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)
(1) Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt (DLR)
(2) Max Planck Institut für Sonnensystemforschung (MPS)
(3) Zentrum für Astronomie und Astrophysik, Technische Universität Berlin (TUB)
(4) Institut für Verkehrsforschung, Deutsches Zentrum für Luft- und Raumfahrt (DLR)
(5) Institut für Weltraumforschung, Österreichische Akademie der Wissenschaften
(6) Abteilung für Weltraumforschung und Planetologie, Universität Bern
(7) Laboratory for Atmospheric and Space Physics, University of Colorado
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 2008/06/22-24
Pascal Hedelt
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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 2008/06/22-24
Pascal Hedelt
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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 2008/06/22-24
Pascal Hedelt
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Input Data: H & CH4 profiles
Methane
Hydrogen
Lammer Model
Chamberlain
model:
Lammer
modelrbits included
Bound
Bound orbits excluded
Lammer MC model
Methane profile
700 – 2,000 km:
INMS TA, TB, T5 data
(De la Haye,et al. 2007)
Chamberlain model
2,000km – 30,000 km:
Particle MC model
(Lammer & Wurz, 2003)
Exobase
Yung
model
INMS
data
Hydrogen profile
700 – 1,500 km:
Rough fit to Yung ‘84
model
1,500 – 30,000 km
Particle MC model
(Lammer & Wurz, 2003)
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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Lammer MC Model
– At exobase: 3D Maxwellian velocity distribution
3D random angle distribution
– 2D calculation of trajectories
1D density distribution
Photoionization is included but unimportant at Titan
Radiative pressure forcing not included
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Monte Carlo Model: Output
Sun
• Output: scattering positions, direction before/after scattering, wavelength
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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Data Sampling Model
• Uses output from MonteCarlo model
• For every Cassini position during T9:
Absorption
at
Absorption function
– Calculates opt.
depthfunction
to each
scattering
pointatin FOV
beginning of flyby
end of flyby
 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
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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Data Sampling Model:
How it works
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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HDAC observations
Cassini closest
approach
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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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 2008/06/22-24
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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 2008/06/22-24
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Best parameter set
(so far…)
Input: TExo = 150K, Tcell=300K, H/CH4: Lammer, nH,Exobase= 4.0E4 cm-3
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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I. Exospheric temperature
Input: Tcell=300K, H/CH4: Lammer, nH,Exobase= 4.0E4 cm-3
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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II. Hydrogen density
Input: TExo = 150K, Tcell=300K, H/CH4: Lammer
Replace by newer plot!!!
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
1
0
-
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III. Hydrogen profile
Fixed density
Input: TExo = 150K, Tcell=300K, nH,Exobase= 1.0E4 cm-3
Replace by newer plot!!!
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
Pascal Hedelt
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III. Hydrogen profile
Variable densities
Input: TExo = 150K, Tcell=300K
-
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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VI. Cell temperature
Input: TExo = 150K, H/CH4: Lammer, nH,Exobase= 4.0E4 cm-3
Replace by newer plot!!!
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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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
cm
• Hydrogen density at exobase has strong impact n(Yung,= 4.2x10
1984)
 Best fitting value close to nH,Exobase= 4.0E4 cm-3 n = 1.0x10 cm
(Broadfoot, et al. 1981)
• Celltemperature has only little impact
H
3
-3
4
-3
H
•
Using HDAC data we are able to determine the hydrogen
density & distribution in Titan’s exosphere!!!
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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Outlook
• Find best fitting parameter sets
• Use HDAC again during another flyby!
• Publish…
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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Thanks for your attention!
UVIS Team Meeting, Boulder, Colorado 2008/06/22-24
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