Phase locking among Saturn radio emissions revealed byCassiniobservations

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.

Ray‐Tracing Analysis for the Propagation of Saturn Narrowband Emission Within the Saturnian Magnetosphere

“…The injected hot plasma also provides favorable conditions for the excitation of the lower hybrid waves and whistler mode chorus emissions, which may further erode the temperature anisotropy (Wing et al., 2020). Notably, the NB emissions usually show modulation features (Wang et al., 2010; Wu, Ye, et al., 2023; Ye et al., 2016), which exhibit a good correlation to the periodic plasma injections (Kinrade et al., 2023; Wing et al., 2020). Hence, the source regions and the free energy of the wave generation for the NB emissions may also be tied to these hot plasma injection events.…”

“…The detailed propagation of these Z mode waves within the trapping regions is still unknown. The presence of dual modulation periods in NB emissions observed within a single hemisphere (Wu, Ye, et al., 2023; Ye, Gurnett, et al., 2010), as opposed to the modulation of SKR which shows one period per hemisphere (Fischer et al., 2015; Gurnett et al., 2009; Lamy, 2011; Ye et al., 2016), implies that Z mode NB emissions in the TR may traverse to the opposite hemisphere before mode converting to L‐O mode.…”