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

Abstract

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 hour 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 present a survey of these relativistic electron injections observed during the first 10 years of the Cassini orbital mission. During this period, 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. A statistical analysis of these pulsed events is carried out to investigate their properties. This analysis reveals that the mean interpulse period is 68 ± 19 minutes and that the events are made up of less than 9 pulses in general, but they can include up to 19 pulses. We have also investigated the signatures of each electron injection event in the observations acquired by other Cassini's instruments, in particular 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 multi-instrument approach sets constraints on the origin and significance of the pulsed events.

Pulsations of the polar cusp aurora at Saturn

“…In their survey based on 720 electron pulsed events identified over 10 years of Cassini/LEMMS observations, Palmaerts et al [2016] found an averaged interpulse period of 68 ± 10 min, similar to the periodicities appearing in the cusp aurora brightness. Some of the quasiperiodic ∼68 min electron pulsations are observed at high magnetospheric latitudes (| | > 40 ∘ ) between 05 and 12 LT.…”

“…Furthermore, the magnetically mapped position of Cassini position (shown in Figure 1) indicates that the spacecraft is not crossing the flux tubes connected to the cusp aurora. These pulsations in the energetic electron fluxes are common and observed globally in Saturn's magnetosphere Palmaerts et al, 2016]. These perturbations in B are possibly the signatures of field-aligned currents crossed by Cassini.…”

“…Although this energy range includes the energy of the primary electrons producing Saturn's aurorae [Gérard et al, 2004[Gérard et al, , 2009Lamy et al, 2013], the detected particles are not in the field-aligned direction and the electron pulsations do not magnetically map to the cusp emission. However, ∼1 h fluctuations in charged particle fluxes, in the azimuthal magnetic field component, as well as in the broadband electromagnetic plasma waves are known phenomena in Saturn's magnetosphere [e.g., Mitchell et al, 2009[e.g., Mitchell et al, , 2016Badman et al, 2012;Palmaerts et al, 2016]. However, ∼1 h fluctuations in charged particle fluxes, in the azimuthal magnetic field component, as well as in the broadband electromagnetic plasma waves are known phenomena in Saturn's magnetosphere [e.g., Mitchell et al, 2009[e.g., Mitchell et al, , 2016Badman et al, 2012;Palmaerts et al, 2016].…”

“…However, ∼1 h fluctuations in charged particle fluxes, in the azimuthal magnetic field component, as well as in the broadband electromagnetic plasma waves are known phenomena in Saturn's magnetosphere [e.g., Mitchell et al, 2009[e.g., Mitchell et al, , 2016Badman et al, 2012;Palmaerts et al, 2016]. These quasiperiodic injections of electrons, mainly observed at high energies (from 100 keV to several MeV), occur in Saturn's outer magnetosphere over a wide range of latitudes and local times and at all pitch angles simultaneously, as revealed by two long-term surveys of these events Palmaerts et al, 2016]. In particular, in situ measurements of electron intensities displayed in Figures 4 and 5 include different examples of quasiperiodic 1 h pulsations in the electron fluxes.…”

Quasi-periodic magnetospheric activities of Jupiter and Saturn and magneto{inertial oscillations of their inner radiation belts (extended abstract).

“…The low-frequency receiver of JUNO is capable of detecting QP-40 radio bursts from both Jovian poles but without polarization information. QP-40 bursts of relativistic electrons with energy E ≥ 10 MeV may penetrate the shield for protecting the instruments onboard.In reference to the model scenario and physical interpretations for Jupiter's QP-40 phenomena [Lou 2001;Lou et al, 2012] and especially by striking similarities in several major aspects Palmaerts et al, 2016], we advance here an analogous scenario of global magneto-inertial oscillations of Kronian inner radiation belt (KIRB) for QP-60 phenomena of Saturn as widely detected by various diagnostics of Cassini in-situ observations since early July 2004 [Badman et al, 2016;Mitchell et al, 2016; Palmaerts et al, 2016 and references therein]. With a weaker field, KIRB traps relativistic electrons (up to E ∼ 12 MeV and likely higher) and energetic protons within ∼ 5 R S (Saturn radii) [Kollmann et al, 2011], and the KIRB electron number density is ∼ 50−100 cm −3 [Persoon et al, 2005].…”