Recent Knowledge: Plasma Interaction of Titan
Plasma Flow around Titan
Saturn spins around its axis once every 10.2 hours. Titan orbits Saturn in the equatorial plane with a radius of 20.3 Saturn radii and a period of 16 days. The plasma ('gas' of charged particles) populating the magnetosphere of Saturn is driven by electro-magnetic forces and moves nearly with the same angular velocity as Saturn. Friction and outward directed mass transport decelerate this co-motion in such a way that the plasma streams towards the backside (referring to the motion of Titan) of Titan with a relative speed of 120 km/h (appr. 60% of co-rotation velocity of Saturn). Since the interaction of the magnetospheric plasma with Titan shows some interesting features, many researchers including the institute of Geophysics and Meteorology of the University of Cologne investigate the dynamics close to this satellite.
The interaction is determined by the properties of the magnetospheric plasma and the composition of the dense atmosphere covering Titan's surface. Under certain circumstances the properties of Titan's solid body can also affect the interaction. In the following we are going to describe some features of the interaction.
The Frozen-In Magnetic Field
A magnetic field is influenced by the motion of charged particles via the Lorentz force, leading to a gyration of the particles around the magnetic field lines. If the conductivity of the plasma is sufficiently large and thereby the coupling of electric and magnetic fields is sufficiently strong, the motion can be described as the motion of the magnetic field lines with the mean velocity of the plasma particles. The field lines are said to be 'frozen' in the plasma. Therefore the plasma passing Titan carries the magnetic field lines of the Saturnian field.
Mass Loading
When the magnetospheric plasma approaches Titan's atmosphere, the fast charged particles of the ionospheric plasma interact with the ionospheric plasma and the neutral atmosphere of Titan. The hot electrons ionize neutral particles of the atmosphere. However, the main source of ionization and thereby the main cause for the formation of Titan's ionosphere is the EUV radiation of the sun. The ionospheric particles, independent of their origin, interact with the Saturnian magnetic field. Thus the mass of the magnetospheric plasma enlarges close to Titan. This effect is called mass loading. Conservation of energy and momentum forces the plasma to slow down when its mass is increased.
Draping
The field lines of the Saturnian magnetic field are frozen in the magnetospheric plasma. Thus, the part of the field lines that intersect the mass loaded part of the plasma must slow down velocity together with the plasma particles, and the parts of the field lines intersecting that parts of the plasma where no mass loading takes place try to keep velocity leading to a deformation of the field lines. For this situation there are two limiting cases depending on the ratio of the kinetic energy of the plasma particles and the magnetic field energy. In a strong magnetic field there is only a small deformation of the magnetic field lines by the plasma. In a weak magnetic field the plasma forces the magnetic field lines to drape around the body (see picture). In the case of Titan the magnetic field is relatively weak.
Some Special Features of Titan
Finally, we want to point out some special features of Titan.
Apart from the plasma velocity the velocities of waves propagating in the plasma are important to characterize the plasma interaction. In the case of Titan the three velocities, the slow and fast magnetosonic velocity and the Alfvén velocity, are of the same order of magnitude. Thus, Titan is the only body in our solar system with a distinct atmosphere and this kind of plasma interaction.
The sun is the main ionization source for the formation of Titan's ionosphere. Therefore the ion density of the ionosphere on the sun-facing side of Titan differs significantly from the ion density on the dark side of Titan. The side of Titan facing the magnetospheric flow varies with a period of 16 days (see picture). One question is under which circumstances the ion density in the ionosphere is sufficient to prevent the Saturnian magnetic field to reach the surface of Titan by forming a magnetopause. If the magnetic field reaches the surface, induction by conducting layers of Titan becomes important.
A further peculiarity of Titan is his orbit near the magnetopause of the Saturnian magnetosphere towards the solar wind near the local noon. In times of high solar activity the magnetopause is displaced towards Saturn and Titan is outside of the Saturnian magnetosphere in the solar wind. This situation is comparable to that of Venus.
Titan bares a lot of scientific questions. Soon the Cassini/Huygens mission will give us answers to, hopefully, most of them. The Institute of Geophysics and Meteorology of the University of Cologne is involved in the Cassini/Huygens misssion by a participation in the magnetometer experiment (MAG) and the atmospheric structure experiment (ASI).
Plasma Parameter of Titan:
Plasma Velocity Background Magnetic Field | 80 - 150 km/s 5 nT |
Electron Number Density H+ Number Density N+ Number Density | 0.3 cm-3 0.1 cm-3 0.2 cm- |
Electron Temperature H+ Temperature N+ Temperature | 200 eV 210 eV 2.9 keV |
Autor: Heiko Backes