Excitation of extraordinary modes inside the source of Saturn's kilometric radiation

TL;DR

This study investigates the excitation of extraordinary modes, including the relativistic R mode, in Saturn's kilometric radiation sources using particle-in-cell simulations, revealing conditions for mode dominance and aligning with observed emissions.

Contribution

It introduces the relativistic R mode into ECMI analysis and explores its excitation conditions, providing new insights into SKR emission mechanisms.

Findings

The R mode is most unstable at a 24% energetic electron density ratio.

X-mode emissions dominate when energetic electrons exceed 90%.

Results align with linear kinetic theory and observed SKR properties.

Abstract

The electron cyclotron maser instability (ECMI) of extraordinary mode waves was investigated with the parameters observed in Saturn's kilometric radiation (SKR) sources. Previous studies employed simplified dispersion relations, and did not consider the excitation of the relativistic (R) mode. This mode is introduced by considering the relativistic effect in plasmas consisting of both cold and hot electrons. Using particle-in-cell simulations, we investigated the excitation of R and X modes based on the measured data. Using the reported value of the density ratio of energetic to total electrons $n_e/n_0=24\%$, the most unstable mode is the R mode. The escaping X-mode emissions are amplified only if the energetic electrons are dominant with $n_e/n_0 \ge 90\%$. For these cases, only the X mode is excited and the R mode disappears due to its strong coupling. The results are well in line with the linear kinetic theory of ECMI. The properties of both the R and X modes are consistent with the observed SKR emissions. This raises questions about the nature of the measured electric field fluctuations within ``presumed'' SKR sources. The study provides new insights into the ECMI process relevant to SKR emission mechanisms.