Initial interpretation of Titan plasma interaction as observed by the Cassini plasma spectrometer: Comparisons with Voyager 1

The Cassini plasma spectrometer (CAPS) instrument made measurements of Titan's plasma environment when the Cassini Orbiter flew through the moon's plasma wake October 26, 2004 (flyby TA). Initial CAPS ion and electron measurements from this encounter will be compared with measurements made by the Voyager 1 plasma science instrument (PLS). The comparisons will be used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements. The plasma wake trajectories of flyby TA and Voyager 1 are similar because they occurred when Titan was near Saturn's local noon. These similarities make possible direct, meaningful comparisons between the various plasma wake measurements. They lead to the following: (A) The light and heavy ions, H⁺and N⁺/O⁺, were observed by PLS in Saturn's magnetosphere in the vicinity of Titan while the higher mass resolution of CAPS yielded H⁺ and H₂⁺as the light constituents and O⁺/CH₄⁺ as the heavy ions. (B) Finite gyroradius effects were apparent in PLS and CAPS measurements of ambient O⁺ ions as a result of their absorption by Titan's extended atmosphere. (C) The principal pickup ions inferred from both PLS and CAPS measurements are H⁺, H₂⁺, N⁺, CH₄⁺ and N₂⁺. (D) The inference that heavy pickup ions, observed by PLS, were in narrow beam distributions was empirically established by the CAPS measurements. (E) Slowing down of the ambient plasma due to pickup ion mass loading was observed by both instruments on the anti-Saturn side of Titan. (F) Strong mass loading just outside the ionotail by a heavy ion such as N₂⁺ is apparent in PLS and CAPS measurements. (G) Except for the expected differences due to the differing trajectories, the magnitudes and structures of the electron densities and temperatures observed by both instruments are similar. The high-energy electron bite-out observed by PLS in the magnetotail is consistent with that observed by CAPS.     

Ion and neutral sources and sinks within Saturn's inner magnetosphere: Cassini results

Using ion-electron fluid parameters derived from Cassini Plasma Spectrometer (CAPS) observations within Saturn's inner magnetosphere as presented in Sittler et al. [2006a. 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_IONL², for protons, H⁺, and water group ions, W⁺, as a function of radial distance or dipole L shell. In Sittler et al. [2005. 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 (T_⊥/T_∥)_W⁺∼5 and (T_⊥/T_∥)_H⁺∼2. 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_IONL² 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∼10²⁷ ions/s as function of dipole L using N_H⁺, N_W⁺ and the time scale for ion loss due to radial transport τ_D and ion-electron recombination τ_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, τ_ION, and charge exchange, τ_CH. The estimated source rate for water molecules shows a pronounced peak near Enceladus’ L shell L∼4, with a value S_W∼2×10²⁸ mol/s.     

Earth-Affecting Solar Causes Observatory (EASCO): A potential International Living with a Star Mission from Sun–Earth L5

This paper describes the scientific rationale for an L5 mission and a partial list of key scientific instruments the mission should carry. The L5 vantage point provides an unprecedented view of the solar disturbances and their solar sources that can greatly advance the science behind space weather. A coronagraph and a heliospheric imager at L5 will be able to view CMEs broadsided, so space speed of the Earth-directed CMEs can be measured accurately and their radial structure discerned. In addition, an inner coronal imager and a magnetograph from L5 can give advance information on active regions and coronal holes that will soon rotate on to the solar disk. Radio remote sensing at low frequencies can provide information on shock-driving CMEs, the most dangerous of all CMEs. Coordinated helioseismic measurements from the Sun–Earth line and L5 provide information on the physical conditions at the base of the convection zone, where solar magnetism originates. Finally, in situ measurements at L5 can provide information on the large-scale solar wind structures (corotating interaction regions (CIRs)) heading towards Earth that potentially result in adverse space weather.     

