Journal Article

Formation of an Extended Equatorial Shadow Zone for Low‐Frequency Saturn Kilometric Radiation

Kurth, William S., Fischer, Georg, Wang, Mengmeng, Cecconi, Baptiste, Wu, Siyuan, Jackman, Caitriona M., Long, Minyi, Baskevitch, Claire, Ye, Shengyi, Zarka, Philippe, Taubenschuss, Ulrich, Ning, Hao

2024

Overview

Saturn Kilometric Radiation (SKR), being the dominant radio emission at Saturn, has been extensively investigated. The low‐frequency extension of SKR is of particular interest due to its strong association with Saturn's magnetospheric dynamics. However, the highly anisotropic beaming of SKR poses challenges for observations. In most cases, the propagation of SKR is assumed to follow straight‐line paths. We explore the propagation characteristics of SKR across different frequencies in this study. An extended equatorial shadow region for low‐frequency SKR is identified, resulting from the merging of the Enceladus plasma torus and the previously known equatorial shadow zone. Ray‐tracing simulations reveal that low‐frequency (≲ $\\lesssim $100 kHz) SKR is unable to enter the shadow region and is instead reflected toward high latitudes. In contrast, high‐frequency SKR (≳ $\\gtrsim $100 kHz) generally propagates without hindrance. Observations suggest that some low‐frequency SKR can enter the shadow region through reflection by the magnetosheath or leakage from the plasma torus. Plain Language Summary Saturn Kilometric Radiation (SKR) is a natural electromagnetic wave generated in Saturn's high‐latitude region along its magnetic field lines. Variations in SKR frequency could offer insights into Saturn's magnetic conditions, especially its interaction with the solar wind. However, the observed frequency characteristics of SKR depend on viewing geometry due to its directional nature. While past studies assumed SKR travels in straight lines, this may not hold true for low‐frequency SKR. These emissions can change direction when they encounter dense plasma, similar to light reflecting off a mirror or bending when entering water. At Saturn's equatorial region, the plasma torus created by Enceladus, one of Saturn's moons, contains dense plasma and significantly affects radio wave propagation. Our study investigates the distribution of SKR at different frequencies and identifies a shadow region where low‐frequency SKR emissions are rarely seen. Using numerical simulations of ray propagation paths, we discover that low‐frequency SKR emissions cannot reach these shadow regions because they are reflected by the dense plasma torus. However, occasionally, we observe low‐frequency SKR in the shadow region, suggesting the possibility of reflection by Saturn's magnetosheath or leakage through the plasma torus. Key Points The propagation characteristics of Saturn Kilometric Radiation (SKR) are established statistically and by ray‐tracing A shadow region of the low‐frequency SKR near the equatorial region at large radial distances is discovered and discussed Low‐frequency SKR may enter the shadow region due to torus leakage or reflection at the magnetosheath

• Publisher: John Wiley & Sons, Inc,American Geophysical Union,Wiley
• Subject: Deformation / Dense plasmas / Electromagnetic radiation / Emission analysis / Emissions / Enceladus / Equatorial regions / Latitude / Leakage / Magnetic field / Magnetic fields / Magnetosheath / magnetosphere / Magnetospheres / Magnetospheric dynamics / Numerical simulations / Planetary magnetic fields / Plasma / plasma torus / Propagation / Radiation / Radio emission / Radio wave propagation / Radio waves / Ray tracing / Reflection / Saturn / Saturn satellites / Sciences of the Universe / Shadows / Solar magnetic field / Solar wind / Straight lines / Toruses / Wave propagation / Wave reflection