Statistical analysis and multi-instrument overview of the quasi-periodic 1-hour pulsations in Saturn's outer magnetosphere

The in-situ exploration of the magnetospheres of Jupiter and Saturn has revealed different periodic processes. In particular, in the Saturnian magnetosphere, several studies have reported pulsations in the outer magnetosphere with a periodicity of about 1 h in the measurements of charged particle fluxes, plasma wave, magnetic field strength and auroral emissions brightness. The Low-Energy Magnetospheric Measurement System detector of the Magnetospheric Imaging Instrument (MIMI/LEMMS) on board Cassini regularly detects 1-hour quasi-periodic enhancements in the intensities of electrons with an energy range from a hundred keV to several MeV. We extend an earlier survey of these relativistic electron injections using 10 years of LEMMS observations in addition to context measurements by several other Cassini magnetospheric experiments. The one-year extension of the data and a different method of detection of the injections do not lead to a discrepancy with the results of the previous survey, indicating an absence of a long-term temporal evolution of this phenomenon. We identified 720 pulsed events in the outer magnetosphere over a wide range of latitudes and local times, revealing that this phenomenon is common and frequent in Saturn's magnetosphere. However, the distribution of the injection events presents a strong local time asymmetry with ten times more events in the duskside than in the dawnside. In addition to the study of their topology, we present a first statistical analysis of the pulsed events properties. The morphology of the pulsations shows a weak local time dependence which could imply a high-latitude acceleration source. We provide some clues that the electron population associated with this pulsed phenomenon is distinct from the field-aligned electron beams previously observed in Saturn's magnetosphere, but both populations can be mixed. We have also investigated the signatures of each electron injection event in the observations acquired by the Radio and Plasma Wave Science (RPWS) instrument and the magnetometer (MAG). Correlated pulsed signatures are observed in the plasma wave emissions, especially in the auroral hiss, for 12% of the electron injections identified in the LEMMS data. Additionally, in about 20% of the events, such coincident pulsed signatures have been also observed in the magnetic field measurements, some of them being indicative of field-aligned currents. This analysis combined with the multi-instrument approach sets constraints on the origin and significance of the pulsed events. Hence, our results suggest that the acceleration process providing the quasi-periodic relativistic electrons takes place at high-latitudes. (C) 2016 Elsevier Inc. All rights reserved.