1 / 66

Millimeter Line Calibration

Millimeter Line Calibration. Bertrand Lefloch (IPAG, France). Introduction. Introduction : my first steps as a radioastronomer … . June 1988 : Commissioning of the Plateau de Bure antennas …. A pointed single dish - telescope does not image the sky !

eleazar
Download Presentation

Millimeter Line Calibration

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Millimeter Line Calibration Bertrand Lefloch (IPAG, France) IRAM Summer School 2011

  2. Introduction IRAM Summer School 2011

  3. Introduction : my first steps as a radioastronomer… June 1988 : Commissioning of the Plateau de Bure antennas… A pointed single dish-telescopedoes not image the sky ! Planets are used for pointing and calibration… Atmosphereis not alwayscooperativeat mm wavelengths… IRAM Summer School 2011

  4. Noise and Signal in Radioastronomy Astrophysicalsignals are extremelyweak : flux unit : 1Jy = 10-23 erg s-1 cm-2 Hz-1 sr-1 Radioastronomicalsignals are largelydominated by the noise of the electronicchain amplification by manyorders of magnitude requiredsensitive and stable detectors IRAM Summer School 2011

  5. Introduction : Instrumentation in Radioastronomy • In the mm and submmwindows, line observations are carried out by means of SIS heterodynereceivers, allowing a high spectral resolution : • R = Δn/n = 10 -7 •  velocity resolution Δv= 0.03 km/s • c= (kT/µmH ) ½ = 0.18 km/s (10K) • Comparison : • Optical/mid IR (H2) • Gratings in the optical : R = Δn/n = 10 -4 • VelocityresolutionΔv= 30 km/s • PACS : R = 1500 – 2000 • SPIRE : R = 40 - 1000 mm submm far IR mid IR (FXD’s talk) PACS A. Navarini’s talk Heterodynereceiversallow to study the gaskinematics (subsonic motions, turbulence) at a resolution out of reach to anyother instruments. IRAM Summer School 2011

  6. Introduction: Imaging with Single-DishTelescopes • Single-dishtelescopes are able to producehigh-quality images of molecularlinesatmillimeter and submillimeterwavelengths.: • More sensitive heterodynereceivers • (200  50K,… • on the way to the quantum limit ! ) • Heterodynearrays Direct comparison with optical/near IR images isnow possible and meaningful, allowing to bring more constraints on excitation conditions and source modelling. IRAM Summer School 2011

  7. Outline • Outline of the Lecture: • 0. Introduction • 1. Calibration • 2. HeterodyneDetection • 3. Spectral Line Observations: spectral surveys and mapping • 3.1 Observing modes and Time estimates (proposals !) IRAM Summer School 2011

  8. Calibration IRAM Summer School 2011

  9. The Framework Atmosphere corrected for atmospheric absorption Source Brightness Distribution Telescope Beam ⊗ IRAM Summer School 2009

  10. Whenwe talk of calibration, wewant to • Determine the properties of the antenna : antenna calibration (efficiency) • Characterize the behaviour of the receiver and the electronicchain (instrumental calibration) • Characterize the behaviour of the atmosphere • - How to convert Voltage (receiver output) into a Power (K) ? • - How to correct for the effect of the atmosphere ? • How to retrieve TBfrom TA’ ? IRAM Summer School 2011

  11. The NyquistTheorem The NyquistTheorem relates the electric power dissipated in a device and the thermal agitation measured by temperature T Energyavailable in the resistance is : E = kT (2 deg of freedom) Exchange on a timescalet, hence the power dissipation : P = E/t  kT Dn Dn (bandwidth in whichenergy is dissipated), the exchangeable power is : <i> = 0 <i2> ≠ 0(e- thermal motion) Pn = kT Dn Proportional to itstemperature : Noise Temperature ReciprocityTheorem(based on the Maxwell equations): it is always possible to express the properties of an antennaeither as a transmitter or as a receiver, withoutdistinguishingbetween the two. (think of your mobile phone !) IRAM Summer School 2011

