Lecture 13. Temperature Lidar (2) Resonance Fluorescence Doppler Tech

Size: px

Start display at page:

Download "Lecture 13. Temperature Lidar (2) Resonance Fluorescence Doppler Tech"

Kelley Sullivan
5 years ago
Views:

1 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Lecture 13. Temperature Lidar () Resonance Fluorescence Doppler Tech Resonance Fluorescence Na Doppler Lidar Na Structure and Spectroscopy Scanning versus Ratio Techniques Principle of Doppler ratio technique Three-frequency ratio technique Comparison of calibration curves Other resonance fluorescence Doppler lidars K Doppler Lidar Fe Doppler Lidar Summary 1

2 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Na Doppler Wind and Temperature Lidar Na Doppler lidar is one of the most successful lidars. Textbook Chapter 5 by Chu and Papen

3 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Na Atomic Energy Levels 3

4 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Doppler Effect in Na D Line Resonance Fluorescence Na D absorption linewidth is temperature dependent Na D absorption peak freq is wind dependent 4

5 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Na Atomic Parameters 5

6 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Doppler-Limited Na Spectroscopy Doppler-broadened Na absorption cross-section is approximated as a Gaussian with rms width σ D 1 e 6 f σ abs (ν) = A n exp [ν n ν(1 V R /c)] πσ D 4ε 0 m e c n=1 σ D (13.1) Assume the laser lineshape is a Gaussian with rms width σ L The effective cross-section is the convolution of the atomic absorption cross-section and the laser lineshape σ eff (ν) = where 1 πσ e e f 4ε 0 m e c 6 n=1 A n exp [ν n ν(1 V R /c)] σ e k B T σ e = σ D + σ σ D = (13.3) L and (13.4) The frequency discriminator/analyzer is in the atmosphere! 6 Mλ 0 (13.)

7 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Doppler Scanning Technique N Na (λ,z) = P L(λ)Δt σ eff (λ)n Na (z)δz hc λ ( ) N R (λ,z R ) = P L (λ)δt σ R (π,λ)n R (z R )Δz hc λ σ eff (λ,z) = C(z) T c (λ,z) ( ) N Na (λ,z) N R (λ,z R ) (13.7) A 4πz A z R η(λ)t a (λ)t c (λ,z)g(z) ( ) η(λ)t a (λ,z R )G(z R ) ( ) (13.5) (13.6) where C(z) = σ R(π,λ)n R (z R ) n Na (z) 4πz z R (13.8) Least-square fitting gives temp [Fricke and von Zahn, JATP, 1985] 7

8 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Scanning Na Lidar Results 8

9 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Doppler-Free Na Spectroscopy f - f a f + See detailed explanation on Na Doppler-free saturation-fluorescence spectroscopy in Textbook Chapter ν a ν c ν b ν c =(ν a +ν b )/ (13.9) 9

10 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 -Frequency Doppler Ratio Technique R T (z) = N norm( f c,z,t 1 ) N norm ( f a,z,t ) = σ eff ( f c,z)n Na (z,t 1 ) σ eff ( f a,z)n Na (z,t ) σ eff ( f c,z) σ eff ( f a,z) (13.10) N norm ( f,z,t) N Na ( f,z,t) N R ( f,z,t)t c ( f,z) z z R (13.11) N norm ( f,z,t) = σ eff ( f )n Na (z) 1 σ R (π, f )n R (z R ) 4π (13.1) 10

11 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 3-Frequency Doppler Ratio Technique R T (z) = N norm ( f +,z,t 1 ) + N norm ( f,z,t ) N norm ( f a,z,t 3 ) σ eff ( f +,z) + σ eff ( f,z) σ eff ( f a,z) (13.13) R W (z) = N norm ( f,z,t ) N norm ( f +,z,t 1 ) σ eff ( f,z) σ eff ( f +,z) (13.14) 11

