Plasma Heating In The Saturn’s Magnetosphere (original) (raw)
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Sources and losses of energetic protons in Saturn's magnetosphere
Icarus, 2008
We present Cassini data revealing that protons between a few keV and about 100 keV energy are not stably trapped in Saturn's inner magnetosphere. Instead these ions are present only for relatively short times following injections. Injected protons are lost principally because the neutral gas cloud converts these particles to energetic neutral atoms via charge exchange. At higher energies, in the MeV to GeV range, protons are stably trapped between the orbits of the principal moons because the proton crosssection for charge exchange is very small at such energies. These protons likely result from cosmic ray albedo neutron decay (CRAND) and are lost principally to interactions with satellite surfaces and ring particles during magnetospheric radial diffusion. A main result of this work is to show that the dominant energetic proton loss and source processes are a function of proton energy. Surface sputtering by keV ions is revisited based on the reduced ion intensities observed. Relatively speaking, MeV ion and electron weathering is most important closer to Saturn, e.g. at Janus and Mimas, whereas keV ion weathering is most important farther out, at Dione and Rhea.
Composition and dynamics of plasma in Saturn's magnetosphere
Science, 2005
During Cassini's initial orbit, we observed a dynamic magnetosphere composed primarily of a complex mixture of water-derived atomic and molecular ions. We have identified four distinct regions characterized by differences in both bulk plasma properties and ion composition. Protons are the dominant species outside about 9 RS (where RS is the radial distance from the center of Saturn), whereas inside, the plasma consists primarily of a corotating comet-like mix of water-derived ions with∼ 3% N+. Over the A and B rings, we ...
Energetic ions trapped in Saturn's inner magnetosphere
Planetary and Space Science, 2009
The low-energy magnetospheric measurement system (LEMMS) of the magnetosphere imaging instrument (MIMI) aboard the Cassini orbiter observed energetic ions and electrons during Saturn orbit insertion (SOI) of July 1, 2004. Salient features of the trapped ion fluxes observed in the L ¼ 2-4R S region include the occurrence of two distinct components of the energy spectrum of energetic protons. We shall refer to protons below 10 MeV as the low-energy component and above 10 MeV as the highenergy component. The low-energy component has a power law energy spectrum that falls at approximately E À2.5 . At about 1 MeV/nucleon, the ion pitch angle distributions tend to peak along and opposite to the magnetic field. The high-energy component has a separate peak in energy at about 20 MeV/nucleon and a pitch angle distribution that peaks at 901 to the magnetic field direction. The pitch angle distributions intermediate in energy evolve systematically from peaking along the field at low energies through isotropy to peaking perpendicular to the field at high energies. Ion species heavier than protons are present at energies from several MeV/nucleon up to 25 MeV/nucleon. Oxygen is separately observed to be present. Molecular hydrogen, H 2 and H 3 and helium are also present although the LEMMS instrumentation is not capable of unambiguously separating these species at multi-MeV energies. These species are measured separately in the outer magnetosphere (L ¼ 6.3-11R S ) with the MIMI CHEMS instrument at energies from 1 to 100 keV/nucleon. This paper will report details of the observations and the results of modeling the abundances of the inner magnetosphere ions to determine constraints on source material and acceleration processes.
Electron sources in Saturn's magnetosphere
Journal of Geophysical Research, 2007
We investigate the sources of two different electron components in Saturn's inner magnetosphere (5 < L < 12 Rs) by performing phase space density (f(v)) analyses of electron measurements made by the Cassini CAPS instrument (1 eV to 28 keV). Because pitch angle distributions indicate that the traditional single particle invariants of gyration and bounce are not appropriate, we use a formulation of the isotropic invariant derived by Wolf (1983) and Schulz (1998) and show that it is similar in functional form to the first adiabatic invariant. Our f(v) analyses confirm that the cooler electrons (<100 eV) have a source in the inner magnetosphere and are likely products of neutral ionization processes in Saturn's neutral cloud. The mystery is how the electrons are heated to energies comparable to the proton thermal energy (which is approximately equal to the proton pickup energy), a process that reveals itself as a source of electrons at given invariant values in our f(v) analyses. We show that Coulomb collisions provide a viable mechanism to achieve the near equipartition of ion and electron energies in the time available before particles are lost from the region. We find that the source of the hotter electron component (>100 eV) is Saturn's middle or outer magnetosphere, perhaps transported to the inner magnetosphere by radial diffusion regulated by interchange-like injections. Hot electrons undergo heavy losses inside L $ 6 and the distance to which the hot electron component penetrates into the neutral cloud is energy-dependent, with the coolest fraction of the hot plasma penetrating to the lowest L-shells. This can arise through energy-dependent radial transport during the interchange process and/or loss through the planetary loss cone.
