1. Latest Results of HDAC analysis
Pascal Hedelt(1), Yuichi Ito(2), H. U. Keller(2), H. Lammer(5), Heike Rauer(1,3), Ralf Reulke(4), P. Wurz(6), L. Esposito(7)
Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt (DLR)
Max Planck Institut für Sonnensystemforschung (MPS)
Zentrum für Astronomie und Astrophysik, Technische Universität Berlin (TUB)
Institut für Verkehrsforschung, Deutsches Zentrum für Luft- und Raumfahrt (DLR)
Institut für Weltraumforschung, Österreichische Akademie der Wissenschaften
Abteilung für Weltraumforschung und Planetologie, Universität Bern
Laboratory for Atmospheric and Space Physics, University of Colorado

2. Aims & Scope
Using HDAC data gathered during T9, the distribution of atomic hydrogen in Titan‘s exosphere is investigated:
Calculate exospheric emission of resonantly scattered Hydrogen Ly-Alpha from Titan
Simulate HDAC measurement during the Cassini/Titan T9 encounter
Little is known about Titan‘s hydrogen exosphere
Vary input parameters
Determine exospheric parameters
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3. HDAC noise reduction Problem: Undersampled CELL OFF data
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4. Noise reduction Problem in difference signal:
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5. Noise reduction Solution: Take average of datapoints nearby
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6. Parameter Variation Exosphere hydrogen distribution:
Particle Monte Carlo model (Wurz & Lammer, 2003)
Chamberlain model (Chamberlain, 1963)
Exospheric temperature
Exobase hydrogen density
 Fit to data
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7. Profile Variation Methane Hydrogen Particle MC model Chamberlain model
Exobase
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8. Model description Chamberlain model Particle MC model
Maxwellian velocity distribution at exobase
Use Liouville theorem to derive exospheric densities
Static contribution of ballistic/escaping orbits
Particle MC model
Dynamic MC approach, single particles started at exobase with random energy & ejection angle
Independent of partition functions
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9. Profile Variation Exobase H density: 1x104 cm-3
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10. Temperature Variation
Exosphere temperatures in the literature:
De la Haye et al. (2008):
152.8 ± 4.6 K (Ta)
149.0 ± 9.2 K (Tb)
157.4 ± 4.8 (T5)
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11. Temperature Variation
Exobase H density: 8x104 cm-3, Particle MC model profile
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12. Density Variation Exobase densities in the literature
nH = 4.2x103 cm-3 (Yung, model)
nH = 8x103 cm-3 (Toublanc, model)
nH = 1x104 cm-3 (Broadfoot, et al., data)
nH = 8x104 cm-3 (De la Haye, et al., model)
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13. Density Variation
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14. Discussion: Why signal decreases when increasing density?
In the model, HDAC flyby takes place within model boundaries
Model boundaries: 700 – 30,000 km
HDAC: 25,468 km (Start) – 12,985 km (lowest)
Increasing density
 More photons scattered above HDAC
 Fewer photons arriving and scattered below
 Decreasing signal
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15. Fit to data UVIS Team Meeting 2009 Pascal Hedelt
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16. Best fitting H profiles
Chamberlain profile:
nExo,H= 1x104 cm-3
Particle MC profile:
nExo,H= 8x104 cm-3
Cassini/HDAC measuring altitude
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17. Discussion HDAC signal strength depends on H column density above HDAC
Both profiles nearly parallel in this region
Measurement nearly independent of density distribution below

18. Summary & Conclusion Using HDAC data we are able to determine atomic
Plausible agreement between model and data
Exospheric temperature has no influence
Best fitting hydrogen density at lowest measurement altitude (12,985 km): approx. 400 cm-3
Depending on hydrogen density profile, exobase values vary strongly
 Best fit using Chamberlain model: nH,Exobase= 1.0x104 cm-3
validated for Earth only, static model
 Best fit using Particle MC model: nH,Exobase= 8.0x104 cm-3
validated for Mercury & Mars, dynamic model  Better choice (?)
From latest calculations (De la Haye, et al. 2007): nH,Exobase= 8.0x104 cm-3
 Good agreement with Particle MC model distribution
Using HDAC data we are able to determine atomic
hydrogen distribution in Titan’s exosphere!!!
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19. Thanks for your attention!
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20. Density regions crossed by HDAC
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