Magnetospheric period magnetic field oscillations at Saturn: Equatorial phase “jitter” produced by superposition of southern and northern period oscillations

Abstract

[1] We investigate magnetic field oscillations near the planetary rotation period in Saturn's magnetosphere observed during the initial near-equatorial phase of the Cassini mission. Phase determinations on 28 periapsis passes during this ∼2 year interval display pronounced nonrandom “jitter” relative to the ∼10.8 h modulations in the dominant southern Saturn kilometric radiation (SKR) emissions. Phase deviations in the radial and azimuthal components are strongly positively correlated, while being anticorrelated with the phase deviations in the colatitudinal component. This suggests the presence in the equatorial magnetosphere of superposed weaker field oscillations at the ∼10.6 h period of the northern SKR modulations, the phase deviations being shown to be periodic near the corresponding ∼23 day “beat” period. Modeling the effect of the northern period oscillations shows that their amplitude is ∼30%–40% of the southern period oscillations, producing phase deviations of ∼±25°. The relative phasing of the northern period radial and azimuthal fields is such as to form a rotating quasi-uniform field, as for the southern period oscillations, while the phasing of the colatitudinal component indicates perturbation field lines arched with apices pointing to the south, opposite to the southern period field lines that are arched with apices pointing to the north. The northern period field points sunward at northern SKR maxima, consistent with previous observations of the northern polar oscillations and opposite to the southern period field that points tailward at southern SKR maxima. The results support the view that the field oscillations are due to two auroral current systems that rotate with differing periods in the two hemispheres.