LIMB SKIM SATURN DATA:
Study of H2 airglow emission
J. Gustin - I. Stewart
UVIS Team Meeting - Goslar - June 11-13 2007
1. UVIS Limbskim dataset overview + viewing geometry
2. Model of reflected sunlight component
3. Comparison between H2 observed spectra and model
(Moses atmosphere with limb line of sight)
4. Conclusion
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
-
3. Comparison UVIS H2/ synthetic H2
-
4. Conclusions
Start time
Integration (s)
Number of records
Mean latitude, longitude (deg)
1.
Oct. 29 2005 19:27:45
120
4
-17, 26.5
2.
Oct. 29 2005 19:35:33
30
77
3-16.5, 35
3.
Nov. 27 2005 08:57:53
75
7
-17.5, 133.5
4.
Nov. 27 2005 09:07:07
18
68
-18, 140
5.
Nov. 27 2005 13:17:55
75
7
-17.5, 227.5
6.
Nov. 27 2005 13:27:07
18
88
-18, 238
 Total number of spatial pixels x readouts = 14133 spectra, for EUV & FUV low res. slit
Viewing geometry:
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
-
3. Comparison UVIS H2/ synthetic H2
-
4. Conclusions
processing of data:
- rtg (0.0005 cnts s-1 pixel-1)
- flatfield with NaN & interpolation at evil pixels
- red patch
- Ly  background
- sum of spectra  bins of 50 km (altitude range: -1350 km, 4100 km)
- remove bgd  average spectrum of last 10 high-alt. spectra
example of spectrum:
reflected sunlight
airglow H2
Overlap of emissions for spectra
between -1350 and 700 km 
need to build a model of Saturn’s
reflectivity to separate the two
contributions to remove the
reflected  emission and get
“pure” H2 emission
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
-
3. Comparison UVIS H2/ synthetic H2
-
4. Conclusions
Model:
- viewing geometry  need a model with spherical atmosphere and limb LOS
- Wallach & Hapke paper (Icarus 63, 354-373, 1985) : “Light scattering in spherical, exponential
atmosphere”
- application to Saturn spectra with reflected  component (fit of [1500 - 1880Å] window):
 multiple scattering version of the theory
 atmosphere is supposed homogeneous
 H2 as scatterer, mixing ratio fixed to 0.975
 absorbers as parameters: * C2H2 m. r., with C2H4, C3H6p, CH3C2H, CH2CCH2, C4H2,
C4H8 absorbers included, tied to C2H2 (average ratio to C2H2 from Moses et al.
2000)
* C2H6 m.r., with C3H8 and C4H10 included, tied to C2H6
 Synthetic H2 spectrum included to fit [1500 - 1650Å] window
 each UVIS spectrum = sum of spectra from 6 observations ( multiple , 0 )
 model applied 6 times and weighted sum compared to UVIS spectrum
 reflected  in the [1500 - 1880Å] window is derived and removed from data
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
-
3. Comparison UVIS H2/ synthetic H2
-
4. Conclusions
Result:
- fit of 40 spectra from -1320 km to 625 km
kR/ Å
- corresponding C2H2 m. r. from 2.3x10-7
to 6.4x10-6
- Moses at 300 km (1mbar) : 5x10-7
Wavelength (Å)
- HC m.r. of bestfit used to build Saturn’s
kR/ Å
reflectivity from 1500 to 1880 Å
- multiplied by solar spectrum
- removed from UVIS spectra to get “pure”
H2 spectra
Wavelength (Å)
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
-
3. Comparison UVIS H2/ synthetic H2
-
4. Conclusions
Comparison between UVIS H2 spectra and H2 synthetic airglow spectra:
Observed emergent H2 spectrum = sum of H2 emissions along LOS, attenuated by HC
U
V
LOS
I
z0
min rayheight
S
•
1 bar level
Saturn
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
-
3. Comparison UVIS H2/ synthetic H2
 (s) 
-
4. Conclusions

n
HC
(s')  HC ds'
s

LOS
z0
s

z
H2(z0)
 (s) = (z)
H2 vol. em. rate
Rsat
H 2 (z0 ) 

  (s) e 
 (s)
ds
S

H2 unabsorbed synthetic spectrum  provided by J. Hallet
Needed:
atmospheric model  Moses et al., 2000
H2 volume emission rate vs altitude  build from UVIS spectra
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
-
3. Comparison UVIS H2/ synthetic H2
-
4. Conclusions
Synthetic volume emission rate:
- for all UVIS spectra: sum of the [1550-1620 Å] region (smallest influence of HC) and plot vs z
- observed brightness profile:
B

: H2 volume emission rate

S: path along los
  (s) ds
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
- fit of B with  as a Chapman function:
-
3. Comparison UVIS H2/ synthetic H2

   m exp{ f (1   e )}
-
4. Conclusions
 = (z-zm)/H reduced height
f = shape factor
H = scale height

. f = 0.43,
. m= 0.70,
. zm = 968 km,
. HH2 = 136.7 km
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
-
3. Comparison UVIS H2/ synthetic H2
-
4. Conclusions
Preliminary results:
- Use of 9 HC: CH4, C2H2, C2H6, C3H8, C4H10, C2H4, C4H2, CH3C2H, C3H6p
- Synthetic spectra absorbed from -1325 to 1025 km
775 km
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
1375 km
-1025 km
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2. Reflected 
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3. Comparison UVIS H2/ synthetic H2
-
4. Conclusions
Systematic excess of intensity
in [1235-1275A] region for
observed spectra above 1075 km
Systematic excess of intensity
in [1560-1630A] region for
observed spectra below 525 km
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
-
3. Comparison UVIS H2/ synthetic H2
-
4. Conclusions
Consequences:
Study of color ratio: - I [1550-1620Å] / I [1230-1300Å]
- measure of CH4 absorption (strong  < 1400 Å)
- CR = 1.5 for unabsorbed spectrum
Low s/n  datapoints scattered
around theoretical value
Observed CR too small
due to unexpected
signal ~ 1250 Å
Increasing of observed CR
due to increase of unexpected
signal ~ 1600 Å
UVIS Team Meeting - Goslar - June 11-13 2007
1. Overview
-
2. Reflected 
-
3. Comparison UVIS H2/ synthetic H2
-
4. Conclusions
Conclusions:
Reflected sunlight
- good fits of reflected sunlight component for low altitude UVIS spectra
- C2H2 observed mixing ratios in the range of previous studies/ models
- observed/modeled C2H2 can be compared for each altitude: need more work to be interpreted
H2 airglow emission
- use of a model with realistic geometry
- Moses atmosphere is a first step
- first of all: need to understand the emissions at ~1250 Å and ~1600 Å
UVIS Team Meeting - Goslar - June 11-13 2007