University of Kansas
Image courtesy of NASA/JPL-Caltech.
T. E. Cravens, J. Vann, J. Clark, J. Wu, C. N. Keller, and C. Brull
The final version of this paper appears in Advances in Space Research, 33, 212, 2004 (special issue on Planetary Atmospheres, Ionospheres and Plasma Interactions, edited by E. Kallio and H. Shinagawa).
Abstract with link to full article through ScienceDirect.
Abstract. Titan has an atmosphere consisting mainly of molecular nitrogen and methane. Solar extreme ultraviolet and x-ray radiation and energetic electrons from Saturn's magnetosphere interact with the upper atmosphere producing an ionosphere. This paper describes improvements to earlier models of Titan's ionosphere. In particular, we consider in more detail ion production from solar ionizing radiation for solar zenith angles beyond the terminator, and a higher spectral resolution soft x-ray flux is adopted in the ion production rate calculations. We demonstrate that significant photoionization takes place well beyond the terminator. K-shell photoionization is also included, and this process adds Auger electrons to the ionospheric photoelectron spectrum, which we model using the two-stream transport code. Our calculated photoelectron spectrum shows a distinct Auger electron peak near an energy of 400 eV.
|Figure 1. Upward photoelectron flux as a function of electron energy at an altitude of 1220 km and for a solar zenith angle of 60 deg.|
|Figure 2. Production rate of N2+ ions versus altitude for 60 deg. solar zenith angle. The primary (i.e., from photons), secondary (i.e., from superthermal photoelectrons), and total ionization rates are shown.|
|Figure 3. N2+ production rate for a range of solar zenith angles. The curves are labeled with the solar zenith angle in units of degrees.|
|Figure 4. CH4+ production rate for a range of solar zenith angles.|
|Figure 5. Electron density altitude profiles for several solar zenith angles for the solar-only conditions of this paper. The highest production rates are for 0 deg. solar zenith angle.|
|Figure 6. HCNH+ density profiles for several solar zenith angles.|
Acknowledgments: Support is acknowledged from NASA Planetary Atmospheres grant NAG5-11038 and from the NASA Cassini project via a University of Michigan subcontract to the University of Kansas.
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