Saturnian magnetospheric dynamics: Elucidation of a camshaft model

摘要

Periodic modulation of magnetospheric phenomena at Earth and Jupiter results principally from the tilt of the dipole axis relative to the rotation axis. Saturn's nearly aligned dipole moment is tilted by less than 0.5° from the spin axis, yet the power of radio-frequency emissions, the orientation of the magnetic field, and many properties of the magnetospheric plasma vary periodically at the approximate rate of Saturn's rotation. Here we examine properties of the periodic magnetic signal detected in the magnetospheric regions inside ～12–15 R S . We show that it is associated with a rotating nonaxisymmetric system of field-aligned currents flowing on magnetic shells bounding the region where the signals are seen. Magnetohydrodynamic ideas suggest that these currents would drive rotating plasma flow patterns in the northern and southern ionospheres, with the flows oppositely directed in the two hemispheres. On magnetic shells beyond the sheets of field-aligned current, the magnetic perturbations generate an effective rotating equatorial dipole moment that when added to the planetary dipole moment, produces a dipole moment tilted relative to the spin axis at an angle of order 12–15°. The overt source of the north–south asymmetric ionospheric circulation could link to different ionospheric conductances that result from nonuniform solar illumination. Other possible sources are discussed but, although we can elucidate much, the origin of the cam signal (as well as other phenomena such as the Saturn kilometric radio emission with close to the same period) remains enigmatic.