Model Calculations of the Ionosphere of Titan during Eclipse Conditions 2006-03-21 Karin Ågren IRF-U, LTU

Contents Introduction Neutral atmosphere Ionisation calculations Cassini Titan Neutral atmosphere Ionisation calculations Chemistry in Titan’s ionosphere Ion and electron spectra Conclusion

Cassini ESA/NASA-project Launch: Oct 1997 Saturn: July 2004 RPWS Langmuir probe INMS CAPS Huygens

Cassini 44 planned flybys of Titan T5 – April 16 2005 Closest approach: 1025 km INMS, CAPS, RPWS data collected rather deep flyby nightside pass

Titan r = 2575 km 2nd largest moon in the solar system Dense atmosphere Mainly N2 Orbits Saturn at a distance of 20.3 Rs

Titan Ionisation sources: EUV radiation Photoelectrons Magnetospheric e- Cosmic rays Proton and ion precipitation

Titan We would like to investigate whether magnetospheric impacting electrons alone can account for the observed ionisation during T5

Neutral atmosphere Based on work by Roger Yelle N2 - main constituent CH4 H2 - escapes the moon

Neutral atmosphere Minor constituents HCN Values achieved from Ingo Müller-Wodarg C2H4 HC3N Based on models by Yung and Toublanc

Ionisation calculations M. H. Rees 200 eV < E < 50 keV the effective range is dependent on the assumption that the average energy loss per ionising collision is constant this breaks down for low energies of electrons

Ionisation calculations F = electron flux [cm-2 s-1] E = energy of the incoming electrons [eV] (s/R) = energy dissipation function [-] (z) = mass density [g cm-3] R(E) = effective range [g cm-2] ion = energy loss per ion formation [eV] 37 eV for N2

Ionisation calculations The ionisation rate shown for a unidirectional beam and an isotropic distribution The unidirectional distribution goes straight into the atmosphere shows better agreement to data

Ionisation calculations Model for E < 200 eV achieved from Prof. Dirk Lummerzheim propagates an electron spectra into a neutral atmosphere obtains the ionisation rate as a function of column density known column density as a function of altitude of Titan  ionisation rate as a function of altitude 10, 20, 30, 40, 50, 70, 100 and 150 eV

Ionisation calculations Combining the models peak altitudes as a function of electron energy good agreement can be seen

Chemistry in Titan’s ionosphere Keller et al, 1998

Chemistry in Titan’s ionosphere Main reactions: N2+ + CH4  CH3+ + N2 + H CH3+ + CH4  C2H5+ + H2 C2H5+ + HCN  HCNH+ + C2H4 Recombination reaction: HCNH+ + e-  HCN + H

Chemistry in Titan’s ionosphere Beside the main reactions we look at the formation of C3H2N+ C5H5N+ By adding the ion densities we achieve the total electron density

Ion spectra The Toublanc model shows better agreement to observation This is most clearly seen by looking at the density of heavy ions at low altitudes

Ion spectra A cross-over can be seen at approximately the same altitude in both figures The heavy ions are common at low altitudes, after which they quickly lose importance for the total electron density

Ion spectra A cross-over can be seen at approximately the same altitude in both figures

Ion spectra The heavy ions are common at low altitudes, after which they quickly lose importance for the total electron density

Electron spectra Electron density measured by Cassini Density profiles achieved from our model The upper limit for the electron energy is 450 eV By adding the profiles we achieve an energy range of the incoming electrons of 30 – 450 eV

Electron spectra CAPS ELS data Photoelectrons from the spacecraft Passing of Titan Incoming electrons in an energy range of appr. 10 – 500 eV

Electron spectra CAPS ELS data Photoelectrons from the spacecraft

Electron spectra CAPS ELS data Passing of Titan

Electron spectra CAPS ELS data Incoming electrons in an energy range of appr. 10 – 500 eV

Electron spectra The same energy range of the incoming electrons in the model and in the actual data

Conclusion By using our model we have shown that magnetospheric impacting electrons alone can account for the observed density profile This applies for electron energies and flux

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