12 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Main Ideas Behind Ratio Technique Three unknown parameters (temperature, radial wind, and Na number density) require 3 lidar equations at 3 frequencies as minimum highest resolution. In the ratio technique, Na number density is cancelled out. So we have two ratios R T and R W that are independent of Na density but both dependent on T and W. The idea is to derive temperature and radial wind from these two ratios first, and then derive Na number density using computed temperature and wind at each altitude bin. However, because the Na extinction coefficient is involved, the upper bins are related to lower bins, and extinction coefficient is related to Na density and effective cross-section. The solution is to start from the bottom of the Na layer and then work bin by bin to the layer top. 1

13 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Principle of Doppler Ratio Technique Lidar equation for resonance fluorescence (Na, K, or Fe) N S (λ,z) = P L (λ)δt [ σ eff (λ,z)n c (z)r B (λ) +σ R (π,λ)n R (z)]δz hc λ ( T a (λ)tc (λ,z) )( η(λ)g(z) )+ N B A 4πz R B = 1 for current Na Doppler lidar since return photons at all wavelengths are received by the broadband receiver, so no fluorescence is filtered off. Pure Na signal and pure Rayleigh signal in Na region are N Na (λ,z) = P L(λ)Δt A [ σ eff (λ,z)n c (z)]δz hc λ 4πz T a (λ)tc (λ,z) ( ) η(λ)g(z) ( ) N R (λ,z) = P L(λ)Δt hc λ [ σ R (π,λ)n R (z)]δz A z T a (λ)tc (λ,z) ( ) η(λ)g(z) ( ) So we have N S (λ,z) = N Na (λ,z) + N R (λ,z) + N B 13

14 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Principle of Doppler Ratio Technique Lidar equation at pure molecular scattering region (35-55km) N S (λ,z R ) = P L (λ)δt [ σ R (π,λ)n R (z R )]Δz hc λ A z R T a (λ,z R )( η(λ)g(z R ))+ N B Pure Rayleigh signal in molecular scattering region is N R (λ,z R ) = P L (λ)δt A [ σ R (π,λ)n R (z R )]Δz T a (λ,z R ) η(λ)g(z R ) hc λ z R So we have N S (λ,z R ) = N R (λ,z R ) + N B N R (λ,z) [ N R (λ,z R ) = σ R (π,λ)n R (z) ]T a (λ,z)tc (λ,z)g(z) [ σ R (π,λ)n R (z R )]T a (λ,zr )G(z R ) z R z = n R (z) n R (z R ) ( ) The ratio between Rayleigh signals at z and z R is given by z R z T c (λ,z) Where n R is the (total) atmospheric number density, usually obtained from atmospheric models like MSIS00. 14

15 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Principle of Doppler Ratio Technique From above equations, the pure Na and Rayleigh signals are N Na (λ,z) = N S (λ,z) N B N R (λ,z) Normalized Na photon count is defined as N R (λ,z R ) = N S (λ,z R ) N B N Norm (λ,z) = N Na (λ,z) z N R (λ,z R )T c (λ,z) z R From physics point of view, the normalized Na count is N Norm (λ,z) = N Na (λ,z) = σ eff (λ,z)n c (z) 1 N R (λ,z R )T c (λ,z) σ R (π,λ)n R (z R ) 4π From actual photon counts, the normalized Na count is N Norm (λ,z) = N Na (λ,z) z N R (λ,z R )T c (λ,z) = N S (λ,z) N B N R (λ,z) z R N R (λ,z R )T c (λ,z) = N S (λ,z) N B z N S (λ,z R ) N B z R z z R 1 T c (λ,z) n R (z) n R (z R ) 15

16 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Principle of Doppler Ratio Technique From physics, the ratios of R T and R W are then given by R T = N Norm( f +,z) + N Norm ( f,z) N Norm ( f a,z) σ eff ( f +,z)n c (z) σ = R (π, f + )n R (z R ) + σ eff ( f,z)n c (z) σ R (π, f )n R (z R ) σ eff ( f a,z)n c (z) σ R (π, f a )n R (z R ) = σ eff ( f +,z) +σ eff ( f,z) σ eff ( f a,z) R W = N Norm ( f +,z) N Norm ( f,z) N Norm ( f a,z) σ eff ( f +,z)n c (z) σ = R (π, f + )n R (z R ) σ eff ( f,z)n c (z) σ R (π, f )n R (z R ) σ eff ( f a,z)n c (z) σ R (π, f a )n R (z R ) = σ eff ( f +,z) σ eff ( f,z) σ eff ( f a,z) Here, Rayleigh backscatter cross-section is regarded as the same for three frequencies, since the frequency difference is so small. Na number density is also the same for three frequency channels, and so is the atmosphere number density at Rayleigh normalization altitude. 16