Ion and neutral sources and sinks within Saturn's inner magnetosphere: Cassini results
Planetary and Space Science, 2008
Using ion-electron fluid parameters derived from Cassini Plasma Spectrometer (CAPS) observations within Saturn's inner magnetosphere as presented in Cassini observations of Saturn's inner plasmasphere: Saturn orbit insertion results. Planet. Space Sci., 54, 1197-1210], one can estimate the ion total flux tube content, N ION L 2 , for protons, H + , and water group ions, W + , as a function of radial distance or dipole L shell. In Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32(14), L14S04), it was shown that protons and water group ions dominated the plasmasphere composition. Using the ion-electron fluid parameters as boundary condition for each L shell traversed by the Cassini spacecraft, we self-consistently solve for the ambipolar electric field and the ion distribution along each of those field lines. Temperature anisotropies from Voyager plasma observations are used with ðT ? =T k Þ W þ 5andðT?=TkÞHþ5 and ðT ? =T k Þ H þ 5andðT?=TkÞHþ2. The radio and plasma wave science (RPWS) electron density observations from previous publications are used to indirectly confirm usage of the above temperature anisotropies for water group ions and protons. In the case of electrons we assume they are isotropic due to their short scattering time scales. When the above is done, our calculation show N ION L 2 for H + and W + peaking near Dione's L shell with values similar to that found from Voyager plasma observations. We are able to show that water molecules are the dominant source of ions within Saturn's inner magnetosphere. We estimate the ion production rate S ION 1027ions/sasfunctionofdipoleLusingNHþ,NWþandthetimescaleforionlossduetoradialtransporttDandion−electronrecombinationtREC.TheionproductionshowslocalizedpeaksneartheLshellsofTethys,DioneandRhea,butnotEnceladus.Wethenestimatetheneutralproductionrate,SW,fromourionproductionrate,SION,andthetimescaleforlossofneutralsbyionization,tION,andchargeexchange,tCH.TheestimatedsourcerateforwatermoleculesshowsapronouncedpeaknearEnceladus′LshellL10 27 ions/s as function of dipole L using N H þ , N W þ and the time scale for ion loss due to radial transport t D and ion-electron recombination t REC . The ion production shows localized peaks near the L shells of Tethys, Dione and Rhea, but not Enceladus. We then estimate the neutral production rate, S W , from our ion production rate, S ION , and the time scale for loss of neutrals by ionization, t ION , and charge exchange, t CH . The estimated source rate for water molecules shows a pronounced peak near Enceladus' L shell L1027ions/sasfunctionofdipoleLusingNHþ,NWþandthetimescaleforionlossduetoradialtransporttDandion−electronrecombinationtREC.TheionproductionshowslocalizedpeaksneartheLshellsofTethys,DioneandRhea,butnotEnceladus.Wethenestimatetheneutralproductionrate,SW,fromourionproductionrate,SION,andthetimescaleforlossofneutralsbyionization,tION,andchargeexchange,tCH.TheestimatedsourcerateforwatermoleculesshowsapronouncedpeaknearEnceladus′LshellL4, with a value S W $2 Â 10 28 mol/s.
OH in Saturn's magnetosphere: Observations and implications
Journal of Geophysical Research, 1998
The discovery of OH in Satum's inner magnetosphere changed our view of this region from one where plasma dominated the physics to one where neutrals are dominant. We revisit Hubble Space Telescope observations of OH and derive revised OH brightnesses for observations in 1992, 1994, and 1995. These OH observations as well as Voyager observations are used as constraints on a model of neutral and plasma interactions. We find that the neutral source required to produce the observed OH brightnesses is 1.4x1027H20 s -•, with a sharp peak in the neutral source rate near 4.5 Rs. A good fit to the data requires OH densities of over 700 cm -3 at 4.5 Rs. Rapid diffusion times, about 5 days at 6 Rs, are required to match the observed ion densities. We find that the plasma and neutral composition vary with distance from Saturn, and make predictions for the ion and neutral densities as a function of radius. ; Richardson and Eviatar; 1988; Richardson and Sittier, 1990; Gan-Baruch et al., 1994] with plasma temperatures, densities, velocities, and thermal anisotropies all known, at least to some extent, along the spacecraft trajectories. The major exception is the ion composition; the distribution of the heavy ions among N, O, OH, H20, and H30 cannot be determined from the Voyager observations. The neutral populations are less well measured. The Voyager ultraviolet spectrometer (UVS) detected neutral H [Broadfoot et al., 1981; Sandel et al., 1982] in the vicinity of the rings and near Titan. Subsequent analysis of these data [Shemansky and Hall, 1992] suggested that an H cloud extends throughout the magnetosphere with densities of the order of 100 cm -3 in the region near Tethys and Dione (at 4.9 and 6.3 Saturn radii, respectively, where 1 Rs = 60330 km). These results provoked controversy, as large H densities were inconsistent with the plasma observations given the published source rates of neutrals due to sputtering [Richardson and Eviatar, 1987; Barbosa, 