  12. The Black Body Law and the Rayleigh-Jeans Approximation A system in thermodynamicalequilibriumwithitsown radiation (absorption emission of photons) ischaracterized by a radiation spectrumwhichfollows the Planck law: Atlowfrequency (radio) : Rayleigh-Jeans regime 2hn3 1 2kT n2 2kT c2exp(hn/kT) –1 c2l2 In = Bn(T) =_____________________ = _________ = ____ Definition of Source BrightnessTemperature The power receivedisdirectlyproportional to the brightnesstemperature T At mm and submml, the RJ approx. is not alwaysvalid. At 230 GHz, T=10K deviation by about 10 % One introduces : J n (TB) =  Bn (TB) [K] l2 2 k Athigh-frequency : Wien regime IRAM Summer School 2011

  13. The Beam Pattern The IRAM 30m is a Nasmythtelescope: Parabolic main reflector: widelyfavored to collect the astronomical signal (stigmatism, polarization) Advantages: Focus close to vertex of primarydish easyaccess to receivers Large FoV use of severalreceiverssimultaneously Drawbacks : Receiver must move in elevationwhen the telescopetracks the source on the sky : Cassegrain  Nasmyth Illumination losses : blockage of the quadrupod+ secondary + standing waves Losses by diffraction (spillover) are oriented to the sky, colderthanground IRAM Summer School 2011

  14. The Beam Pattern The Beam (Power) Pattern P(Ω)is the response of the antenna as a function of direction. NormalizedBeam Pattern : Pn (Ω) = P(Ω) / Pmax TR Beam solid angle A =∫∫Pn(,φ)d The Beam Pattern P(Ω) can beexpressed as the sum of 2 factors: P(Ω) = P1 (Ω) + Pe (Ω) P1 (Ω) : diffracted beam, formed by radiation coherently focused in the focal plane P1 (Ω) = Pml (Ω) + Psl (Ω) Main diffraction lobe Secondary diffraction lobes Pe (Ω) : error beam, formed by spillover + scattered radiation in the focal plane (FT)  gradingfunction : response of the antenna in the aperture plane (collecting surface) UsefulReferences: Born M. & Wolf E.« Principles of Optics » Greve A., Kramer C., Wild W., 1998, A&A Supp. Series, 133, 271 Kutner M.L. & Ulich B.L., 1977, ApJ 250, 341 IRAM Summer School 2011

  15. AntennaTemperature 1 A 1 A  ∫∫B(,φ)Pn(0-,φ0-φ) d = S(0,φ0) = - B * Pn The AntennaTemperatureis the equivalent noise temperature of a resistanceradiating the same power availableat the output terminals of the receiver. W = ½ AeB*Pn= kTA (Nyquist) Ae= l2/A (an important relation between the effective collecting surface and the beamsolid angle) 1 l2 A2k TA = ∫∫ B(,φ). Pn(0-,φ0-φ) d Here, TAis the antennatemperatureuncorrected for the rearwardlosses. Brightnesstemperature TB: temperature of the equivalentblackbodythatradiates the samespecificintensityatn B(TB,) = Bn (TB). () IRAM Summer School 2011

  16. AntennaTemperature Rayleigh-Jeans approximation : Bn(TB,) = 2 k TB() l2 1 A TA(Ω0)= ∫∫TB(). Pn(0-).d General case : J n (TB) =  Bn (TB) [K] or Jn (TB) = hn 1 k exp(hn/kTB) - 1 l2 2 k 1 A TA’(Ω0)= ∫∫Jn(TB) Ψ(). Pn(0-).d TA ‘ is the antennatemperature and has been correctedfromatmosphericattenuationexp(- t A) Jn(TB,) is the source brightness distribution Pn (Ω) is the normalized antenna beam pattern : Pn (0) = 1 TA (antennatemperature) has nothing to do with the actualtemperature of the structure of the antenna. TBis the thermodynamicaltemperature of the radiatingmaterialonly for an opticallythick and thermalized layer Relation canbeinvertedifPn() and TA() are fullyknown large scalemaps+ lowrms Only an approximate (de) convolution is performed IRAM Summer School 2011

