Galactic Cosmic Rays Access to the Magnetosphere of Saturn

Kotova, Anna; Roussos, E.; Kollmann, P.; Krupp, N.; Dandouras, I.

doi:10.1029/2018ja025661

Cited by 12 publications

(7 citation statements)

References 37 publications

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“…Thus, the noise level due to RTG and GCR is retrieved from the G1 measurements at L ≥ 15. Note that the intensities of GCR decrease as approaching the planet, starting from L ∼6, as shown in Kotova et al (). From a set of orbits late in the mission during 2017, this GCR decrease could be fully resolved and empirically quantified.…”

Section: Data Set and Data Processingsupporting

confidence: 66%

Sustaining Saturn's Electron Radiation Belts Through Episodic, Global‐Scale Relativistic Electron Flux Enhancements

et al. 2020

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The L shell distributions of MeV electrons in Saturn's radiation belt are investigated orbit by orbit for the 13-year exploration of Cassini. It is found that in addition to the monotonic decrease profiles, there are orbits showing superimposed transient extensions. The extensions are found to stand out above the background population during 50% of belt crossings. We estimate that there is high probability (>72%) that transient extensions contribute more than 10% electron content in the radiation belt. The high occurrence frequency of one extension every 2-3 weeks, together with the relative content, demonstrate that the extensions constitute a regular and fundamental process populating and sustaining the electron belts of Saturn. The transients regularity excludes interplanetary coronal mass ejections as dominant trigger and implies corotation interaction regions and/or internal processes as candidates. Statistical results suggest that the communication of electrons between the middle magnetosphere and the radiation belts is largely through convective radial transport, which produces transient radiation belt extensions. Plain Language SummaryThe electrons with velocities comparable to light speed residing in Saturn's magnetosphere, namely its electron radiation belt, are highly variable. One prominent feature of this variability is that the shape of permanently trapped population can be severely distorted by episodic, global-scale enhancements. A case study suggested that such an enhancement can be rapidly transmitted from large distances to the radiation belts by global-scale flows. These flows act on electrons in a similar manner engine impulses propel a spacecraft on a different orbit around a celestial body. It is of interest the role played by these enhancements on the evolution of the radiation belt, and which dynamic processes are responsible. In this study, enhancements are sought in the radial distributions of radiation belt electrons for the full, 13-year exploration of Cassini spacecraft. It is demonstrated that these enhancements constitute a regular and fundamental process that populates and sustains the radiation belts of Saturn. The enhancements' regularity implies periodic solar wind perturbations and/or internal magnetospheric dynamics as their major triggers. Among the large number of enhancements analyzed, we find evidence for several traveling radially inward, supporting the scenario that electrons are transported from large distances under the influence of global-scale flows.

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Section: Data Set and Data Processingsupporting

confidence: 66%

Sustaining Saturn's Electron Radiation Belts Through Episodic, Global‐Scale Relativistic Electron Flux Enhancements

et al. 2020

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“…The GCR signal in LEMMS can be directly isolated for times that magnetospheric MeV electron or ion fluxes were below LEMMS's detection threshold, essentially when Cassini was away from Saturn's radiation belts (Appendix A). Because crossings of the radiation belts were brief (typically lasting less than 12 hr), and since GCRs can easily penetrate into the outer and middle magnetosphere of Saturn (Kotova et al 2019), monitoring of GCRs with LEMMS was nearly continuous for 13 yr, even within the boundaries of Saturn's magnetosphere.…”

Section: Instrumentationmentioning

confidence: 99%

Jovian Cosmic-Ray Protons in the Heliosphere: Constraints by Cassini Observations

Roussos

Krupp

Dialynas

et al. 2019

ApJ

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Measurements of >82 MeV Galactic cosmic-ray (GCR) protons at Earth indicate that they may be mixed with protons that leak into the heliosphere from Jupiter's magnetosphere (Jovian cosmic-ray protons (JCRPs)). A ∼400 day periodicity in these proton fluxes, which is similar to the synodic period between Jupiter and Earth, and an excess proton flux observed when Jupiter and Earth can be connected through the interplanetary magnetic field were the basis for this claim. Using nearly 13 yr of GCR measurements at Saturn with Cassini's Magnetosphere Imaging Instrument, we show that the ∼400 day periodicity is also present in 100 MeV protons at ∼9.6 au, although the synodic period between Saturn and Jupiter is ∼20 yr. We also find that the features responsible for this periodicity were convected from 1 au to Saturn's distance with the solar wind velocity. Their origin is therefore heliospheric, not Jovian. We attribute these features to quasi-biennial oscillations, observed in the solar magnetic field and various heliospheric indices. This finding indicates that fluxes of JCRPs at 1 au, if present, are considerably overestimated, because the signal originally attributed to them represents the amplitude of the ∼400 day periodic GCR oscillation. This oscillation has to be subtracted before the resulting proton GCR flux residuals are analyzed in the context of a possible Jovian source. A confirmation of the presence of JCRPs over extended regions in the heliosphere and a constraint on their fractional abundance in GCR spectra may therefore require further validation and analysis, and several options are proposed for this purpose.

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“…We use the particle tracing approach and calculated the trajectories of the particles (electrons and ions), which enter the apertures of MIMI/LEMMS and potentially hit the detector. To trace the high‐energy particles, we used the numerical method based on the Boris scheme (Boris, 1970), modified for relativistic energies as described in Kotova (2016), Kotova et al (2019), and references therein. Based on the position and the pointing of MIMI/LEMMS, we traced the trajectories of particles backwards in time for every second along the orbit of Cassini in order to verify if the particles could freely pass the moon or if they would have been lost on it.…”

Section: Usage Of Aikef Hybrid Code and Particle Tracing Methodsmentioning

confidence: 99%

Magnetospheric Interactions of Saturn's Moon Dione (2005–2015)

et al. 2020

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The moon Dione orbits Saturn at 6.2 Saturn radii R S deep in the Kronian magnetosphere. In situ studies of the moon-magnetosphere interaction processes near Dione were possible with the Cassini/Huygens mission which flew by close to Dione five times at distances between 99 and 516 km. In addition, Cassini crossed Dione's L-shell more than 400 times between 2004 and 2017 and documented the variability of Saturn's magnetosphere. Different flyby geometries allowed to study the interaction processes upstream, in the low-energy wake, and above the north pole of Dione. We describe here the energetic particle measurements from the Low Energy Magnetospheric Measurement System (LEMMS), part of the Magnetosphere Imaging Instrument (MIMI) onboard Cassini. We also use hybrid simulation results from "A.I.K.E.F." to interpret the signatures in the particle fluxes. This paper is a continuation of Krupp et al.

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Galactic Cosmic Rays Access to the Magnetosphere of Saturn

Cited by 12 publications

References 37 publications

Sustaining Saturn's Electron Radiation Belts Through Episodic, Global‐Scale Relativistic Electron Flux Enhancements

Sustaining Saturn's Electron Radiation Belts Through Episodic, Global‐Scale Relativistic Electron Flux Enhancements

Jovian Cosmic-Ray Protons in the Heliosphere: Constraints by Cassini Observations

Magnetospheric Interactions of Saturn's Moon Dione (2005–2015)

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