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

This study investigates the propagation characteristics of Saturn's Narrowband (NB) emissions using a 3D ray-tracing code incorporating Saturn's magnetic field and electron density parameters. The potential source regions and propagation zones of the L-O mode, Z mode and whistler mode NB emissions are distinguished. The L-O mode NB emissions, generated along local electron plasma frequency surfaces through mode conversion, exhibit straight-line propagation but undergo reflections between the ionosphere, the plasma torus, and the magnetosheath. The slot region, characterized by a lower electron density distributed around the plasma torus boundary, significantly influences emission propagation, potentially leading to trapping and depolarization. The 5 kHz Z mode NB emissions propagate and fill the trapping region delineated by lower cut-off and upper hybrid resonance frequencies. In contrast, the 20 kHz Z mode NB emissions are primarily confined near source regions at the north and south edges of the plasma torus, with the possibility of escaping under variable plasma conditions. Scientists have discovered low-frequency radio waves near Saturn, roughly at 5 and 20 kHz, using data from the Voyager and Cassini spacecrafts. These waves are referred to as narrowband (NB) emissions. The NB emissions are thought to originate from Saturn, but exhibit a puzzling propagation pattern when they propagate far away from their source region. Recent studies propose that the 5 kHz waves might be bouncing off a dense plasma area in Saturn's magnetosheath, introducing intriguing yet not fully understood propagation characteristics. This study employs a numerical approach, that is, the ray-tracing technique, to trace the likely paths of these waves. Drawing on earlier theoretical studies, we pinpoint the potential origins of these waves. Our results indicate that most of these waves, particularly the so-called L-O mode emissions, tend to travel toward Saturn's high latitudes. This directional preference is due to interference from a dense plasma torus in Saturn's equatorial region. The boundary of this torus can trap these waves. Meanwhile, the so-called Z mode waves are confined to a specific region near Saturn. Our study unveils intricate features such as multiple reflections and refractions of these waves near Saturn, shedding light on various data observations. A 3D ray-tracing analysis is conducted for Saturn Narrowband (NB) emissions The L-O mode NB emissions propagate toward high latitudes and the Z mode NB emissions are trapped near Saturn Small electron density structures on the plasma torus can lead to trapping and depolarization of NB emissions