Source locations of narrowband radio emissions detected at Saturn

Abstract

Since Cassini's arrival at Saturn in 2004, the Radio and Plasma Wave Science instrument has detected numerous narrowband (NB) radio emission events. These emissions, mostly detected around 5 and 20 kHz, usually occur periodically for several days after intensifications of Saturn kilometric radiation. We present calculations based on an electron density profile of Saturn's plasma torus and a dipole magnetic field model showing that the NB emissions originate from the northern and southern edges of Saturn's plasma torus at L shells ∼ 8 to 10 for 5‐kHz NB and L ∼ 4 to 7 for 20‐kHz NB. In many cases, Cassini passes through the source region of the 20‐kHz NB, as indicated by intense electrostatic upper hybrid (ESUH) waves in close proximity to electromagnetic emissions on spectrograms. The positions of the spacecraft when intense ESUH waves are observed agree with the model predictions of the NB source locations. Source locations determined by goniopolarimetric (also known as direction‐finding) analysis of the NB emissions also support the above results, although sometimes the directions of arrival point toward the region interior to Saturn's plasma torus. A polarization reversal technique is applied to localize the NB emissions observed during spacecraft rotation, on the basis of the fact that the source is within the antenna plane when the apparent circular polarization degree switches sign. The NB emissions are found to be L‐O mode polarized, which is consistent with the prediction of linear/nonlinear mode conversion theory. It is also found that sometimes right‐hand polarized NB emissions are generated at second harmonic frequencies of the 20‐kHz NB; in which case, wave‐wave interactions between oppositely propagating ESUH waves may play an important role in the mode conversion process.