A comparison of induced magnetotails of planetary bodies: Venus, Mars, and Titan

Abstract

The Pioneer Venus orbiter (PVO), PHOBOS 2, and Voyager 1 spacecraft have together provided observations of three planetary bodies with induced magnetotails: Venus, Mars, and Titan. During the extended mission of PVO, the tail of Venus was probed at an altitude of ∼1.3 planetary radii, which provided a more appropriate basis for comparison with the Mars data (at ∼2.7 planetary radii), and Titan data (∼2.5 planetary radii downstream), than the previously analyzed Venus tail data obtained near PVO apoapsis (∼12 planetary radii). A parallel examination of the magnetic properties of these tails at downstream distances within 3 planetary radii reveals the following similarities and differences. In the cases of Venus and Mars, which are always embedded in the supermagnetosonic solar wind flow, the tail lobe fields are smoothly joined to the draped magnetosheath fields at their outer boundaries, but separated in the center by a distinct, and sometimes narrow, current sheet. The tail of Mars has a cross section that is wider, when scaled by the planet radius, than that at Venus (as found by earlier MARS spacecraft experiments), a lobe field strength that is about the same as that at Venus in spite of the factor of ∼3 smaller interplanetary field at Mars, and a cross‐tail field strength that exceeds that at Venus by ∼1.5 times. The tail of Titan appears similar to the others except that there is no bow shock and little or no draped magnetosheath field signature since the surrounding magnetospheric plasma flow is submagnetosonic (although super‐Alfvenic). The lobe field strengths are about half those at Venus and Mars, while the cross‐tail field is almost negligible. The near‐Titan tail diameter is close to the body diameter. In place of the smooth transition to a draped magnetosheath field at the tail boundaries, as seen at Venus and Mars, the Titan observations show current sheets where the field rotates to its external orientation. It is shown that the Titan wake magnetic signature can be simulated with a model field composed of a cylindrical boundary containing antiparallel axial “tail lobe” fields, surrounded by a field described by streamlines of incompressible flow around a cylinder. Simulation of the magnetic fields observed at Mars and Venus, on the other hand, requires a draped magnetosheath field model with an appropriately oriented comet‐tail‐like model in its interior.