Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere

摘要

Saturn's magnetospheric structure and the intensity of radio frequency emissions from immediate surroundings are modulated at close to the planet's rotation period. alogous rotation-modulated variations at Jupiter are readily interpreted as effects of the on-axisymmetric intrinsic magnetic field. At Saturn, to the contrary, the high level of axial mmetry in the intrinsic field suggests that the periodicity is not internally imposed. number of mechanisms have been proposed to account for the observations. Each model plains a subset of the observations in a qualitative manner, but no quantitative models yet ist. Here, using a magnetohydrodynamic simulation, we investigate the magnetospheric rturbations that arise from a localized vortical flow structure in the ionosphere near 0° S-latitude that rotates at roughly the rate of planetary rotation. The model reproduces early quantitatively a host of observed magnetospheric periodicities associated with the riod of the dominant (southern) radio frequency emissions during the Cassini epoch eluding rotating, quasi-uniform magnetic perturbations in the equatorial plane, rotating ass density perturbations, periodic plasmoid releases that we associate with observed ursts of energetic neutral atoms (ENAs), periodic oscillations of magnetospheric undaries, current sheet flapping, and periodic modulation of the field-aligned currents ed to Saturn's kilometric radiation (SKR). The model is not unique but is representative f a class of models in which asymmetric flows in the (as yet unmeasured) upper atmosphere couple to the ionosphere and generate currents that flow into the magnetosphere. It can be extended to include the second periodicity that has been associated with SKR emissions in the northern hemisphere.