17 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Principle of Doppler Ratio Technique From actual photon counts, the ratios R T and R W are R T = N Norm ( f +,z) + N Norm ( f,z) N Norm ( f a,z) N S ( f +,z) N B z N S ( f +,z R ) N B z = R 1 T c ( f+,z) n R (z) n R (z R ) + N S ( f a,z) N B z N S ( f a,z R ) N B z R N S( f,z) N B z N S ( f,z R ) N B z R 1 T c ( fa,z) n R (z) n R (z R ) 1 T c ( f,z) n R (z) n R (z R ) R W = N Norm ( f +,z) N Norm ( f,z) N Norm ( f a,z) N S ( f +,z) N B z N S ( f +,z R ) N B z = R 1 T c ( f+,z) n R (z) n R (z R ) N S ( f a,z) N B z N S ( f a,z R ) N B z R N S( f,z) N B z N S ( f,z R ) N B z R 1 T c ( fa,z) n R (z) n R (z R ) 1 T c ( f,z) n R (z) n R (z R ) 17

18 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 How Does Ratio Technique Work? From physics, we calculate the ratios of R T and R W as R T = σ eff ( f +,z) +σ eff ( f,z) σ eff ( f a,z) R W = σ eff ( f +,z) σ eff ( f,z) σ eff ( f a,z) From actual photon counts, we calculate the ratios as R T = N Norm ( f +,z) + N Norm ( f,z) N Norm ( f a,z) = N S ( f +,z) N B z N S ( f +,z R ) N B z R R W = N Norm( f +,z) N Norm ( f,z) N Norm ( f a,z) = N S ( f +,z) N B z N S ( f +,z R ) N B z R 1 T c ( f+,z) n R (z) n R (z R ) + N S ( f a,z) N B z N S ( f a,z R ) N B z R 1 T c ( f+,z) n R (z) n R (z R ) N S ( f a,z) N B z N S ( f a,z R ) N B z R N S ( f,z) N B z N S ( f,z R ) N B z R 1 T c ( fa,z) n R (z) n R (z R ) N S ( f,z) N B z N S ( f,z R ) N B z R 1 T c ( f,z) n R (z) n R (z R ) 1 T c ( f,z) n R (z) n R (z R ) 1 T c ( fa,z) n R (z) n R (z R ) 18

19 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 How Does Ratio Technique Work? Compute Doppler calibration curves from physics Look up these two ratios on the calibration curves to infer the corresponding Temperature and Wind from isoline/isogram. -40 m/s Isoline / Isogram 180 K 19

20 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 UIUC Na Doppler Lidar SOR 0

21 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Comparison of Calibration Curves Different metrics of R W result in different wind sensitivities The ratio R W = N + /N - has inhomogeneous sensitivity 1

22 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Comparison of Calibration Curves The ratio R W = (N + - N - )/N a has much better uniformity than the simplest ratio

23 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Comparison of Calibration Curves The ratio R W = ln(n - /N + )/ln(n - N + /N a ) has good uniformity 3

24 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 More Resonance Fluorescence Doppler Lidars MSIS90 Temperature Mesopause Thermosphere Besides Na, there are more metal species (K, Fe, Ca, Ca +, Mg, Li, ) from meteor ablation. They can be used as tracers for Doppler lidar measurements in MLT region Altitude (km) Stratopause Mesosphere Stratosphere Ca K Na Fe 0 Tropopause Temperature (K) Troposphere Altitude / km Concentration / cm -3 4

