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Analysis of the Terrestrial Thermospheric Dayglow of the N2 c’1Σu+ (0) and b’1Σu+ (1) - X 1Σg + Emission and Implications for Cassini UVIS Titan Data Analysis Xianming Liu and Donald Shemansky Space Environment Technologies Alan, N. Heays and Brenton R. Lewis Australian National University Paul D. Feldman Johns Hopkins University Status of N2 parameters required for modeling Cassini UVIS observations (I) • Photoabsorption cross sections – High-resolution measurements by Stark et al. for strong bands – Other low & medium resolution measurements (>10,700 cm-1 or < 935 Å)- saturation! – Couple-channel Schrödinger Equation (CSE) model of Lewis’ group • reproduces Stark et al’s measurements within 10%, eg. the b(3)-X(0) and photodissociation by solar Lyman-γ lines. • predicts line oscillator strengths, predissociation rates, photodissociation cross section for both strong and weak bands for levels below 10,700 cm-1 (>935Å) over wide temperature range • needs to be examined and refined by emission measurement • requires additional high-resolution experimental measurements and information for F and G 3Πu for higher levels Status of N2 radiative parameters required for modeling Cassini UVIS observations (II) • Emission cross section – very limited – measurements by Walter and Cosby on predissociation yields for a few isolated bands with laser spectroscopy – e+N2 measurements by Ajello et al. (1989), James et al. (1990) and Liu et al. (2005) – errors up to 25%, and Liu et al. (2007) • Problems for accurate modeling – Strong variation of oscillator strength and predissociation rates with ro-vibrational quantum numbers. • Cross section strong temperature dependence • Experimental emission cross section inapplicable unless temperatures are close to each other – Reliable absorption cross section not available until recently – Strongly coupled system requiring systematic experimental measurement and joint theoretical investigation Status of N2 radiative parameters required for modeling Cassini UVIS observations (III) • Estimated time for accurate N2 radiative parameters (≤12% error – Levels below 10,700 cm-1 (>935Å) • • • • Some available now 1-3 years for all strong or moderately strong bands 2-5 years for all VUV bands 935-1800Å Requires additional funding from NSF and NASA – Levels above 10,700 cm-1 (<935Å) • Requires high-resolution photoabsorption from other groups (eg. Collaborator Stark et al) Potential Energy Curves of N2 Valence-Rydberg States Classification of N2 Singlet-ungerade states • 2 valence states: b’1Σu+ and b 1Πu states. • 3 Rydberg series: npσ, npπ and nsσ. – npσ and npπ series converge to the N2+ X 2Σg + state and are designated as c’n+11Σu+ and cn 1Πu. – nsσ series converges to the N2+ A 2 Πu state and is designated as on 1Πu. Adiabatic and Diabatic Curves of Singlet-ungerade States Stahel et al, J. Chem. Phys, 79, 2541 (1983). FUSE-Earth vs UVIS-Titan Dayglow • FUSE higher resolution – 0.1-0.4 Å vs 2.5-3 Å for UVIS – Easier separation of atomic and ionic emission features from the N2 features – A very important step towards accurate analysis of the Titan dayglow spectrum • Limited spectral coverage – 904 to 1188 Å with 11 Å gap from 1083 to 1094 Å • Stronger atomic and ionic oxygen emission