Titan UVIS Airglow Spectra:
Modeling and Laboratory
Studies
JOSEPH AJELLO
JPL
MICHAEL STEVENS
NRL
IAN STEWART
KRIS LARSEN
LARRY ESPOSITO
JOSH COLWELL
WILLIAM MCCLINTOCK
WAYNE PRYOR
GREG HOLSCLAW
LASP/CU
UVIS TITAN AIRGLOW PRIMARY GOALS
• PRESENT FIRST CALIBRATED UV SPECTRA
(800-1900 Å)
• IDENTIFY SPECTRAL CONTENT -c'(0,0) 958Å?
• IDENTIFY SOURCES BY MODEL SPECTRUM
o
o
o
o
o
PHOTODISSOCIATIVE IONIZATION EXCITATION (PDI)
PHOTOELECTRONS(PE) + N2
MULTIPLE SCATTERING IN c'(0,v'')PROGRESSION
MAGNETOSPHERIC ELECTRONS (ME)+ N2
ATOMIC N +e (PE or ME)
• DEVELOP PE MODEL FOR 200 PERTURBED &
STRONG RYDBERG BANDS N2 (b,b',c'-X)
CASSINI UVIS OBSERVATION
• Inbound Orbit Tb 13dec04
EUV/FUV AIRGLOW
5 km, 200 km resolution
•
10
• 560-1180 & 1120-1910Å
• 1024 (spectral)x64 (spatial)
• 2.0 mrad x 59 mrad FOV
EUV DAYGLOW OBSERVATION IN N2
RYDBERG BANDS (b 1Pu, b' 1Su, c 1Su  X 1Sg)
• Dayside (900-1100 Å)
03:43 UT, 13DEC04
• phase angle of 16.5o
• distance 163,000km
• Nightside 14:13 UT
• 163o phase angle
• distance 50,500km
• Resolution ~100 km
The EUV Airglow of Titan
Titan V1 UVS airglow data suggested that
the N2 c(0,0) and (0,1) bands dominated
the EUV due to similarities with electron
impact spectra from the lab.
However, (0,0) is optically thick near peak
photoelectron exciation. Newer model
results showed that (0,0) is weak and
many other features are there instead,
including NI excited by photodissociative
ionization.
UVIS data from December, 2004 now show
the EUV airglow of Titan in detail.
(0,0) is indeed weak or absent and the
complexity of the spectrum is revealed.
Red curve is the composite of a four
component linear regression…
Spectral Fitting of EUV Airglow
• Electron impact laboratory spectrum without c(0,0-2).
• Relative intensities of NI and NII PDI [Bishop and Feldman, 2003].
• Relative intensities of (0,0-2) based on model results [Stevens, 2001].
• HI Lyman- at 1026 Å fit separately.
Photon loss and redistribution in c(0)—X shown by comparing blue dotted and green lines.
COMPARISON CASSINI EUV TO
LABORATORY SPECTRUM
• 5
5.6Å FWHM
Feature 6
100eV
FWHM=0.2Å
COMPARIONS OF EUV DAGLOW & NIGHTGLOW
SPECTRA (800-1140 Å) WITHIN SATURN
MAGNETOSPHERE
IDENTIFICATION OF FUV SPECTRUM
Dayglow
900km
LBH 68R
HI 266R
NI 30R
CH4~1017cm2
Nightglow
GLOBAL MODEL OF N2 DISSOCIATION FOR FAST N (~1 eV) CHEMISTRY
FROM EUV INTENSITY
MAJOR RESULTS OF UVIS
13DEC04 DAYGLOW ANALYSIS
•
•
c’(0,0) IS ABSENT, NI lines are present at 952, 964Å
MAIN EUV/FUV PROCESSES INVOLVE N2
– PDI(XUV 5-410Å),
– PE(e~23 eV),
– PDE PROCESSES(XUV 820-1000Å)
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ALTITUDE OF EUV DAYGLOW 1100 KM
ALTITUDE OF FUV DAYGLOW 900 KM
EUV OUTPUT 2.9 X 107 WATTS-measure of Titan atmos. N2
Dissociation and fast N-atoms
FUV OUPUT (LBH) 3.6 X 107 WATTS-measure of Solar XUV input
FUV OUTPUT (NI,HI,CI) 1 X 108 WATTS
N ATOMS ~1 X 1027 GLOBAL ATOMS/S
MAGNETOSPHERE EXCITATION WEAKER BY ~20:1
UVS & UVIS IN GOOD CROSS CALIBRATION –SOLAR MAX/MIN~ 3:1
TECHNIQUE & CROSS SECTION FOR N2 PLANETARY ATMOSPHERES
COMPARISON OF VOYAGER UVS AND
CASSINI UVIS - 24 YEARS APART
21R
8.9R