Titan Airglow: High Resolution
Laboratory UV Spectroscopy for
Cassini UVIS Model
Joseph Ajello
JPL
Alan Heays
Leiden
Observatory
TITAN PROTON PLASMA (0.1 keV-4 MeV)
& SOLAR XUV GENERATES 2-level
IONOSPHERE (300-1500 KM)
S. PolSS. POLE
South
shelf
-Solar XUV Excitation –Strong
Top Ionosphere
-Magnetospheric: p and O+
Plasma Sporadic low
ionosphere (Cravens et al.,
2008).
N2 Cross section Analysis for Dayglow
VI (2)→UVIS (16)→Lab(e +N2 500 features)
V(z)=g(z) x w MODEL
gj (z) = F (e) x n(z) x s(e)
Dl=0.2Å
Dl=30Å
Dl=5Å
DD
ISS & UVIS OBSERVATION OF
TITAN SOLAR ECLIPSE @ 600K km
UV- VIS-near
IR (200-1200)
nm N2 night
glow to 1000
km
plasma + N2
EUV DARK SIDE RECORD 1
LIMB & DISK SPECTRA
DOY110-2009 @15K km
-Spectrum =Eclipse at 20 x closer
-4-minute records
-Record 0 MRH = surface to 800
km
-Record 1;MRH = 800-1100 KM)
-NO Rayleigh scattering
Plasma excitation : 500-1200 km
-Molecular bands and NI ,II lines
-1/10 Intensity of Dayglow
H IGH R ESOLUTION UV S PECTROSCOPY
FOR C ASSINI UVIS
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GOAL AND RESULT:
Obtain BOTH High Resolution LABORATORY AND FIRST MODEL EUV
Spectra of N2 by Electron Impact (FWHM= 0.2Å) to establish UV
EMISSION CROSS SECTIONS .
AND Identify ~800 N2 (~500) and N I,II (~300) features (also under
16 features in Cassini UVIS Titan EUV Airglow Spectrum -FWHM 4Å).
Produce First Emission ab initio Quantum Model of Electron Excited
Induced UV Fluorescence of N2 of 9 electronic states (b 1Πu, b' 1Σu+,
c4' 1Σu+ c3 1Πu, o3 Πu, c4 1Πu, c5 1Πu, c5 '1Σu+ c6'Σu+  X 1Σg+ from 1215 eV for UVIS model. All UV Emission states are perturbed (>20%
admixture) and weakly-to-strongly predissociated.
Determine from Laboratory Spectra Rotational Predissociation and
Emission Yields for complete & accurate N2 model.
COMPARISON HIGH RESOLUTION LAB
SPECTRUM & TITAN LOW RESOLUTION
UVIS LIMB SPECTRUM
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Lab Spectrum
0.2Å FWHM vs
Titan Airglow 4-5Å
FWHM
10-40 N2
Molecular Bands
or NI,II lines per
UVIS feature
16 UVIS features
vs 800 Lab
features (8001350 Å)
Quantum Model (Coupled-Schroedinger
Equation [CSE]) Based on High Resolution
Laboratory Spectra
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Observed 20 and 100
eV Lab Emission
(black) Compared to
CSE Model of N2
Excitation,
Predissociation, &
Fluorescence (red).
Output quantummechanical model
simulates electronimpact excitation and
fluorescence (grey =
each band)– to aid the
assignment of complex
and blended UVIS
spectrum.
Accurately Modeled
optical excitation (hi
energy)
Molecular emission from
c4 ‘ 1S+u (0,0-12) to FUV
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more members
of c4' (0, v'') than
previous
homogenous
coupling
c4 ‘ 1S+u ~ b' 1S+u
Short EUV l shows Molecular N2+
X(0)+np Rydberg states Identified
for First Time to n=9
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c5(0,0–1)
c6'‘(0,0–1)
c6(0,0),
c7'‘(0,0)
c8(0,0)
c9'(0,0)
Resonances
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2 valence states,
13 Rydberg states
100 eV Emission Cross Sections of
c 4'1S+u X 1S+g Rydberg bands
electron
energies at
Titan 10(100 eV)
 Q=1.1×
10-17 cm2
 FCF vs
b'~c4‘
coupling
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Emission, Predissociation Cross
Sections Five (n=3,4,5) RydbergValence (12-15 eV) States of N2
state
Qex (10-19 cm2)
Qem(10-19 cm2)
Qpre (10-19 cm2)
c 4 ' 1 S u+
155a
115a
40a
b' 1Su+
128b
19.6
107.4a,e
b 1P u
121b
9.7a
111.3a
c3 1P u
161b
~1.7
159.3a
o3 1Pu
75b
~0.4
74.6a
totals
640
146
493
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Average
Predissociation) yield
(Qpre/Qex) =77%, EUV
photon yield=23%,
175K Titan).
Five singlet-ungerade
states predissociate,
eject fast N-atoms
(>1 eV) of N(4So) +
N(2Do) & N(4So) +
N(2Po)