Dynamical and magnetic field time constants for Titan's ionosphere: Empirical estimates and comparisons with Venus

Abstract

Plasma in Titan's ionosphere flows in response to forcing from thermal pressure gradients, magnetic forces, gravity, and ion‐neutral collisions. This paper takes an empirical approach to the ionospheric dynamics by using data from Cassini instruments to estimate pressures, flow speeds, and time constants on the dayside and nightside. The plasma flow speed relative to the neutral gas speed is approximately 1 m s−1 near an altitude of 1000 km and 200 m s−1 at 1500 km. For comparison, the thermospheric neutral wind speed is about 100 m s−1. The ionospheric plasma is strongly coupled to the neutrals below an altitude of about 1300 km. Transport, vertical or horizontal, becomes more important than chemistry in controlling ionospheric densities above about 1200–1500 km, depending on the ion species. Empirical estimates are used to demonstrate that the structure of the ionospheric magnetic field is determined by plasma transport (including neutral wind effects) for altitudes above about 1000 km and by magnetic diffusion at lower altitudes. The paper suggests that a velocity shear layer near 1300 km could exist at some locations and could affect the structure of the magnetic field. Both Hall and polarization electric field terms in the magnetic induction equation are shown to be locally important in controlling the structure of Titan's ionospheric magnetic field. Comparisons are made between the ionospheric dynamics at Titan and at Venus.

Investigation of the force balance in the Titan ionosphere: Cassini T5 flyby model/data comparisons

“…Although the present model does not reproduce the detailed variations in the magnetic field observations very effectively, the analysis of the model results is still useful for understanding the ionospheric dynamics at Titan. Unlike Venus, the substantial thickness of Titan's ionosphere and potential horizontal forces and horizontal plasma motion, especially in the region below the exobase ($1500 km) -as suggested by Cravens et al (2010), Müller-Wodarg et al (2008), and Cui et al (2009) -imply that modeling Titan's interaction and ionosphere requires a 3D treatment. Therefore, 3D numerical experiments such as the one presented here are necessary for analysis of Titan's ionosphere.…”

“…This study is also complementary to recent work by Cravens et al (2010), where an empirical approach is taken to explore the ionospheric dynamics in Titan's ionosphere. In their work, estimates for thermal and magnetic pressures, plasma flow speeds and time constants of different ionospheric processes are obtained by using Cassini data for the T17, T18, and T5 flybys and by estimating pressure gradients using characteristic length-scales.…”

“…(2) amounts to a relatively simple statement of pressure balance. Cravens et al (2010) solved Eq. (2) for the flow velocity and used approximate length-scales to estimate the pressure gradient terms.…”

“…The variation of the angle between the solar radiation and corotating magnetospheric plasma due to Titan's motion around Saturn complicates the nature of the interaction making the ionosphere difficult to analyze. In addition to this complex geometry, intricate ion-neutral chemistry makes the governing ionospheric dynamics even more difficult to understand thoroughly (Fox and Yelle, 1997;Keller et al, 1998;Nagy and Cravens, 1998;Cravens et al, 2010;Sittler et al, 2009;Rymer et al, 2009;Bertucci et al, 2009;Simon et al, 2010).…”

The orientation of Titan’s dayside ionosphere and its effects on Titan’s plasma interaction

“…Localized heating of the atmosphere by ion precipitation may have a significant impact on the neutral density profiles. In addition it has been suggested that the neutral winds may have an influence on how the magnetic field slides around Titan (Cravens et al, 2010). These are just some of the outstanding issues that remain to be addressed.…”