It has been known for many years that Saturn emits intense radio emissions at kilometer wavelengths and that this radiation is modulated by the rotation of the planet at a rate that varies slowly on time scales of years. Recently it has been shown that the radio emission consists of two components that have different rotational modulation rates, one emitted from the northern auroral region and the other emitted from the southern auroral region. In this paper we show using radio measurements from the Cassini spacecraft that the rotational modulation rates of the northern and southern components reversed near Saturn's recent equinox, which occurred on 11 August 2009. We show that a similar reversal was also observed by the Ulysses spacecraft near the previous equinox, which occurred on 19 November 1995. The solar control implied by these reversals has important implications on how Saturn's rotation is coupled into the magnetosphere.

[1] 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 ( f ce ) are >20 dB more intense than the usual L-O mode narrowband emissions observed above local f ce . Polarization measurements show that the narrowband emissions observed below f ce are oppositely polarized to those above f ce , which identifies the emissions below f ce 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.

[1] 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.

The ionized upper layer of Saturn's atmosphere, its ionosphere, provides a closure of currents mediated by the magnetic field to other electrically charged regions (for example, rings) and hosts ion-molecule chemistry. In 2017, the Cassini spacecraft passed inside the planet's rings, allowing in situ measurements of the ionosphere. The Radio and Plasma Wave Science instrument detected a cold, dense, and dynamic ionosphere at Saturn that interacts with the rings. Plasma densities reached up to 1000 cubic centimeters, and electron temperatures were below 1160 kelvin near closest approach. The density varied between orbits by up to two orders of magnitude. Saturn's A- and B-rings cast a shadow on the planet that reduced ionization in the upper atmosphere, causing a north-south asymmetry.

Planetary magnetospheres receive plasma and energy from the Sun or moons of planets and consequently stretch magnetic field lines. The process may last for varied timescales at different planets. From time to time, energy is rapidly released in the magnetosphere and subsequently precipitated into the ionosphere and upper atmosphere. Usually, this energy dissipation is associated with magnetic dipolarization in the magnetosphere.This process is accompanied by plasma acceleration and field-aligned current formation, and subsequently auroral emissions are often significantly enhanced. Using measurements from multiple instruments on board the Cassini spacecraft, we reveal that magnetic dipolarization events at Saturn could reoccur after one planetary rotation and name them as recurrent dipolarizations. Three events are presented, including one from the dayside magnetosphere, which has no known precedent with terrestrial magnetospheric observations. During these events, recurrent energizations of plasma (electrons or ions) were also detected, which clearly demonstrate that these processes shall not be simply attributed to modulation of planetary periodic oscillation, although we do not exclude the possibility that the planetary periodic oscillation may modulate other processes (e.g., magnetic reconnection) which energizes particles. We discuss the potential physical mechanisms for generating the recurrent dipolarization process in a comprehensive view, including aurora and energetic neutral atom emissions. Plain Language SummaryUsing measurements from the Cassini spacecraft, we reveal a new feature of magnetic dipolarization at Saturn, that is, the magnetic signature repeat after one planetary rotation, which is named recurrent dipolarization. Up to hundreds of kiloelectron volt electrons and ions are identified for the recurrent dipolarization events, suggesting that these particles have experienced efficient acceleration and cannot be purely due to planetary modulation. It remains a mystery why the magnetic dipolarization process associated with energetic ions and electrons could reoccur after one planetary rotation. Moreover, dipolarization process in Saturn's dayside magnetosphere is reported for the first time at Saturn, which has no known precedent with terrestrial or other planetary magnetospheric observations. The results demonstrate that magnetosphere-ionosphere coupling dynamics at Saturn and Earth have fundamental similarities and differences.