  17. AntennaTemperature In the case of unpolarizedemission, the antennareceiverwilldetectonlyhalf the incoming signal and the collected power densityis (source spatiallyincoherent : intensities are added). W = ½ Ae∫∫B(,φ).Pn(,φ) d / ∫∫Pn(,φ)d W [erg s-1 Hz-1] Ae Effective aperture [cm-2] B(,φ) is the brightness distribution of the source [erg s-1 cm-2 Hz-1 sr-1] General case : the source and the main beam do not have the same reference. dS(φ) = B(φ). Pn(φ0-φ)dφ ∫∫B(,φ)Pn(0-,φ0-φ) d ∫∫Pn(0-,φ0-φ)d S(0,φ0) = ½ Ae IRAM Summer School 2011

  18. Aperture Efficiency Pc = amount of power collected by an antennailluminated by a plane wave with power density |S|. Effective aperture Ae: equivalent surface detecting Pc In practice, Ae≠ physicalsurface of the antenna= Ag = pD2/4 (707 m2 for the IRAM 30m) Aperture Efficiency Ae Ag ηA = Flux of a point source : W= k TA = ½ Sn Ae = 1/2 A Sn Ag The effective aperture efficiencyusuallydepends on the relative orientation of the antenna and the illuminating radiation. The effective collecting area and the antennabeamsolid angle are related via : Ae . A = l2 You cannot have both a large collecting area AND a large beamsolid angle… The directivity of the antenna and the effective aperture efficiency are not independent : IRAM Summer School 2011

  19. Aperture Efficiency : measurement (Measurements by Juan Penalver in august 2007) Ruze’s Formula e measures the roughness of the surface (errorsrms) IRAM Summer School 2011

  20. Gain and Efficiency Antenna Beam Solid Angle p p/2 A = ∫4pPn(,φ) d= ∫ ∫ Pn(,φ) sinddφ 0 -p/2 Can berepresented as a solidangle with Pn= 1 insideA = 0 elsewhere Main-beamsolid angle The main beamis the regionwhere Pn() > 0.5 mb =∫Pn(,φ) d Pn() > 0.5 Half-Power BeamWidth HPBW That characterizes the resolving power of the telescope IRAM Summer School 2011

  21. Main-BeamEfficiency mb A Beff =  mbis integrated over the main beam : P(Ω) > 0.5 A is integrated over 4 psteradian Beff( mb ) does not depend on the actual value of A (antennabeamsolid angle) but on the quality of the antenna. It shouldbe as large as possible, i.e. the power pattern beconcentrated in the main beam as much as possible. One defines the Main Beambrightnesstemperature Tmb : Tmb A = or TA ‘ mb TA’= Beff Tmb 1l2 mb 2k Tmb = ∫∫ Jn(TB) Ψ(). Pn(0-,φ0-φ) d IRAM Summer School 2011

  22. Main BeamEfficiency B (,φ) = Jn(TB).(,φ) = 2k TB / l2 .(,φ) Tmb = TB . (1/mb ) ∫∫(,φ) . Pn(0-,φ0-φ) d Under “reasonable conditions”, Tmb is a good approximation to TB Source flux density : Sn= ∫s Bn (TB). d = Jn(TB).s TA’ = (1/2k) Ae∫∫ Jn(TB )(,φ) . Pn(0-,φ0-φ) d = (Sn Ae/2k) . (1/ s) ∫∫(,φ) . Pn(0-,φ0-φ) d = K (K 1 if s << mb) At the IRAM 30m, the antenna temperature is corrected for the readward losses : TA* Fe = T’A we obtain the relation : TA *= (Sn K)Ae/2kFe Flux density /beam IRAM Summer School 2011

