Z mode waves as the source of Saturn narrowband radio emissions

Abstract

We report 5 kHz narrowband Z mode emissions observed by the Cassini Radio and Plasma Waves Science (RPWS) instrument during high latitude perikrone passes. The narrowband emissions observed below the local electron cyclotron frequency (fce) are >20 dB more intense than the usual L‐O mode narrowband emissions observed above local fce. Polarization measurements show that the narrowband emissions observed below fce are oppositely polarized to those above fce, which identifies the emissions below fce with Z mode. We propose that the L‐O mode narrowband emissions observed at 5 kHz are mode converted from the Z mode waves at a density gradient or density irregularity. The Z mode to L‐O mode conversion via scattering off of density irregularities can also account for the direction finding results of 5 kHz narrowband emissions which point to a source in the auroral zone. We also present the first magnetic field measurements of Saturn narrowband emissions validating their electromagnetic nature.