Production, ionization and redistribution of O2 in Saturn's ring atmosphere

Molecular oxygen produced by the decomposition of icy surfaces is ubiquitous in Saturn's magnetosphere. A model is described for the toroidal O₂ atmosphere indicated by the detection of and O⁺ over the main rings. The O₂ ring atmosphere is produced primarily by UV photon-induced decomposition of ice on the sunlit side of the ring. Because O₂ has a long lifetime and interacts frequently with the ring particles, equivalent columns of O₂ exist above and below the ring plane with the scale height determined by the local ring temperature. Energetic particles also decompose ice, but estimates of their contribution over the main rings appear to be very low. In steady state, the O₂ column density over the rings also depends on the relative efficiency of hydrogen to oxygen loss from the ring/atmosphere system with oxygen being recycled on the grain surfaces. Unlike the neutral density, the ion densities can differ on the sunlit and shaded sides due to differences in the ionization rate, the quenching of ions by the interaction with the ring particles, and the northward shift of the magnetic equator relative to the ring plane. Although O⁺ is produced with a significant excess energy, is not. Therefore, should mirror well below those altitudes at which ions were detected. However, scattering by ion-molecule collisions results in much larger mirror altitudes, in ion temperatures that go through a minimum over the B-ring, and in the redistribution of both molecular hydrogen and oxygen throughout the magnetosphere. The proposed model is used to describe the measured oxygen ion densities in Saturn's toroidal ring atmosphere and its hydrogen content. The oxygen ion densities over the B-ring appear to require either significant levels of UV light scattering or ion transmission through the ring plane.     

Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols?

Discovery by Cassini's plasma instrument of heavy positive and negative ions within Titan's upper atmosphere and ionosphere has advanced our understanding of ion neutral chemistry within Titan's upper atmosphere, primarily composed of molecular nitrogen, with ~2.5% methane. The external energy flux transforms Titan's upper atmosphere and ionosphere into a medium rich in complex hydrocarbons, nitriles and haze particles extending from the surface to 1200 km altitudes. The energy sources are solar UV, solar X-rays, Saturn's magnetospheric ions and electrons, solar wind and shocked magnetosheath ions and electrons, galactic cosmic rays (GCR) and the ablation of incident meteoritic dust from Enceladus’ E-ring and interplanetary medium. Here it is proposed that the heavy atmospheric ions detected in situ by Cassini for heights >950 km, are the likely seed particles for aerosols detected by the Huygens probe for altitudes <100 km. These seed particles may be in the form of polycyclic aromatic hydrocarbons (PAH) containing both carbon and hydrogen atoms C_nH_x. There could also be hollow shells of carbon atoms, such as C₆₀, called fullerenes which contain no hydrogen. The fullerenes may compose a significant fraction of the seed particles with PAHs contributing the rest. As shown by Cassini, the upper atmosphere is bombarded by magnetospheric plasma composed of protons, H₂⁺ and water group ions. The latter provide keV oxygen, hydroxyl and water ions to Titan's upper atmosphere and can become trapped within the fullerene molecules and ions. Pickup keV N₂⁺, N⁺ and CH₄⁺ can also be implanted inside of fullerenes. Attachment of oxygen ions to PAH molecules is uncertain, but following thermalization O+ can interact with abundant CH₄ contributing to the CO and CO₂ observed in Titan's atmosphere. If an exogenic keV O+ ion is implanted into the haze particles, it could become free oxygen within those aerosols that eventually fall onto Titan's surface. The process of freeing oxygen within aerosols could be driven by cosmic ray interactions with aerosols at all heights. This process could drive pre-biotic chemistry within the descending aerosols. Cosmic ray interactions with grains at the surface, including water frost depositing on grains from cryovolcanism, would further add to abundance of trapped free oxygen. Pre-biotic chemistry could arise within surface microcosms of the composite organic-ice grains, in part driven by free oxygen in the presence of organics and any heat sources, thereby raising the astrobiological potential for microscopic equivalents of Darwin's “warm ponds” on Titan.     

Probing the Fundamental Physics of the Solar Corona with Lunar Solar Occultation Observations

Imaging and spectroscopy of the solar corona, coupled with polarimetry, are the only tools available at present to capture signatures of physical processes responsible for coronal heating and solar wind acceleration within the first few solar radii above the solar limb. With the recent advent of improved detector technology and image processing techniques, broad-band white light and narrow-band multi-wavelength observations of coronal forbidden lines, made during total solar eclipses, have started to yield new views about the thermodynamic and magnetic properties of coronal structures. This paper outlines these unique capabilities, which until present, have been feasible primarily with observations during natural total solar eclipses. This work also draws attention to the exciting possibility of greatly increasing the frequency and duration of solar eclipse observations with Moon orbiting observatories utilizing lunar limb occultation of the solar disk for coronal measurements.     