  23. Efficienciesat the IRAM 30m telescope The aperture efficiencycanbewritten as : TA* Fe = T’A = Beff Tmb : TA* = Beff / Feff Tmb A = (2k/Ag).Fe TA*/Sn,b The choice of the T-scaledoesmatter ! A depends a lot on frequency : itlimits the possibility of observingathigh-frequency (Ruze) IRAM Summer School 2011

  24. ApodizingFunction A Taper is appliedat the edge of the (secondary) reflector in order to decrease the level of the sidelobes (10-15 dB otherwise) The radio feedswhichcollect the signal are usuallyscalarhorns (monomodes) whoselowest mode ( Efield) is approximatelygaussian in the aperture. In otherwords, the gradingfunction is wellapproximated by a Gaussian. FeedHorns have typicaltapers of 10-15 dB. Impact of taper on antennaparameters Improvement of Beff and decrease of ηA IRAM Summer School 2011

  25. Gradings and Electric Field Pattern IRAM Summer School 2011

  26. The Main Beam Pattern GaussianBeam Grading : exp( -0.1/2 (r/R)2) r≤R Power Patter in the image plane : Pn() = exp( -ln2 . (2 /B)2) B is the half-power beamwidth (HPBW) = 1.2l / D Airy disk (uniform illumination) : 1.02 l / D Taper has broadened the beam by 20% but the sidelobelevels has been decreased to lessthan 1% The diffraction losses have diminishedtoo. The beamefficiencyishigher (and the aperture efficiencylower). MB= 1.133 MB2 o2 = B 2 + S2 TMB=TS . S2 / (B2+ S2) Observed Source (gaussian) IRAM Summer School 2011

  27. Source Brightness and Main BeamBrightnessTemperature • The Source flux density Snis thepower radiated per unit area and per unit frequency Rayleigh -Jeans regime (hn << kT) : For a planet of diameters The AntennaTemperaturecorrected for atmosphericlosses is : Brightness Distribution of the source: IRAM Summer School 2011

  28. Source BrightnessTemperature and Main BeamTemperature The beam of the telecope is wellapproximated by a Gaussian with HPFW B : P() = exp -ln2 (2/B)2 BrightnessDistribution of Planet: () = 1  ≤ s/2 = 0 elsewhere s/2 0 1 A T’A =∫ 2p Jn (Tb) exp -ln2 2 sin  d 2 B at the brigntnesspeak   pB2 ln 2 s2 4 ln 2 B2 1 – exp - x Jn(Tb) MB =p B2 / 4 ln 2 MB A T’A =Jn (TB) (1 – exp(x2)) x = √ln 2 .( s/B ) Beff IRAM Summer School 2011

  29. Source BrightnessTemperature and Main-BeamTemperature MB A T’A =Jn (TB) (1 – exp(-x2)) = Beff J n (TB)( 1 – exp(-x2)) Beff Tmb Tmb≠ Jn(TB) in general x = √ln 2 .( s/B ) For an extended sources > 2.6 B , one has Tmb≈ Jn(TB) Tmbisthen a good approximation to the source BrightnessTemperature Beff can bederivedfrom the measuredantennatemperature of a planet, of knownextentand brigthness. The best case if whens  MB It is possible to determineηA andAefffrom planet flux measurements : that is the method adopted in practice ! The scale T’A is adopted at the JCMT IRAM Summer School 2011

