Dual periodicities in the rotational modulation of Saturn narrowband emissions

Abstract

Using the Cassini Radio and Plasma Wave Science (RPWS) instrument it has recently been shown that the rotational modulation period of Saturn kilometric radiation (SKR) has two components, one with a period of 10.6 h and the other with a period of 10.8 h. The longer period is primarily observed in the southern hemisphere and the shorter period is primarily observed in the northern hemisphere. In this paper, the modulation period of 5 kHz Saturn narrowband radio emissions is examined, restricting the spacecraft location to either the northern or the southern hemisphere of Saturn. It is found that in both hemispheres, the modulation period of 5 kHz narrowband emissions has two components that are equal to the SKR periods. It is known that Saturn narrowband emissions are first generated in the auroral regions as Z‐mode and then mode convert to escaping L‐O modes at density gradients. These Z‐mode waves are trapped in a region close to the planet and can propagate from one hemisphere to another before they mode convert to L‐O modes, thereby leading to dual periods of Saturn narrowband emissions in each hemisphere. The phase shift between the SKR and narrowband emissions is around 90° in the longitude, which means that SKR leads narrowband emissions by 2–3 h.