Spontaneous Axisymmetry Breaking of Saturn's External Magnetic Field
Peter Goldreich ( IAS) - Sep 11 at 12:00 noon
Saturn's magnetic field is remarkably axisymmetric. Its dipole axis is
inclined by less than 0.2 degrees with respect to its rotation axis. Rotationally
driven convection of magnetospheric plasma breaks the axisymmetry of its
external magnetic field. Field aligned currents transfer angular momentum
from the planet to a tongue of outflowing plasma. This transfer slows the rate
of rotation of the ionosphere relative to that of the underlying atmosphere.
The currents are the source for the non-axisymmetric components of the field.
The common rotation rates of these components and Saturn's kilometric
radio (SKR) bursts is that of the plasma near the orbit of Enceladus, and by
extension the rotation rate in the ionosphere to which this plasma is coupled.
That rate tells us nothing about the rotation rate of Saturn's deep interior.
Of that we remain ignorant.
Magnetic perturbations with magnitudes similar to those observed by
Cassini are produced for dM/dt ~ 10^4 g/s, a value similar to estimates for the
rate of production of plasma from Saturn's E-ring.
Enhancement of the SKR occurs in a narrow range of longitudes where
the tip of the outgoing plasma stream connects to the auroral ionosphere
via field lines that are bowed outwards by currents that supply the plasma's
centripetal acceleration.
The seminar will be held in 544 Campbell Hall.
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