New Constraints on the Jovian Narrowband Radio Components from Juno/Waves Observations and 3D Geometrical Simulations

Measurements of Waves instrument onboard the Juno spacecraft suggest that narrowband kilometric radiation (nKOM; 20-141 kHz) and narrowband low-frequency radiation (nLF; 5-70 kHz) are generated within the plasma near the Io plasma torus (IPT) in low-latitude regions. While these emissions are thought to result from the conversion of the natural modes of the plasma into escaping radio waves, at either the fundamental or the first harmonic of the plasma frequency, there is no consensus on the specific mechanism involved. Using the electron density and the magnetic field data from the Jovian Auroral Distribution Experiment (JADE) and the FluxGate Magnetometer (FGM), we determine the range of frequencies accessible to different wave modes during Juno's crossing of the plasma disk environment. We classify the observed nKOM and nLF according to their propagation modes: trapped (Z-mode or Whistler), escaping (X-mode or O-mode), and undetermined (either trapped or escaping). We apply the 3D numerical modeling method that was developed in Boudouma et al. (2024) to the escaping and undetermined nKOM and nLF observations, deriving macroscopic constraints on the generation mechanisms, wave modes, characteristic frequencies, beaming and source locations. Our results support the interpretation in which high-latitude nKOM is consistent with O-mode, while low-latitude is rather X-mode. Both nKOM and nLF appear to be generated near the fundamental of the plasma frequency, but only nLF shows compatibility with emission near the first harmonic, suggesting the possible coexistence of both linear and nonlinear generation mechanisms.