Dynamical and Magnetic Field Time Constants for Titan's Ionosphere - Empirical Estimates

CRAVENS, T. E.; RICHARD, M. S.; MA, Y.; BERTUCCI, C.; LUHMANN, J. G.; LEDVINA, S. A.; ROBERTSON, I. P.; WAHLUND, J.; AGREN, K.; CUI, J.; MUELLER-WODARG, I. C.; WAITE, J. H.; DOUGHERTY, M. K.; BELL, J. M.; ULUSEN, D.

Congreso; AGU Fall Meeting 2009; 2009

Plasma in Titan's ionosphere flows in response to forcing from thermal pressure gradients, magnetic forces, gravity, and ion-neutral collisions. Analysis of the coupled ionosphere-magnetosphere dynamics is difficult due to the complex mix of solar and magnetospheric geometries that can exist (i.e., the angle of the ram direction of the external flow relative to the subsolar point), due to the spherical nature of Titan's neutral atmosphere, and due to the complex ion-neutral chemistry. We will present an empirical approach to the ionospheric dynamics by using data from several Cassini instruments to estimate pressures, flow speeds, and time constants for both the dayside and nightside. The coupling of the ionospheric plasma to the neutrals should dominate the plasma transport below an altitude of roughly 1300 km. Vertical or horizontal transport becomes more important than chemistry in controlling ionospheric densities for altitudes above about 1400 - 1500 km, depending on the ion species. The structure of the ionospheric magnetic field should be determined by plasma transport (including any neutral wind effects) for altitude above about 1000 km and by magnetic diffusion at lower altitudes. It is possible that sometimes a velocity shear layer near 1300 km could exist and would affect the magnetic field. Both Hall and polarization electric field terms in the magnetic induction equation can be locally important in determining Titan's ionospheric magnetic field. The dynamics associated with the magnetospheric interaction with Titan's ionosphere is linked with possible loss in the tail.