LIMB SKIM SATURN DATA
J. Gustin - I. Stewart
UVIS Team Meeting - Pasadena - Jan. 8-10
1. Limbskim dataset overview + viewing geometry
2. Goals
3. Emission = H2 dayglow + reflected 
4. Model of Saturn’s reflectivity, applied to disc data
5. Conclusion
UVIS Team Meeting - Pasadena - Jan. 8-10
1.  Dataset overview:
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 - Pasadena - Jan. 8-10
2. Goal: obtain informations on the emission profiles of
H2 dayglow
a. Sort spectra by altitude
U
los
V
I
rayheight
S
•
1 bar level
Saturn
 sum spectra  bins of 50 km ( increase S/N and keep good spatial resolution)
 altitude range : [-1350 km, 4100 km]
UVIS Team Meeting - Pasadena - Jan. 8-10
b. Sum the intensity for selected spectral regions and display the result
versus altitude
 2 spectral regions: 1230-1300 Å and 1550-1620 Å
 In auroral spectroscopy, 1550-1620 Å :”unabsorbed” region; 1230-1300 Å: “absorbed” region.
The unabs/abs ratio defines the color ratio, which is a measure of the intensity absorbed by the HC
overlying the emission peak
* 1550-1620 Å
* 1230-1300 Å
UVIS Team Meeting - Pasadena - Jan. 8-10

 Observed brightness: B 

  (s) ds

(1)
: volume emission rate
S: path along los
 Specify  as a function of z: 2 cases

No absorber between emission and observer (1550-1620 Å)
•
   m exp{ f (1   e )}
 = (z-zm)/H reduced height
f = shape factor
H = scale height
Best fit gives zm , m, f , H
• S=S(z)
• Evaluate (1) and compare with
observed B(z)
UVIS Team Meeting - Pasadena - Jan. 8-10
Absorber between emission and observer (1230-1300 Å)
 Addition of the absorber in (1):
• B

  (s) e 
 (s)
ds (2)

 (s) 


n
HC
(s')  HC ds' (3)
s

n HC (z)  n0 e
zz0
H HC
Best fit gives in addition n0, HHC


• Evaluate (3), (2) and compare
with observed B(z)
UVIS Team Meeting - Pasadena - Jan. 8-10
3. Problem: observed emission at long wavelength
= H2 + reflected sunlight
reflected sunlight
airglow H2
Contribution of reflected  that affects the H2 profile  need to build a model of
Saturn’s reflectivity to remove the reflected  emission and get “pure” H2 emission
UVIS Team Meeting - Pasadena - Jan. 8-10
4. Model of Saturn reflectivity
 simple model: single scattering reflectivity
1
1
 (  )
I
  0   ( )  (1 e   0 )
F
  f  1
a a

  0  1
  f  
s s 



albedo, with f: mixing ratio, : cross-section
  ( )  0.75  (1 cos2 ( ))
phase function, with : phase angle

 For each 50 km bin: spectrum = sum of individual spectra taken at different observation time
 multiple values for ,0 
 For each individual spectrum: reflected  emission is the sum of the emissions along LOS
 also multiple values for ,0 
 Model valid for a plane-parallel slab but not for a limb observation, where the ray path crosses
curved atmospheric layers
 Test of reflectivity model on data with simpler geometry
UVIS Team Meeting - Pasadena - Jan. 8-10
 Application to Saturn disc data (Oct 10, 2005)
Set of 9 observations:
exp. time of 300 s, 2 readouts
Synthetic H2 spectrum at UVIS resolution
added to the fit (between 1500-1620 Å)
Fit better if shift of data of +1.5 Å
UVIS Team Meeting - Pasadena - Jan. 8-10
UVIS
model
Scatterer: H2
Absorbers: C2H2, C3H6p, C2H4, C2H6,
CH3C2H
Vertical columns of best fit:
H2:
C2H2:
C3H6p:
C2H4:
C2H6:
CH3C2H:
1.9 x 1023 cm-2
1.1 x 1018 cm-2
1.5 x 1016 cm-2
3.4 x 1016 cm-2
1.1 x 1020 cm-2
1.5 x 1016 cm-2
Comparison with atm. model (Moses et al, 2000)
H2 as reference:
Observed H2 column  ~315 km & ~0.6 mbar
Observed HC columns are from 10 to 100x
higher than model.
C2H2 as reference:
Observed C2H2 column  ~70 km & ~170 mbar
Other observed HC columns: factor 2 higher for C2H4,
C2H6, factor 10 for CH3C2H, factor 35 for C3H6p
 Early stage: behavior of model is promising
UVIS Team Meeting - Pasadena - Jan. 8-10
5. Conclusions
 Comparison between observed reflected
 data and model, observed
columns and columns from an atmospheric model are already “acceptable” need to compare with other models
 Relative difference between the total intensity in the 1550-1620Å spectral
region for model and disc data < 2%
 applicable to limb skims data
 Model of Saturn reflectivity may be improved by:
- addition of other hydrocarbons
- addition of PH3, NH3, H2O
- addition of haze
- addition of other processes?
 After limbskim study: complete study of disc data: seek for latitudinal and
longitudinal variations of Saturn’s photochemistry
UVIS Team Meeting - Pasadena - Jan. 8-10