features in FUSE-Earth observation Brightness (R/Å) 10 0 1047 5 1048 1049 1050 Wavelength (Å) 15 1051 1005 1052 1006 1053 0 1024 0 1072 1025 1073 1026 1074 1027 10 1075 1028 Wavelength (Å) Wavelength (Å) c4'(0) & b'(1) - X(4) c4'(0) & b'(1) - X(5) 1076 Wavelength (Å) O I (1027.431) N I (1027.15, 1027.24) N I (1026.69, 1026.78) O I (1028.157) N I (1028.357, 1028.449) c4'(1,4) 1077 N I (1029.500, 1029.592) c4'(3,6) c4'(2,5) 5 c4'(3,8) c4'(1,6), c4'(2,7) 40 H I (1025.722) O I (1025.762) 15 CO 3p E(0)-X(0) 1004 b'(4,6) 1003 10 b'(7,5) 50 b'(7,7) 30 Brightness (R/Å) c4'(1,3) 10 Brightness (R/Å) 1002 N I (1051.867, 1051.964) N I (1052.050,1052.149,1052.22 1001 b'(4,5) b'(7,4) 20 b' (7,6) N* 0 1000 Ar I (1048.220) b(5,4) Brightness (R/Å) c4'(0) & b'(1) - X(2) c4'(0) & b'(1) - X(3) 1029 1030 15 5 1078 Brightness (R/Å) 10 5 0 1153 1154 1101 1155 1102 1103 1156 Wavelength (Å) 1157 1104 1158 1105 15 1159 Brightness (R/Å) 5 0 1181 5 1126 1182 1127 15 10 1183 1128 1184 1129 Wavelength (Å) Wavelength (Å) c4'(0) & b'(1) - X(8) c4'(0) & b'(1) - X(9) 1185 Wavelength (Å) O II (1130.147) O II (1129.251) P(1) (1128.507) O II (1128.081) 10 O III (1185.961) N I (1101.291) 15 P(1) (1185.048) 1100 0 1125 O III (1184.174) 0 1099 P(1) (1101.687) 4 O III (1183.150) 6 O III (1182.770) 8 N I (1100.360; 1000.465) 20 b'(4,10) O III (1181.748) c4'(3,12) 10 Brightness (R/Å) 12 P(1) (1156.28) 14 O II (1154.096) 16 N I (1098.954, 1099.152) 18 c4'(4,12) O II (1153.357) b'(4,9) Brightness (R/Å) c4'(0) & b'(1) - X(6) c4'(0) & b'(1) - X(7) 2 1130 1186 1131 1187 Summary I. Model Results • Inferred thermospheric temperature 500 ±50 K • Model reproduces observed brightness of v”=2-9 within FUSE observation error (±15% ) • Reliable estimate for v”>9 level • Radiation loss – 98% for v”=0 level (2115 R expected for optically thin vs 24±7 R observed) • Multiple scattering, predissociation, radiative escaping to v”>0 • Reduction of emission rate and distortion of band shape – 68% for v”=1 level (315 R expected vs 100 R observed) • By predissociative b(2)-X(0) absorption • By self-absorption of c’(0)-X(1) itself (require significant population at v”=1 level -> non-LTE N2(v”) population! Analysis of b(1)-X(v”) will provide definitive answer!) c4'(0) & b'(1) - X(0) c4'(0) & b'(1) - X(1) 80 100 FUSE Model (opt. thin) x 1/30 70 60 Brightness (R/Å) 50 Brightness (R/Å) 80 P(1) (958.602) 60 FUSE Model (opt. thin) x 2/7 P(1) (980.502) 70 90 40 30 20 50 40 30 20 10 10 -0 -0 -10 957 -10 958 959 Wavelength (Å) 960 961 979 980 981 982 Wavelength (Å) 983 Summary II. Neutral N2 Excitation mechanisms in thermospheres of Earth and Titan • Excitation by Photoelectrons – Principal mechanism for every singlet-ungerade levels except where resonant solar photoexcitation is important • Resonant photoexcitation by solar radiation – – – – – b(3) by H Ly-γ (complete predissociation) b(6) by H Ly-δ, b(10) by H Ly (n=9) b’(4) by H Ly-ε, b’(6) by H Ly (n=8), b’(7) by H Ly (n=12) b’(8), o(2), and b(12) by H Ly (n>15) b(12) also by H ionization continuum • Resonant photoexcitation by strong N2 emission – b(2) from v”=0 by c’1Σu+ (0)-X 1Σg +(1) transition – b’(4) from v”=1 by c’1Σu+ (0)-X 1Σg +(0) transition [non-LTE X(v”)] Summary III. Prominent N2 dayglow emission features from the FUSE and implication for UVIS-Titan modeling • c’1Σu+ - X 1Σg + band system (v’=0, 1, 3, 4, 5, and 6) – v’ = 0 can be accurately modeled – v’ = 1-4 have good oscillator strengths but unreliable predissociation rates – v’ = 5 & 6 require both oscillator strengths and predissociation rate • b’1Σu+ - X 1Σg + band system (v’= 1, 4, 6, 7, 9, 12, and 14) – v’ = 1 can be accurately modeled – Others require accurate predissociation rates • b 1Πu - X 1Σg + band system (v’ = 1, 6, 7, and 10) – Most levels require accurate predissoiciation rates • c 1Πu - X 1Σg + band system (v’ = 0 and 1) (relatively weak) – Requires accurate diabatic transition moment Example of strong temperature dependence and the importance of accurate predissociation rates at rotational levels I. II. III. IV. V. Fig 1. Comparison of 150 K laboratory measurement and model spectrum using CSE calculation Fig 2. Comparison of 150 K lab measurement and adjusted CSE model. The adjustment factors are listed at top right corner Fig 3. Comparison of 300 K lab measurement with model using adjustment factors obtained at 150K Fig 4. Comparison of 300 K lab spectrum with model using a new set of adjustment factors Fig 5. Comparison of FUSE spectrum (~500K) with model using adjustment factors obtained at 300 K e+N2 (150K, 100 eV) Model based on CSE calculation with no adjustment 2.5 Calibrated Intensities (arb unit) 100eV, 4.6E-4 Torr model (no self-abs) 2.0 1.5 0.9 0.4 -0.1 1000.0 1001.0 1002.0 1003.0 1004.0 1005.0 1006.0 Wavelength(A) 1007.0 1008.0 1009.0 1010.0 e+N2 (150K, 100 eV) Model based on CSE cal. with adjusted transition moments 2.2 2.0 c'(1) x 0.35; c'(3) x 0.70; c'(4) x 0.47; b(6) x 2.5; 100eV, 4.6E-4 Torr model (no self-abs) b(7) x 2.5; b'(7)x 2.5; b'(9) x 0.8; b'(12)x0.75, b'(14)x0.3 Calibrated Intensities (arb unit) 1.8 1.6 1.4 1.2 0.9 0.7 0.5 0.3 0.1 -0.1 1000.0 1001.0 1002.0 1003.0 1004.0 1005.0 1006.0 Wavelength(A) 1007.0 1008.0 1009.0 1010.0 e+N2 (100 eV, 300K, Δλ=0.3A) Mar. 29, 2007 3:47:09 PM 1500 Aug-08-2006 MOD_300K Aug-21-2006 c'(1) x 0.35; c'(3) x 0.70; c'(4) x 0.47; b(6) x 2.5; Calibrated Intensities (counts) 1300 b(7) x 2.5; b'(7)x 2.5; b'(9) x 0.8; b'(12)x0.75, b'(14)x0.3 1100 900 700 500 300 100 -100 1000.0 1001.0 1002.0 1003.0 1004.0 1005.0 1006.0 Wavelength(A) 1007.0 1008.0 1009.0 1010.0 e+N2 (100 eV, 300K, Δλ=0.3A) Mar. 29, 2007 6:49:33 PM 1500 1300 Aug-08-2006 MOD_300K Aug-21-2006 c'(1) x 0.45; c'(4) x 0.65; b(1)x1.5; b(6) x 3; Calibrated Intensities (counts) b(7) x 2.5; b'(7)x 2.5; b'(12)x0.75, b'(14)x0.3 1100 900 700 500 300 100 -100 1000.0 1001.0 1002.0 1003.0 1004.0 1005.0 1006.0 Wavelength(A) 1007.0 1008.0 1009.0 1010.0 FUSE Thermospheric N2 Dayglow Emission Spectrum Based on scaling factors for 300 K May 15, 2007 12:05:26 PM 70 c'(1)x0.45; c'(4)x 0.65;b'(2)x3; b(1)x1.5; b(6)x3; 60 FUSE_COMPOSITE Model (100eV, 500K) b(7)x2.5; b'(7)x2.5; b'(12)x0.75, b'(14)x0.3 Brightness (R/Å) 50 40 30 20 10 0 1000.0 1001.0 1002.0 1003.0 1004.0 1005.0 1006.0 Wavelength (Å) 1007.0 1008.0 1009.0 1010.0 Conclusion • Laboratory emission cross section is not directly applicable unless the measurement and thermospheric temperatures are close to each other • Accurate modeling requires reliable predissociation rates at rotational level. • CSE predissociation rates need to be examined and refined by high-resolution lab measurement at 300 K.