  30. Planets as Calibrators Someplanets are good calibratorsbecausetheirtemperatures and fluxes are wellknown. Secondarycalibrators : compact HII regions,etc… Somefrequencies are to beavoidedwhenusingplanets as calibrators (planetary atm lines): CH4 (82.0,82.9,94.1,95.2,98.3) HCN (88.6,265.9,345.5,..) 13CO (110.2,220.4,330.6), 12CO (115.3,230.5,345.8)….. Mercury, Venus : large diameter variations. Fast and large phase effects for mercury not used. Mars : VERY important ! Nearly a BB in the mm and submm : solid surface, tenuous atm Itstemperature varies with heliocentric distance : TB = <TB> (1.524/R) ½ (Ulich, 1981) Jupiter : diameter 32-42’’. Strongatmosphericlineswhichmay influence observations with smallbandwidth. Flux density is large and constant with time. Saturn : changing tilt angle of the rings wrtEarth Emission variations : not negligible ! Strongellipticity (e = 0.096) Uranus : small (3’’), weak. Atm absorp. Lines of CH4. Neptune : Weakerthan Uranus. Small (3’’). Broad CO absorption and HCN emissionlines (Marten et al. 1993). (1) Ulich et al. (1980); (2) Ulich et al. (1981); (3) empirical fit by Griffin & Orton (1993); (4) Griffin et al. (1986); (5) Hildebrand et al. (1985). PlanetaryBrightnessTemperaturesderivedfrombolometricmeasurements with bandwidthsof several 10 GHz. The Martiantemperatures are givenat the mean distance of 1.524 AU IRAM Summer School 2011

  31. Summary of Lecture 1 : BeamPattern - The Beam Pattern is the response of the Antenna as a function of direction Twocontributingfactors : P(Ω) = P1 (Ω) + Pe (Ω) P1 (Ω) : diffracted beam, formed by radiation coherently focused in the focal plane P1 (Ω) = Pml (Ω) + Psl (Ω) Main diffraction lobe Secondary diffraction lobes Pe (Ω) : error beam, formed by spillover + scattered radiation in the focal plane Main Beam: Pn (Ω) > 0.5 HPBW ( resolving power of the instrument)  1.2 l/D (almost Airy disk) IRAM : HPBW= 2460’’ / n(GHz) Main Beamefficiency : Beff = Ωmb / ΩA Beff = 0.81 86 GHz 0.74 145GHz 0.53 260 GHz A = ∫4pPn(,φ) d0.32 330 GHz IRAM Summer School 2011

  32. Summary of Lecture 1 : Temperatures • The AntennaTemperatureis the equivalent noise temperature of a resistanceradiating the same power availableat the output terminals of the receiver. • Power collected by the Antennais the convolution of the beam pattern Pn (response) with the SkyBrightness Distribution B: • W = ½ Ae B*Pn = kTA (Nyquist) • In the general case, after correction for atm. Absorption: • How to retrieve TB (or Jn (TB)) from TA’ ? • Special case : Veryextended, uniform source with s = A Ψ() = 1 for  < A : TA’ =Jn(TB) • Main BeamBrightnesstemperature (Tmbmb =TA A ) • For sources of extent s > 2.6 B , Tmb ≈ Jn(TB) • For small sources s << B , Tmb = Jn(TB) . Ωs/Ωmb  dilution factor TA’(Ω0)= ∫∫Jn(TB) Ψ(). Pn(0-).d 1 A TA’= Beff Tmb IRAM Summer School 2011

  33. The surface of a radiotelescopecannotbeperfect! Primarily, deformations of the main reflector surface large-scaledeformations : gravity, wind, ∆T (day/night) small-scaledeformations : surface errors (Main dish) Large-scaledeformations : Gravity homology (van Hoerner, 1967) : the antenna surface maintainsitsparabolicshape and its focal lengthunder the influence of gravity. The orientation of the parabola axis changes. Thermal Gradients in the structure  Coma, Astigmatism Theycanbeeliminated by some active optics design (Greve et al. 1996, Radio Science, 31,5,1053)  Active Surface Corrections at the CSO : 33%  56% at 350µm The large-scaledeformations affect the central part of the beam and maintain the wholebeam structure : primary+secondary lobes, whereassmall-scaledeformationsproduce one or severalextendederrorbeams. Beam Pattern and telescopedeformations Effelsbergbeamat 7mm IRAM Summer School 2011

