1. Model Calculations of the Ionosphere of Titan during Eclipse Conditions
Karin Ågren
IRF-U, LTU

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

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

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

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

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

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

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

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

10. 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

11. 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

12. 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

13. 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

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

15. Chemistry in Titan’s ionosphere
Keller et al, 1998

16. 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

17. 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

18. 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

19. 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

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

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

22. 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 – 450 eV

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

24. Electron spectra
CAPS ELS data
Photoelectrons from
the spacecraft

25. Electron spectra
CAPS ELS data
Passing of Titan

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

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

28. 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

29. Thank you for listening.