Rotational Modulation of the High Frequency Limit of Saturn Kilometric Radiation

The high frequency limit (HFL) of the Saturnian Kilometric Radiation (SKR) can probe the deepest SKR sources, closest to Saturn's ionosphere. In this study, we determined and analyzed the SKR HFL throughout the entire Cassini Saturn orbital tour. The maximum frequency of the northern SKR, whose distribution peaks at similar to 625 kHz, is shifted by +100 to +200 kHz from the distribution of southern SKR HFL, consistent with the magnetic field offset toward the northern hemisphere at Saturn. The uniformly observed SKR HFL in the vicinity of Saturn suggests a broad extent and beaming of the SKR source. When the observer is confined to certain locations, the rotational modulation of the SKR HFL is clearly observed. This modulation feature of the SKR HFL is statistically established and analyzed in this study. The modulation of HFL is best observed at mid-latitudes between 10 degrees and 40 degrees and at almost all local times. We perform a simulation that suggests that the modulation of HFL requires the superposition of a "clock" like and a rotating source behavior. By comparing the derived HFL modulation using different longitudes with variable and fixed rotation periods, we can exclude the existence of a magnetic anomaly that was proposed in a previous study based on the Voyager data. The calculation of the least-square periodogram confirms that the modulation observed in HFL is similar to the ones previously detected at Saturn. Plain Language Summary Auroral radio emission from Saturn, namely the Saturn Kilometric Radiation (SKR), is generated along high latitude magnetic field lines via the resonance between energetic electrons and a wave's electric field. The first work on the high frequency limit (HFL) of SKR dates back to 1991. Using data from the Voyager Saturn fly-by, scientists found an asymmetry when the HFL is organized by the longitude of the Sun. Based on this asymmetry, a hypothesis about the existence of a magnetic anomaly in Saturn's magnetic field was proposed, which was a novel and breakthrough discovery at that time, but the later Cassini measurements did not confirm this magnetic anomaly. Cassini's expedition around Saturn with 13-yr continuous measurements provided an opportunity to re-study the HFL of SKR. The long-term statistics allow us to exclude the magnetic anomaly hypothesis and instead attribute the asymmetry to a modulation which is introduced by an ionospheric/magnetospheric current system at Saturn. A simulation suggests that both temporal and spatial effects play a role to a certain degree. The average frequency and visibility of the HFL are also discussed. These new results provide new insights into the studies of cyclotron maser-related radio emissions.