Seasonal and radial trends in Saturn’s thermal plasma between the main rings and Enceladus

Elrod, M. K.; Tseng, W. L.; Woodson, A. K.; Johnson, R. E.

doi:10.1016/j.icarus.2014.07.020

Cited by 23 publications

(21 citation statements)

References 30 publications

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“…The strong correlation between the F EUV and the plasma density is somewhat surprising since earlier studies have suggested that a varying photoionization rate due to the changing solar EUV flux alone could not result in such large variations as the one seen in Figures and , (e.g., Elrod et al, ). To estimate the change in density due to the changing solar EUV flux, we derive the photoionization rate coefficients κ j , where j = 1,2,3,4, corresponding to (1) the larger flux at date 30 June 2005, (2) the smaller flux at 30 June 2009, (3) larger flux at 30 June 2015, and (4) smaller flux at 30 June 2016.…”

Section: Resultsmentioning

confidence: 86%

“…As the particles are transported outward, due to collisional interactions, a seasonal variation could also be expected in the particle densities in between the main rings and Enceladus. A seasonal variation was also found in the CAPS n i (Elrod et al, , ) and the RPWS n e (Persoon et al, ). With the 11 years RPWS/LP data set we can now extend the results of these studies and also investigate the solar cycle modulation of Saturn's inner plasma disk.…”

Section: Resultsmentioning

confidence: 86%

“…Tseng et al () predicted that the plasma density within 2.5 R S should show a seasonal variation, assuming that photodissociation of ring particles is the main source of O 2 , and subsequent photoionization resulting in O

_{2}^{+}

, in this region. Elrod et al (, ) showed that the CAPS ion density shows a seasonal modulation in the region 2.4 to 3.5 R S , and this was also found in the RPWS n e (Persoon et al, ). However, the change in insolation angle before the year 2015 follows the change that would be detected due to the changing solar EUV flux.…”

Section: Discussionmentioning

confidence: 91%

“…However, the change in insolation angle before the year 2015 follows the change that would be detected due to the changing solar EUV flux. It is therefore difficult to measure the importance of seasonal changes versus solar cycle, as was also discussed by Elrod et al (, ) and Persoon et al (). The RPWS n e and LP n i measurements from 2016, presented in this paper, show a much better correlation with the solar cycle modulation than with the seasonal variation, Figures to .…”

Section: Discussionmentioning

confidence: 99%

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Density Structures, Dynamics, and Seasonal and Solar Cycle Modulations of Saturn's Inner Plasma Disk

et al. 2017

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We present statistical results from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe measurements recorded during the time interval from orbit 3 (1 February 2005) to 237 (29 June 2016). A new and improved data analysis method to obtain ion density from the Cassini LP measurements is used to study the asymmetries and modulations found in the inner plasma disk of Saturn, between 2.5 and 12 Saturn radii (1 RS=60,268 km). The structure of Saturn's plasma disk is mapped, and the plasma density peak, nmax, is shown to be located at ∼4.6 RS and not at the main neutral source region at 3.95 RS. The shift in the location of nmax is due to that the hot electron impact ionization rate peaks at ∼4.6 RS. Cassini RPWS plasma disk measurements show a solar cycle modulation. However, estimates of the change in ion density due to varying EUV flux is not large enough to describe the detected dependency, which implies that an additional mechanism, still unknown, is also affecting the plasma density in the studied region. We also present a dayside/nightside ion density asymmetry, with nightside densities up to a factor of 2 larger than on the dayside. The largest density difference is found in the radial region 4 to 5 RS. The dynamic variation in ion density increases toward Saturn, indicating an internal origin of the large density variability in the plasma disk rather than being caused by an external source origin in the outer magnetosphere.