  34. The surface deformations with respect to the perfectparaboloidshapevary with elevation for a homologoustelescope. Gain – ElevationEffect 0 22.5 Gain-El curvemeasuredat IRAM 30m on quasar and planetsat 3, 2 and 1.3 and 0.8mm 43 Effect more and more pronounced with n : Main beamremainscircular Side lobes levelincreases 67.5 Beam Pattern of IRAM 30m telescope Contours : -21,-18,-15,-12,-9,-6,-3 db 90 IRAM Summer School 2011

  35. ErrorBeam The surface of the primaryreflectorconsists of 210 panels, distributed in 7 rings of frames, 1 frame : 2 panels 1x2m, 15 screws: 5 rows- 0.5m spacing. Two types of surface errors eachpanel : adjustmentdeformation / screw : correlationlengthlc=0.3-0.5m betweentwo panels (displacement, misalignment, etc..): correlateddeformationslc= 1.5-2.0m IRAM Summer School 2011

  36. The antenna gain G = ErrorBeam In pratice, if d ≠0 at x0, thenthere’s an area aroundthat point where the deformation is ≠0too : Correlationlengthlc • = axial displacementwillinduce a change • in the opticalpath of the incomingwavefront the diffraction pattern  beam pattern and the gain d = 4pe/ l P(,φ) ∫∫P(,φ). d Ruze Formula (1952,1966) plc 2 (1- e-d2) lc sin e l d28 e exp - 2 G/G0 = exp(-(4pe/l)2) + Reduction of the axial gain Increase of the error lobe level Errorbeam of widthe ~ l/lc Effect more pronounced with increasingn  (D/lc) HPBW IRAM Summer School 2011

  37. The IRAM 30m Beam Pattern From Scans across the moon (Greve et al. 1998) The Beam Pattern is the sum of all these contributions IRAM Summer School 2011

  38. ErrorBeamat the JCMT At 450 µm, the first errorbeamcan not beneglected in many cases: Relative amplitude : 6-8% ; Size : 30’’… IRAM Summer School 2011

  39. ErrorBeam : Shouldyou care ? For small sources (   a few HPBW) : not really… For veryextended sources : Tmbis no longer a good approximation to the true TB …(Orion) The intensity spatial distribution isalsomodified by the low-level contribution of the errorbeam dramaticwhenmodellingintensity profiles to retrievedensity distribution n(r) in pre/protostellarcores, in molecularclouds. Motte et al. (1998) IRAM Summer School 2011

  40. ErrorBeam : Everybodyshould care ! Observed Spectrum Errorbeam contribution M51 CO 2-1 (Schuster et al. 2007) Garcia-Burillo et al. (1993) IRAM Summer School 2011

  41. Atmospheric Calibration IRAM Summer School 2011

  42. Atmospheric Calibration • Whyshouldwe care ? • emits thermal radiation and eventuallyadd noise • attenuates the incoming radiation • is turbulent : itintroduces time-dependent phase shifts in the propagation of the incomingelectromagnetic radiation, hence affects the intensity of the signal. IRAM Summer School 2011

  43. Atmosphere: the constituents Speciesmolec. weight Volume abundance amu N2 28 0.78084 O2 32 0.20948 Ar 40 0.00934 99.966% CO2 44 3.33 10-4 Ne 20.2 1.82 10-5 He 4 5.24 10-6 CH4 16 2.0 10-6 Kr 83.8 1.14 10-6 H2 2 5 10-7 => evaporated O348 4 10-7 N2O 44 2.7 10-7 H2O 18 a few 10-6 variable! IRAM Summer School 2011

