Phase locking among Saturn radio emissions revealed by Cassini observations

Abstract

Context. Rotational modulation has been observed in different magnetospheric phenomena at Saturn, including radio emissions, which reflect the fundamental plasma processes in key regions. Though previous studies have shown Saturn's kilometric radiation, 5 kHz narrowband emissions, and auroral hiss to be rotationally modulated, the modulation features of its 20 kHz narrowband emissions are still unknown. Aims. This work complements previous modulation analysis of Saturn radio emissions by undertaking the analysis of 20 kHz narrowband emissions and comprehensively comparing the phases among the regularly observed radio components. Methods. We carried out a least-squares analysis using the time series of narrowband emissions, which we derived from an event list based on a previous statistical study on Saturn narrowband emissions. Results. We reveal a "phase-lock"relation between the 5 and 20 kHz narrowband emissions and Saturn's kilometric radiation, which suggests these strongly clock-like modulated emissions are connected to the rotating field-aligned current system, with local time preferences for the generation of the radio emissions. This local time preference cannot be well explained by existing theoretical frameworks. Although the phase-lock relation is relatively stable, it may be disrupted during solar wind compression. Therefore, the phase lock between these radio emissions may become a fundamental phenomenon that could help in establishing a global picture of the large-scale dynamics of Saturn's magnetosphere.