Flux rope structures in the ionospheres of unmagnetized bodies: comparison study between Mars, Venus and Titan

Magnetic flux ropes have the structure of twisted flux tubes. They are created in the ionosphere of unmagnetized bodies such as Mars, Venus and Titan, by the interaction with the solar wind for the former two, and by the interaction with the Saturnian magnetospheric plasma for Titan. They were first detected at Venus by Pioneer Venus Orbiter during solar maximum, and later by Venus Express during solar minimum. At Mars they are detected by Mars Global Surveyor, and at Titan they are detected by Cassini. Comparison study of the flux rope structures in the three ionospheres can improve our understanding of how flux ropes form and what determines their observed properties. The formation of an ionospheric flux rope is thought to first occur near the boundary between the magnetic barrier and the ionosphere and to be twisted by velocity shear across the flux tube. Later it is pulled into the lower ionosphere and gets twisted further. During this process, the curvature force must overcome the buoyancy force to pull it downward. At Venus, there is more rope occurrence during solar maximum than during solar minimum because the ionosphere is more often magnetized during solar minimum which is unfavorable for rope formation. The observations of both developing ropes and mature ropes and the analysis show that the rope axial orientations agree with the above formation mechanism. The buoyancy force near the Venus' ionopause is slightly larger than the curvature force if the flux tube contains no plasma, but photoionization of exosphere neutrals within the flux tube would reduce the buoyancy force so that the flux tube could be dragged to lower altitudes by the curvature force. At Mars, rope occurrence was observed less frequently than for Venus ropes, probably due to the fact that Mars' ionosphere is more often deeply magnetized. But in general the Mars ropes have similar global characteristics of those at Venus in terms of size and axial orientations. There is no rope observed in the southern hemisphere where there are strong crustal fields. At Titan, the rope occurrence is even less because ionosphere is largely magnetized and there are very few observations in the unmagnetized region. The Titan ropes resemble both types of Venus' flux ropes (either developing or mature ones), and the axial orientations also agree with the formation mechanism. The buoyancy force and curvature force in Titan's ionosphere are comparable even for an empty flux tube so it can be readily pulled into lower regions. The Titan rope size and central field strength are significantly different from those of the Venus and Mars ropes and require further investigation.