25 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Metal Species in MLT Region Species Central wavelength (nm) A ki (x10 8 s -1 ) Degeneracy g k / g i Atomic Weight Isotopes Doppler rms Width (MHz) 0 (x10-1 cm ) Abundance (x10 9 cm - ) Centroid Altitude (km) Layer rms Width (km) Na (D ) / Fe / , 56, 57, 58 K (D 1 ) / , 40, 41 K (D ) / , 40, 41 Ca / , 4, 43, 44, 46, 48 Ca / Same as Ca x x x x In principle, all these species can be used as trace atoms for resonance fluorescence Doppler lidar measurements. Whether a Doppler lidar can be developed and used mainly depends on the availability and readiness of laser and electro-optic technologies. In addition, the constituent abundance and absorption cross-section are naturally determined. 5

26 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 K Atomic Energy Levels 6

27 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 K Atomic Parameters 7

28 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 K Doppler Lidar Principle & Metrics Ratio technique versus scanning technique Scanning technique actually has its advantages on several aspects, depending on the laser system used - whether there is pedestal, background problems, etc. Ratio technique usually gives higher resolution. 8

z 5 F o 3d 6 4s4p z 5 D o 3d 6 4s4p 5 o F4 5 o F5 5 D D 5 o 3 o 4 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Fe Doppler Lidar Principles 7166.

29 z 5 F o 3d 6 4s4p z 5 D o 3d 6 4s4p 5 o F4 5 o F5 5 D D 5 o 3 o 4 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Fe Doppler Lidar Principles cm 1 cm cm cm 1 37nm 374nm 386nm 389nm a 5 D 3d 6 4s 5 D3 5 D4 ΔE 416cm cm cm 1 [Chu et al., ILRC, 008] Fe (iron) 37-nm line 9

30 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Effective Cross Section Computation The effective cross section is a convolution of the atomic absorption cross section and the laser line shape. For Gaussian shapes of atomic absorption and laser lineshape: σ e = σ D + σ L How would you calculate the effective cross section when atoms have isotopes, e.g., K (potassium) or Fe (iron)? σ eff (overall) = N [ σ eff,m (isotope) Isotope Abdn ] m m=1 30

31 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Summary (1) Resonance fluorescence Doppler lidars apply Doppler technique to infer temperature and wind from the Doppler-broadened and Dopplershifted atomic absorption spectroscopy. The Doppler-limited atomic absorption spectroscopy is inferred from the returned fluorescence intensity ratios at different frequencies. Both scanning and ratio techniques can work for the Doppler lidar. With scanning technique, the laser will be operated at many different frequencies, and then a least-square fit derives the width of the atomic absorption line, thus deriving the temperature. Its advantage is to provide more than 3 frequency information, so providing checks on more system parameters. But it requires longer integration time. Doppler ratio technique takes advantage of the high temporal resolution feature by limiting the lidar detection to 3 preset frequencies (usually one peak and two wing frequencies) for 3 unknown parameters (T, W, and density). By taking the ratios among signals at these three frequencies, R T and R W are sensitive functions of temperature and radial wind, respectively. 31

32 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 01 Summary () We compute the ratios R T and R W from atomic physics first to form the lidar calibration curves, and then look up the two ratios calculated from actual photon counts on the calibration curves to infer the corresponding temperature T and radial wind W. Different metrics exhibit different inhomogeneity, resulting in different crosstalk between T and W errors. There are several different atomic species (Na, K, Fe, Ca, Ca +, etc.) originating from meteor ablation in the mesosphere and lower thermosphere (MLT) region. They all have the potentials to be tracers for resonance fluorescence Doppler lidars to measure the temperature and wind in the MLT region. Na and K Doppler lidars are currently near mature status and making great contributions to MLT science. Fe Doppler lidar has very high future potential due to the high Fe abundance, advanced alexandrite laser technology, Doppler-free Fe spectroscopy, and bias-free measurements, etc. 3

Lecture 12. Temperature Lidar (2) Resonance Fluorescence Doppler Lidar

Lecture 12. Temperature Lidar (2) Resonance Fluorescence Doppler Lidar LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, FALL 04 Lecture. Temperature Lidar () Resonance Fluorescence Doppler Lidar Resonance Fluorescence Na Doppler Lidar Na Structure and Spectroscopy Scanning