Titan ionospheric conductivities from Cassini measurements

摘要

We present the first results of ionospheric conductivities at Titan based on measurements during 17 Titan flybys from the Cassini spacecraft. We identify an ionospheric region ranging from 1450 ± 95 km (approximately the location of the exobase) to approximately 1000 km where electrical currents perpendicular to the magnetic field may become important. In this region the ionosphere is highly conductive with peak Pedersen conductivities of 0.002-0.05 S/m and peak Hall conductivities of 0.01 -0.3 S/m depending on Solar illumination and magnetospheric conditions. Ionospheric conductivities are found to be typically higher on the sunlit side of Titan. However, Hall and Pedersen conductivities depend strongly on the magnetic field magnitude which is highly variable, both in altitude and with respect to the draping configuration of Saturn's magnetic field around Titan. Furthermore, a consistent double peak nature is found in the altitude profile of the Pedersen conductivity. A high altitude peak is found to be located between 1300 and 1400 km. A second and typically more conductive region is observed below 1000 km, where the magnetic field strength drops sharply while the electron density still remains high. This nature of the Pedersen conductivity profile may give rise to complicated ionospheric-atmospheric dynamics and may be expected also at other unmagnetized objects with a substantial atmosphere, such as e.g. Mars and Venus. Estimates of the total Pedersen conductance are found to range between 1300 and 22,000 S. The Pedersen conductance is always higher than the local Alfven conductance but the difference varies by two orders of magnitude (from a factor 4 to 100). Thus, the conditions for reflection or absorption of Alfven waves in Titans ionosphere are highly variable and depends strongly on the magnetic field strength.