Magnetic field oscillations near the planetary period in Saturn's equatorial magnetosphere: Variation of amplitude and phase with radial distance and local time

Abstract

We present an analysis of the ∼11 h oscillations in Saturn's near-equatorial magnetic field, using Cassini data acquired during 2004-2007. We assume the oscillation period is given by the magnetic phase model derived by Provan et al. (2009) over the same interval, and use this to combine the data to determine the variation of the oscillation amplitude and phase of all three spherical polar field components with radial distance (∼3-30 RS) and local time (RS is Saturn's radius, 60,268 km). The oscillatory field behavior can be divided into two regions at a radial distance of ∼15 RS. In the inner region the radial and azimuthal components form a rotating field that to a first approximation is quasi-uniform, but shows major suppression and deflection effects around the near-planet region. Associated rotating field-aligned currents in the Enceladus torus are estimated to carry ∼±1 MA. In the outer region these field components form a rotating partial twin-vortex centered in the nightside, with associated North-South directed currents carrying ∼±6 MA. Individual current regions of a given sign emerge first at dusk, propagate via midnight, and dissipate near dawn, avoiding the dayside sector of weaker more uniform oscillatory fields. Oscillations in the colatitudinal field are also present throughout, that are generally in phase with the radial component. The oscillation phases of all components are found to increase with radial distance at all local times, indicating outward radial propagation with phase speeds of ∼200 km s-1 on the nightside and ∼500 km s-1 on the dayside.