAS technique…

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

41. 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

42. 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 ...