Planetary induction currents and their effects on magnetospheres

Abstract:

Induced currents control the interaction between most unmagnetized solar system bodies and their plasma environment. This presentation reviews the properties of these induced currents, how they result in different types of interactions and “induced magnetospheres” and how induced currents are also produced within the bodies themselves. These later currents not only affect the magnetospheric interaction, but also allow soundings of the interior structure of solar system bodies, for example the properties of sub-surface oceans inside the Galilean satellites.
One type of current is induced by the convection –u x B electric field of the upstream plasma. This may drive currents through a planet’s (or moon’s) ionosphere, or currents due to pickup ionization in the body’s exosphere. In the case of bodies in a high  plasma, this leads to draping of the upstream magnetic field, diversion of the flow around the body and the formation of a “induced magnetosphere.” This is the case for Venus, Mars, comets and Saturn’s moon Titan. In a low  plasma, such that upstream of Jupiter’s Io and Europa and Saturn’s moon Enceladus, the induced currents produce Alfven wings. A second type of induced entirely with
Currents flowing entirely within a body are due to a time-dependent upstream field. Although the curst of the body may be insulating, the changing magnetic field produces induced currents in any conductive layer within the body. The resulting induced magnetic fields affect the plasma interaction outside the body. In the case of Mercury, this is believed to limit compression of the upstream magnetosphere. In the case of all four of Jupiter’s Galilean moons, the time dependent field is due to the rotation of the planet and the tilt of its magnetic field. Induced signature have been reported for all of these moons, and interpreted as a metallic core, for Io, or a sub-surface ocean, in the case of Europa, Ganymede and Callisto. This allows magnetic field measurements to probe the internal structure of these moons, and is currently the best evidence for a such sub-surface oceans. An active topic of current research is modeling these currents, both external and internal, to refine this magnetic sounding technique.