  44. The Atmosphere 100 km Heating : photoabsorption in UV bands O, O2 Thermosphere O2 + hv (< 175nm)  O(3P) + O(1D)* Schumann – Runge continuum + hv (< 242nm)  O(3P) + O(3P) Herzberg continuum Modelling in ATM at IRAM Heating : photoabsorption in UV bands O2, O3 O3 + hv (<1175nm)  O2 + O Chapuis band + hv( < 310nm)  O2 + O* Hartley band Troposphere: the lowest portion of the atmosphere, containsapproximately 75% of the atmosphere's mass and almost all of its water vapor and aerosols. The averagedepth is about 11 km in the middle latitudes. The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetaryboundary layer. This layer is typically a few hundredmeters to 2 km deepdepending on the landform and time of the day. The temperaturedecreases in the troposphere. Stratosphere: has a positive thermal gradient, henceisdynamically stable. It isheatedfromabove by conduction (from O3 layer that blocks UV radiation) and frombelow by convection, which balances out at the base. IRAM Summer School 2011

  45. The Atmosphere : a simple model The atmosphereis a highlycomplexandnonlinearsystem (weatherforecast) Forourpurposewedescribeitasbeing Staticd / dt = 0and v = 0 1-dimensional f(r,f,q) -> f(z) Plane-parallel Dz / R << 1 In LocalThermodynamic Equilibrium (LTE) attemperatureT(z) Equationofstateideal gas IRAM Summer School 2011

  46. The Atmospheric Model Equationofstate p = (r/M) RT = Spi Hydrostaticequilibrium dp / dz = -r g = - pM / (RT) g dp / p = -gM / (RT) dz p = p0 exp(-z/H) withthepressurescaleheight H = RT/gM (= 6 ... 8.5km for T=210 ... 290K) TemperaturestructureoftheTroposhere dT/dz = -b (= 6.5 K/km) for z < 11 km T = T0 – b (z-z0) IRAM Summer School 2011

  47. Atmospheric Calibration: somekey-players • O2: Mixing ratio (% N2 = 20/79) is constant up to an altitude of 80km, where photodissociation processesstart to become important. MagneticDipole transitions at 60 and 118 Ghz • H2O: concentratedmainlyatlow altitude (a few kms), where the vertical profiles can vary a lot as the physical conditions in the loweratmosphereallow 3 phases of water to coexist. Beyond 15-20km of altitude, the atmosphere is extremelyextremely dry : H2O becomes a very rare gas. Electric Dipole transitions at 22, 183, etc… Ghz • O3: mixing ratio vertical profile displays a maximum in the lowerstratosphere (30-40 km), depending on the geographical latitude. Lessabundantthan O2 and H2O…a forest of lines in the submm and FIR range… • CO: photochemical and biologicalprocesses in the loweratmophere cause itsabundance to increase in the first 5km, thendecreases up to z= 25 km. Itsabundanceincreasesagainathigher altitude. IRAM Summer School 2011

  48. 3 mm 81-116 GHz 2 mm 129-174 GHz 201-256/269 GHz 0.8 mm 277-371 GHz 1 mm Atmospheric Transmission : Windows exp(-t) H2O 368GHz H2O 22GHz O2 O2 60GHz 118GHz 325GHz 380GHz 183GHz IRAM Summer School 2011

  49. Question : Whatlimits the atmospheric transmission atlowfrequency ? IRAM Summer School 2011

  50. Absorption by the ionosphere : νp= 9 ne1/2 Hz = 1/2 in the ionosphere : ne = 1012 cm-3 νp= 9 MHz The atmospheric opacity varies between day and night, due to recombinations in the plasma at night, so that ne decreases tν= 0.046 (ν/100MHz) -2 day tν= 0.0046 (ν/100MHz) -2 night Atmospheric Windows: the lowfrequencylimit 1 neqe2 2p me e0 Plasma Frequency of the ionosphere IRAM Summer School 2011

More Related