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

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

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

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

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 2008/06/22-24 Pascal Hedelt

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

Monte Carlo Model: Output Output: scattering positions, direction before/after scattering, wavelength Sun UVIS Team Meeting, Boulder, Colorado 2008/06/22-24 Pascal Hedelt

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 2008/06/22-24 Pascal Hedelt

Data Sampling Model: How it works UVIS Team Meeting, Boulder, Colorado 2008/06/22-24 Pascal Hedelt

HDAC observations Cassini closest approach UVIS Team Meeting, Boulder, Colorado 2008/06/22-24 Pascal Hedelt

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 Pascal Hedelt

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 Pascal Hedelt

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 Pascal Hedelt

I. Exospheric temperature Input: Tcell=300K, H/CH4: Lammer, nH,Exobase= 4.0E4 cm-3 UVIS Team Meeting, Boulder, Colorado 2008/06/22-24 Pascal Hedelt

II. Hydrogen density Input: TExo = 150K, Tcell=300K, H/CH4: Lammer - Replace by newer plot!!! - UVIS Team Meeting, Boulder, Colorado 2008/06/22-24 Pascal Hedelt

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 2008/06/22-24 Pascal Hedelt

III. Hydrogen profile Variable densities Input: TExo = 150K, Tcell=300K - - UVIS Team Meeting, Boulder, Colorado 2008/06/22-24 Pascal Hedelt

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 2008/06/22-24 Pascal Hedelt

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 2008/06/22-24 Pascal Hedelt

Outlook Find best fitting parameter sets Use HDAC again during another flyby! Publish… UVIS Team Meeting, Boulder, Colorado 2008/06/22-24 Pascal Hedelt

Thanks for your attention! UVIS Team Meeting, Boulder, Colorado 2008/06/22-24 Pascal Hedelt