A radiation belt of energetic protons located between Saturn and its rings

Roussos, E.; Kollmann, P.; Krupp, N.; Kotova, Anna; Regoli, Leonardo; Paranicas, C.; Mitchell, D. G.; Krimigis, S. M.; Hamilton, D. C.; Brandt, P. C.; Carbary, J. F.; Christon, S. P.; Dialynas, K.; Dandouras, I.; Hill, M. E.; Ip, W.-H.; Jones, G. H.; Livi, S.; Mauk, B. H.; Palmaerts, Benjamin; Roelof, E. C.; Rymer, A. M.; Sergis, N.; Smith, H. T.

doi:10.1126/science.aat1962

Cited by 31 publications

(51 citation statements)

References 53 publications

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“…Such signatures point to a significant but variable dynamic role in these processes for the D ring material and its interaction with the equatorial atmosphere. Following Brice and McDonough (1973), it is further conceivable that the related variable flows thus envisaged play a role in the transport and loss of the trapped energetic charged particles also observed on intra-D ring field lines (Roussos et al, 2018).…”

Section: Possible Origins Of Intra-d Ring Azimuthal Field Variabilitymentioning

confidence: 95%

Variability of Intra–D Ring Azimuthal Magnetic Field Profiles Observed on Cassini's Proximal Periapsis Passes

et al. 2019

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We overview the properties of the azimuthal magnetic fields observed during the periapsis passes of the final 23 orbits of the Cassini spacecraft, including the partial orbit at end of mission, on near equatorial field lines passing inside of Saturn's D ring. The signatures are variable in form and amplitude, though generally approximately symmetric about the point where the spacecraft trajectory lies tangent to a flux shell, corresponding to where the ionospheric field line feet map closest to the equator, consistent with the effect of interhemispheric field‐aligned currents. The perturbations usually begin and end near symmetrically at some point on field lines threading the D ring and extend into the interior region, but in no case do they clearly extend outward onto field lines passing through the C ring. About 35% of cases display a ~20‐40 nT single positive central field peak indicative of southward field‐aligned current flow, while a further ~30% display two or three weaker ~10‐20 nT positive peaks indicative of multiple sheets of northward and southward current. Significant smaller‐scale >5 nT peak‐to‐peak field fluctuations are commonly superposed. A further ~20% of cases exhibit unique profiles within the data set, including two with ~20‐30 nT negative fields and two with only <10 nT fluctuating fields. The variable nature of the signatures is not connected with the pass altitude, local time, planetary period oscillation phase, or D68 ringlet phase but may relate to variable structured thermospheric winds and/or ionospheric conductivities that suggest a significant dynamical role for D ring‐atmosphere interactions.

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Section: Possible Origins Of Intra-d Ring Azimuthal Field Variabilitymentioning

confidence: 95%

Variability of Intra–D Ring Azimuthal Magnetic Field Profiles Observed on Cassini's Proximal Periapsis Passes

et al. 2019

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“…In this near-equatorial region containing Saturn's inner radiation belt, the counting rates on INCA's stop MCP are driven primarily by penetrating particles. The inner radiation belt (18) is composed primarily of very-high-energy protons (≥300 MeV) that easily penetrate the walls of the INCA sensor and produce counts in the detector proportional to both the start and stop areas. The correspondence between the stop rates and the fraction of the start rates that are driven by penetrators can be used to predict and then subtract the contribution from penetrators to the start rates, producing a background-corrected version of the start rates that we attribute to dust impacts (16).…”

Section: Dust Measurements In the Gap Between Saturn And Its D-ringmentioning

confidence: 99%

“…There was no discernable signal in any of the CHEMS stop rates or solid-state detector rates, other than the backgrounds from penetrating particles. This region is devoid of any trace of energetic charged particles in the design range for CHEMS or INCA (18), and at this altitude, the intensity of the energetic radiation belt particles is also very low, resulting in background low enough to measure this relatively weak signal. We have inferred that the response in T2 was produced by positively charged grains with E/q > 40 keV/e, up to the maximum measurable value of 220 keV/e.…”

Section: Dust At Lower Altitudesmentioning

confidence: 99%

Dust grains fall from Saturn’s D-ring into its equatorial upper atmosphere

Mitchell

Perry

Hamilton

et al. 2018

Science

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The sizes of Saturn’s ring particles range from meters (boulders) to nanometers (dust). Determination of the rings’ ages depends on loss processes, including the transport of dust into Saturn’s atmosphere. During the Grand Finale orbits of the Cassini spacecraft, its instruments measured tiny dust grains that compose the innermost D-ring of Saturn. The nanometer-sized dust experiences collisions with exospheric (upper atmosphere) hydrogen and molecular hydrogen, which forces it to fall from the ring into the ionosphere and lower atmosphere. We used the Magnetospheric Imaging Instrument to detect and characterize this dust transport and also found that diffusion dominates above and near the altitude of peak ionospheric density. This mechanism results in a mass deposition into the equatorial atmosphere of ~5 kilograms per second, constraining the age of the D-ring.

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“…This proton response becomes dominant when fluxes of MeV electrons are negligible, as will be the case with the data we present here. Because of that, the proton energy coverage of LEMMS extends above 300 MeV, beyond the instrument's design capabilities (Roussos et al 2018b).…”

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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A radiation belt of energetic protons located between Saturn and its rings

Cited by 31 publications

References 53 publications

Variability of Intra–D Ring Azimuthal Magnetic Field Profiles Observed on Cassini's Proximal Periapsis Passes

Variability of Intra–D Ring Azimuthal Magnetic Field Profiles Observed on Cassini's Proximal Periapsis Passes

Dust grains fall from Saturn’s D-ring into its equatorial upper atmosphere

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

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