1990]. This controversy was resolved when Shemansky et al. [1993] used the Hubble Space Telescope (HST) to observe the hydroxyl radical, OH, in emission at 3500 ]k They reported an abundance of 160 cm -3 at 4.5 Rs on the basis of a brightness of 37 R. Observations taken in 1994 by the same group were presented by Matheson and Shemansky [1995] but never published. These data are now in the public domain and are reanalyzed as part of this work. During the ring plane crossing of 1995, HST observations were taken of the ring region. Densities near 2 Rs were of the order of a few hundred [Hall et al., 1996]. These observers also included a vertical scan at 1.8 Rs from which a scale height of H=0.45 Rs was derived. The neutral sources are thought to be primarily energetic particle sputtering and micrometeorite impact on the moons and rings. The sources of neutrals and plasma have been calculated for the satellites [Johnson et al., 1989] and rings [Pospieszalska and Johnson, 1991] and have recently been revised upward by Shi et al. [1995]. The source determined by these calculations (which should be considered a lower limit) is of the order of 1.4x10•H20 s 4. Models have been used to try to explain the observations and to fill gaps in our knowledge, such as the ion composition, the densities of other neutral species, the total plasma source rate, and the plasma transport rate [Richardson et al., 1986; Barbosa, 1990; Shemansky and Hall, 1992; Richardson, 1992]. Although these models have grown more complex, all have had significant omissions and none have previously coupled the neutrals and plasma in a model including radial dependences of plasma parameters. , V. M. Vaylifinas,
Modeling the electron and proton radiation belts of Saturn
Geophysical Research Letters, 2003
1] We present results from a three-dimensional model of the Saturnian radiation belts. This model draws on preexisting physical radiation-belt models, developed to study the belts of the Earth and Jupiter. In the present work, transport processes and interactions with dust and gas clouds, moons, and plasma are considered to determine the trapped particle distribution in Saturn's inner magnetosphere. The results of the modeling are: absorption by dust is the dominant loss effect in the innermost region while local losses from satellites act as the prominent physical process in the outer part of the inner magnetosphere. Results suggest strong energetic neutral atom emission and weak synchrotron emission. Comparisons between data and model results are presented.
Planetary and Space Science, 2009
The origin of the events could not be determined with certainty because of lack of particle charge state and species measurements at lower (o300 keV) energies, which dominate the spectra. High sensitivity observations of energetic ion directional intensities, energy spectra, and ion composition were obtained by the Ion and Neutral Camera (INCA) of the Magnetospheric IMaging Instrument (MIMI) complement, with a geometry factor of 2.5cm2srandsomecapabilityofseparatinglight(H,He)andheavier(C,N,O)iongroups(henceforthreferredtoas′hydrogen′and′oxygen′,respectively).ChargestateinformationwasprovidedwherepossiblebytheCharge−Energy−MassSpectrometer(CHEMS)overtherange2.5 cm 2 sr and some capability of separating light (H, He) and heavier (C, N, O) ion groups (henceforth referred to as 'hydrogen' and 'oxygen', respectively). Charge state information was provided where possible by the Charge-Energy-Mass Spectrometer (CHEMS) over the range 2.5cm2srandsomecapabilityofseparatinglight(H,He)andheavier(C,N,O)iongroups(henceforthreferredtoas′hydrogen′and′oxygen′,respectively).ChargestateinformationwasprovidedwherepossiblebytheCharge−Energy−MassSpectrometer(CHEMS)overtherange3-235 keV per charge, and magnetic field (IMF) data by the MAG instrument on Cassini. The observations revealed the presence of distinct upstream bursts of energetic hydrogen and oxygen ions whenever the IMF connected the spacecraft to the planetary bow shock to distances 480 R S. The events exhibited the following characteristics: (1) hydrogen ion bursts are observed in the energy range 3-220 keV (and occasionally to E4220 keV) and oxygen ion bursts in the energy range 32 to $700 keV. (2) Pitch angle distributions are initially anisotropic with ions moving away from the bow shock along the IMF, but tend to isotropize as the event progresses in time. (3) The duration of the ion bursts is several minutes up to 4 h. (4) The event examined in this study contains significant fluxes of singly charged oxygen. (5) Ion bursts are accompanied by distinct diamagnetic field depressions with b410, and exhibit wave structures consistent with ion cyclotron waves for O + and O ++. Given the magnetic field configuration during the detection of the events and that energetic ions trapped within the magnetosphere of Saturn are H + , H 2 + and various water products including O + , O ++ , we conclude that O-rich upstream events must be particles leaking from Saturn's magnetosphere under favorable IMF conditions. The spectral evolution of the upstream events and their anisotropy characteristics are discussed in the context of current models.