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Section: Resultsmentioning

confidence: 86%

Section: Resultsmentioning

confidence: 86%

_{2}^{+}

Section: Discussionmentioning

confidence: 91%

Section: Discussionmentioning

confidence: 99%

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Density Structures, Dynamics, and Seasonal and Solar Cycle Modulations of Saturn's Inner Plasma Disk

et al. 2017

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“…Johnson et al () computed a photolytic flux of 10 6 /cm 2 s for neutral O 2 emission from the rings from solar ultraviolet irradiation but found the radiolytic flux from GCR interaction products to be an order of magnitude lower. However, strong seasonal variability up to several orders of magnitude (Christon et al, , ; Elrod et al, , ) of the magnetospheric O 2 + and other heavy ion population beyond the rings argues against the GCR energy source that varies only slightly over decadal time scales at energies above 20 GeV. Roussos et al () report long‐term changes of CRAND proton fluxes by less than a factor of 2 during the Cassini mission; the GCR flux at the ring neutron source for CRAND in the radiation belts would vary comparably.…”

Section: Discussionmentioning

confidence: 99%

Energetic Radiation From Galactic Cosmic Ray Interactions With Saturn's Main Rings

Cooper

Sturner

2018

JGR Space Physics

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Saturn's main rings have an energetic particle and gamma ray photon radiation environment produced by ring interactions of galactic cosmic ray (GCR) protons and heavier ions penetrating the planetary dipolar magnetic field. Accurate models of this radiation environment are important for interpretation of Pioneer 11 and Cassini in situ measurements near the rings and for constraints on radiolytic contributions to neutral gas production and ice chemistry. A GEANT (GEometry ANd Tracking) based simulation is used to model flux spectra of protons, electrons, positrons, charged pions, neutrons, and gamma ray photons emitted from GCR interactions with H2O ice spheres approximating the ring material. Dependent on location in the A to D rings within the planetary magnetic field of Saturn, only GCR protons above respective energies of 20 to 72 GeV can reach the rings without being deflected away by the magnetic field. Calculated differential and integral fluxes from our simulations have good agreement with in situ Pioneer‐11 measurements in selected energy channels. The charged particle and neutral radiation measurements are sensitive, respectively, to the sizes and areal mass densities of ring bodies. Computed gamma ray emission fluxes are 8% of our calculated limit for detection from the Earth by the Fermi Large Area Telescope. Addition of charged particle sensors and neutron‐photon imaging spectrometers to a future Saturn Ring Observer mission would provide valuable information on the ring mass structure. The present paper provides a foundation for modeling of Pioneer 11 and Cassini radiation measurements across the main rings and future measurements of radiation from the rings.

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Evidence for a seasonally dependent ring plasma in the region between Saturn's A Ring and Enceladus' orbit

et al. 2015

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Equatorial electron density measurements from the Cassini Radio and Plasma Wave Science experiment are derived from the upper hybrid resonance frequency from Saturn Orbit Insertion (SOI) on 1 July 2004 through 21 May 2013. These densities are used to determine the characteristics of the plasma in the inner magnetosphere of Saturn between the outer edge of the A Ring and the orbit of Enceladus. Electron densities obtained when Cassini first arrived at Saturn on 1 July 2004 showed a plasma distribution decreasing radially outward from Saturn in the direction of Enceladus, the expected distribution of a centrifugally driven plasma expanding radially outward from a source in the main rings. We examine equatorial electron densities in the region between 2.4 and 4.0 Rs and show that the density measurements in this region exhibit a strong seasonal dependence resulting from photon‐induced decomposition of icy particles on the ring surfaces, a decomposition process which is controlled by the solar incidence angle. This seasonal dependence will have plasma density implications for Cassini when the spacecraft returns to the region just beyond the A Ring in 2016.

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Seasonal and radial trends in Saturn’s thermal plasma between the main rings and Enceladus

Cited by 23 publications

References 30 publications

Density Structures, Dynamics, and Seasonal and Solar Cycle Modulations of Saturn's Inner Plasma Disk

Density Structures, Dynamics, and Seasonal and Solar Cycle Modulations of Saturn's Inner Plasma Disk

Energetic Radiation From Galactic Cosmic Ray Interactions With Saturn's Main Rings

Evidence for a seasonally dependent ring plasma in the region between Saturn's A Ring and Enceladus' orbit

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