1. COST 723 UTLS Summerschool Cargese, Corsica, Oct. 3-15, 2005 Stefan A. Buehler Institute of Environmental Physics University of Bremen www.sat.uni-bremen.de OBS 1: Microwave Limb Sounding

2. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 20052

3. 3 Overview Basics of limb sounding instruments Basics of the measurement Past, present, and future instruments Summary

4. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 20054 Overview Basics of limb sounding instruments Geometry Antenna Radiometry Basics of the measurement Past, present, and future instruments Summary

5. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 20055 h o :Platform altitude θ: Scan angle h t : Tangent altitude typically: R E = 6000 km h o = 600 km h t = 6-60 km Geometry Measure thermal radiation from the atmosphere (passive!) Good altitude resolution, because we can scan vertically

6. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 20056 Small ∆θ ↔ large ∆h t. Accurate Pointing necessary. Narrow Field of View necessary. (Figure: Oliver Lemke)

7. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 20057 Antenna Field of view diameter = “beam width”, even for passive instrument Given by angle of half power of received (or transmitted) radiation Diffraction theory: θ HPBW ~ Wavelength / Antenna size (Figure: Oliver Lemke)

8. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 20058 Antenna Technology θ HPBW ~ Wavelength / Antenna size Needs large antenna, particularly for low frequencies Scan angle small Needs accurate scanning mechanism (or wobble the whole satellite) The Odin reflector mounted on the spacecraft body. (Source: PREMIER mission proposal)

9. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 20059 The Radiometer Challenge The absolute power of thermal radiation in the mm- wave spectral range is low.

10. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200510 (Kraus, J. D., Radio Astronomy, McGraw-Hill Book Company, 1966)

11. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200511 The Radiometer Challenge The absolute power of thermal radiation in the mm- wave spectral range is low. (The peak of the Planck function is in the infrared.) Need to amplify the signal by many orders of magnitude for detection. No good amplifiers for frequencies above approximately 100 GHz (technology constantly moving) State of the art: Heterodyne Receivers

12. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200512 A Typical Heterodyne Radiometer RF = Radio frequency LO = Local oscillator IF = Intermediate frequency (Figure: Oliver Lemke)

13. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200513 The Heterodyne Principle Mixer generates signal with ν IF = | ν RF - ν LO | This can then be amplified and analyzed with a spectrometer Intensity unit: Brightness temperature = The temperature a black body would need to generate the same intensity of radiation (Figure: Oliver Lemke)

14. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200514 The Radiometer Formula T NET = Noise equivalent temperature (noisiness of individual measurement) T Sys = System noise temperature (characteristic noise of measurement system) ΔB= Frequency bandwidth Δt= Integration time

15. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200515 The Radiometer Challenge (2) We cannot make ΔB and Δt as large as we want. (We want spectral resolution, and the satellite flies by fast.) Need low noise receivers. Mixer and first amplifier most critical, because their noise is amplified by subsequent stages. Cool mixer to low operation temperature. Best: Superconducting (SIS) mixers at 4 K. Cooled HEMT amplifiers.

16. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200516 Submillimeter-wave Sensor at –269 C 640 GHz SIS Mixer 4 K Mechanical Cooler 4 K 20 K 100 K 0.4 mm SIS: Superconductor-Insulator-Superconductor Superconductive Device: Nb/AlOx/Nb SMILES (Figures: SMILES Team)

17. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200517 Overview Basics of limb sounding instruments Basics of the measurement Spectroscopy Limb Spectra Clouds Past, present, and future instruments Summary

18. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200518 Microwave Spectroscopy Absorption by a gas (Lambert-Beer’s Law):

19. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200519 Absorption coefficient α determined by Continua Electronic transitions (10 15 Hz, UV visible) Vibrational transitions (10 13 Hz, Infrared) Rotational transitions (10 11 Hz, mm / sub-mm)

20. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200520 Model: ARTS (www.sat.uni- bremen.de/arts)

21. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200521

22. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200522 Absorption coefficient α given by

23. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200523

24. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200524 Limb Spectra H2OH2OO3O3 (Emde et al., J. Geophys. Res., 109(D24), D24207, 2004)

25. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200525 Comparison to IR Limb Sounder Annual global mean probability of limb transmittance >3%, as estimated from ECMWF fields of temperature, water vapour, water and ice clouds sampled globally one day in ten over a year (Kerridge et al., ESA UTLS study final report). Clouds significant, but less critical than for IR

26. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200526 Overview Basics of limb sounding instruments Basics of the measurement Past, present, and future instruments UARS-MLS and MAS (past) EOS-MLS and Odin (present) SMILES and PREMIER (future) Summary

27. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200527 The first two Microwave Limb Sounders O 2 63 GHz H 2 O 183 GHz O 3 184 GHz ClO205 GHz MASMLS ‘Millimeterwave Atmospheric Sounder’‘Microwave Limb Sounder’ On the Space ShuttleOn the UARS satellite Atlas 1, March 1992 1991-1998 Atlas 2, April 1993 Atlas 3, November 1994

28. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200528

29. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200529

30. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200530

31. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200531 A sample MAS H 2 O Measurement MAS Water Vapor Band MMC 20075, 27.3.1992, 16.59 GMT 59° N 021° W, Tangent altitude range: 6-54 km Retrieval = Calculate trace gas profile from measured spectra Requires radiative transfer model = forward model

32. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200532 Retrieval by Optimal Estimation

33. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200533 H 2 O Retrieval Example

34. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200534 UARS/MLS Highlights Stratospheric ozone and chlorine chemistry research Impact of volcano eruptions on the stratosphere (SO 2 loading) Tropical dynamics (‘tape recorder’ effect in the tropical lower stratosphere). Publication overview at: http://mls.jpl.nasa.gov/joe/um_pubs.html

35. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200535 (Source: MLS Website)

36. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200536 (Source: MLS Website. See also: Read et al., Geophys. Res. Lett. 20, 1299- 1302, 1993.)

37. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200537 The Tropical LS Taperecorder Transit time from 100 to 46 hPa > 6 months (Data created by H. Pumphrey. See also: Mote et al. J. Geophys. Res., vol. 101, 3989-4006 [1996]) Deviation from mean VMR [ppm]

38. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200538 EOS MLS The next Generation of the Microwave Limb Sounder MLS Launched July 15, 2004, on the Aura satellite (Figure: MLS Webpage)

39. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200539 EOS MLS Observed Species (WATERS, et al.: EOS MLS ON AURA, IEEE GRS submitted 2005)

40. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200540 Odin Small satellite with just two limb sounders, one sub-mm, one UV-visible The whole platform is moved, not just the antenna Launched February 20, 2001 Frequencies: 118.25 - 119.25 GHz 486.10 - 503.90 GHz 541.00 - 580.40 GHz

41. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200541 JEM / SMILES SIS mixer, cooled to < 4 Kelvin Proof of concept for later instruments of this type Very accurate ozone and chlorine species data Measurements at 625 and 650 GHz ‘Superconducting Sub- Millimeter Wave Limb Emission Sounder’ Launch 2008 on the ‘Japanese Experimental Module’ (JEM) of the International Space Station

42. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200542 PREMIER Combined IR and sub-mm limb imagers Proposed to current ESA call, possible launch 2013 Focus on chemistry climate interaction in the UTLS Sub-mm measurements at 320-360 GHz Array detectors Main Products: O 3, H 2 O, CO, N 2 O, HNO 3, ClO, CH 4, CFC11, CFC12, C 2 H 6, SF 6

43. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200543 Overview Basics of limb sounding instruments Basics of the measurement Past, present, and future instruments Summary

44. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200544 Summary (1) Limb sounding is well suited for the measurement of trace gases in the stratosphere and upper troposphere. Particularly good for chlorine species and humidity. (Also many other trace gases.) Good altitude resolution (1–2 km). Horizontal resolution is ≈ 100 km. We look at emission lines of trace gases. Emission means continuous measurement, day & night.

45. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200545 Summary (2) Less affected by clouds than UV and IR techniques. Cirrus clouds must be taken into account in the UT. Past: UARS-MLS and MAS. Present: EOS-MLS and Odin. Future: SMILES? PREMIER? Third generation of MLS? Technological challenges are low noise receivers with large bandwidth. Antenna size is also always a cost factor.

46. Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 200546 Thanks for your attention. Questions?...