1. HDAC analysis: Hydrogen in Titan‘s exosphere
Pascal Hedelt(1), Yuichi Ito(2), Heike Rauer(1,3), Ralf Reulke(4), H. U. Keller(2), H. Lammer(5), 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 Titans 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
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

3. Overview of this talk 3D Monte Carlo Model Data Sampling Model
HDAC observations
Parameter variations & comparison with HDAC Data
Hydrogen distribution
Exosphere temperature
Cell temperature
Conclusions
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

4. Monte Carlo Model
Investigate scattering of solar Lyα radiation on H atoms in Titan’s exosphere
3D model
Scattering medium: H; absorbing medium: CH4
Altitude range considered: 700 – 30,000km
Resonance scattering (isotropic):
Redistribution function from Henyey 1940
Considers Maxwellian motion of H atoms
Follow 2,500,000 photons within one quarter of the model sphere until they leave at upper/lower boundary or are absorbed; then mirror to get the whole sphere
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

5. Input Data: H & CH4 profiles
Methane profile 700 – 2,000 km: INMS TA, TB, T5 data (De la Haye,et al. 2007)
2,000km – 30,000 km: Particle MC model (Lammer & Wurz, 2003)
Hydrogen profile
700 – 1,500 km: Rough fit to Yung ‘84 model
1,500 – 30,000 km
Particle MC model (Lammer & Wurz, 2003)
Methane
Hydrogen
Lammer Model
Chamberlain model: Bound rbits included
Bound orbits excluded
Lammer MC model
Lammer model
Chamberlain model
Exobase
Yung
model
INMS data
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

6. Lammer MC Model
At exobase: 3D Maxwellian velocity distribution D random angle distribution
2D calculation of trajectories
1D density distribution
Photoionization is included but unimportant at Titan
Radiative pressure forcing not included

7. Monte Carlo Model: Output
Output: scattering positions, direction before/after scattering, wavelength
Sun
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

8. Data Sampling Model Uses output from MonteCarlo model
For every Cassini position during T9:
Calculates opt. depth to each scattering point in FOV  probability for photon to reach detector
Sum up all photons within FOV within discrete wavelength bins
Incorporate FOV sensitivity
Multiply with cell absorption function
Integrate over wavelength
Absorption function at
beginning of flyby
Absorption function at
end of flyby
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

9. Data Sampling Model: How it works
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

10. HDAC observations
Cassini closest
approach
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

11. Comparison with HDAC Data
Compare model & measurement:
Take difference: CELL OFF - H CELL ON
Do the same for simulated data…
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

12. Parameter Variations
Vary exospheric temperature: T = 149 – 157.4K (De la Haye, et al. 2007)
Vary exosphere hydrogen number density: At Exobase: nH = 4.2x103 cm-3 (Yung, 1984) nH = 1.0x104 cm-3 (Broadfoot, et al. 1981)
Vary exospheric distribution of H Lammer MC model / Chamberlain model
Vary cell temperature
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

13. Best parameter set (so far…)
Input: TExo = 150K, Tcell=300K, H/CH4: Lammer, nH,Exobase= 4.0E4 cm-3
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

14. I. Exospheric temperature
Input: Tcell=300K, H/CH4: Lammer, nH,Exobase= 4.0E4 cm-3
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

15. II. Hydrogen density Input: TExo = 150K, Tcell=300K, H/CH4: Lammer-
Replace by newer plot!!! -
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

16. III. Hydrogen profile Fixed density
Input: TExo = 150K, Tcell=300K, nH,Exobase= 1.0E4 cm-3
Replace by newer plot!!!
Replace by newer plot!!!
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

17. III. Hydrogen profile Variable densities
Input: TExo = 150K, Tcell=300K - -
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

18. VI. Cell temperature
Input: TExo = 150K, H/CH4: Lammer, nH,Exobase= 4.0E4 cm-3
Replace by newer plot!!!
Replace by newer plot!!!
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

19. Summary & Conclusion
Principal agreement between model and data (work in progress)
Exospheric temperature has no visible influence
Hydrogen density profile has strong impact  Lammer model more realistic
Hydrogen density at exobase has strong impact  Best fitting value close to nH,Exobase= 4.0E4 cm-3
Celltemperature has only little impact
Using HDAC data we are able to determine the hydrogen density & distribution in Titan’s exosphere!!!
nH = 4.2x103 cm-3 (Yung, 1984) nH = 1.0x104 cm-3 (Broadfoot, et al. 1981)
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

20. Outlook Find best fitting parameter sets
Use HDAC again during another flyby!
Publish…
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt

21. Thanks for your attention!
UVIS Team Meeting, Boulder, Colorado 2008/06/ Pascal Hedelt