An SLS4 Longitude System Based on a Tracking Filter Analysis of the Rotational Modulation of Saturn Kilometric Radiation

Abstract

Saturn has been known for over thirty years to emit an intense radio emission at kilometer wavelengths called Saturn Kilometric Radiation (SKR) that is modulated by the rotation of the planet. Although the period of this modulation was initially thought to represent the rotation period of the planet, it is now known that the radiation has two distinctly different rotational modulation periods that vary by on the order of one percent on times scales of years. One component originates primarily from the northern auroral region, and the other originates primarily from the southern auroral region. The differences in the modulation periods are believed to be due to latitudinal variations in the slippage of the magnetosphere relative to the interior of planet, apparently controlled by the seasonal variation in the tilt of Saturn’s rotational axis. Since other magnetospheric phenomena display similar complicated rotational modulation effects, there is a need to define north and south longitude systems based on the variable SKR modulation periods in the two hemispheres. Because the SKR signal received by the spacecraft often includes both components it is sometimes difficult to separate the phases of the two components. In this paper we describe a method of determining the two phases based on a tracking filter approach that can separately track the modulation waveforms of the two components. The phases of the two waveforms can then be used to define a new longitude system for the northern and southern components that we call the SLS4 longitude system. This is an extension of the previous SLS2 and SLS3 longitude systems, which only described phase variations of the southern component.