Cassini observations of Saturn's inner plasmasphere: Saturn orbit insertion results

We present new and definitive results of Cassini plasma spectrometer (CAPS) data acquired during passage through Saturn's inner plasmasphere by the Cassini spacecraft during the approach phase of the Saturn orbit insertion period. This analysis extends the original analysis of Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32, L14S07, doi:10.1029/2005GL022653] to L∼10 along with also providing a more comprehensive study of the interrelationship of the various fluid parameters. Coincidence data are sub-divided into protons and water group ions. Our revised analysis uses an improved convergence algorithm which provides a more definitive and independent estimate of the spacecraft potential Φ_SC for which we enforce the protons and water group ions to co-move with each other. This has allowed us to include spacecraft charging corrections to our fluid parameter estimations and allow accurate estimations of fluctuations in the fluid parameters for future correlative studies. In the appendix we describe the ion moments algorithm, and minor corrections introduced by not weighting the moments with sinθ term in Sittler et al. [2005] (Correction offset by revisions to instruments geometric factor). Estimates of the spacecraft potential and revised proton densities are presented. Our total ion densities are in close agreement with the electron densities reported by Moncuquet et al. [2005. Quasi-thermal noise spectroscopy in the inner magnetosphere of Saturn with Cassini/RPWS: electron temperatures and density. Geophys. Res. Lett. 32, L20S02, doi:10.1029/2005GL022508] who used upper hybrid resonance (UHR) emission lines observed by the radio and plasma wave science (RPWS) instrument. We show a positive correlation between proton temperature and water group ion temperature. The proton and thermal electron temperatures track each with both having a positive radial gradient. These results are consistent with pickup ion energization via Saturn's rotational electric field. We see evidence for an anti-correlation between radial flow velocity V_R and azimuthal velocity V_φ, which is consistent with the magnetosphere tending to conserve angular momentum. Evidence for MHD waves is also present. We show clear evidence for outward transport of the plasma via flux tube interchange motions with the radial velocity of the flow showing positive radial gradient with functional dependence for 4<L<10 (i.e., if we assume to be diffusive transport then D_LL∼D₀L¹¹ for fixed stochastic time step δt). Previous models with centrifugal transport have used D_LL∼D₀L³ dependence. The radial transport seems to begin at Enceladus’ L shell, L∼4, where we also see a minimum in the W⁺ ion temperature . For the first time, we are measuring the actual flux tube interchange motions in the magnetosphere and how it varies with radial distance. These observations can be used as a constraint with regard to future transport models for Saturn's magnetosphere. Finally, we evaluate the thermodynamic properties of the plasma, which are all consistent with the pickup process being the dominant energy source for the plasma.     

Saturn's magnetospheric interaction with Titan as defined by Cassini encounters T9 and T18: New results

We present new results of Cassini's T9 flyby with complementary observations from T18. Based on Cassini plasma spectrometer (CAPS) and Cassini magnetometer (MAG), compositional evidence shows the upstream flow for both T9 and T18 appears composed of light ions (H⁺ and H₂⁺), with external pressures ∼30 times lower than that for the earlier TA flyby where heavy ions dominated the magnetospheric plasma. When describing the plasma heating and sputtering of Titan's atmosphere, T9 and T18 can be considered interactions of low magnetospheric energy input. On the other hand, T5, when heavy ion fluxes are observed to be higher than typical (i.e., TA), represents the limiting case of high magnetospheric energy input to Titan's upper atmosphere. Anisotropy estimates of the upstream flow are 1<T_⊥/T_‖<3 and the flow is perpendicular to B, indicative of local picked up ions from Titan's H and H₂ coronae extending to Titan's Hill sphere radius. Beyond this distance the corona forms a neutral torus that surrounds Saturn. The T9 flyby unexpectedly resulted in observation of two “wake” crossings referred to as Events 1 and 2. Event 2 was evidently caused by draped magnetosphere field lines, which are scavenging pickup ions from Titan's induced magnetopause boundary with outward flux ∼2×10⁶ ions/cm²/s. The composition of this out flow is dominated by H₂⁺ and H⁺ ions. Ionospheric flow away from Titan with ion flux ∼7×10⁶ ion/cm²/s is observed for Event 1. In between Events 1 and 2 are high energy field aligned flows of magnetosphere protons that may have been accelerated by the convective electric field across Titan's topside ionosphere. T18 observations are much closer to Titan than T9, allowing one to probe this type of interaction down to altitudes ∼950 km. Comparisons with previously reported hybrid simulations are made.     

Jovian plasma torus interaction with Europa: 3D hybrid kinetic simulation. First results

The hybrid kinetic model supports comprehensive simulation of the interaction between different spatial and energetic elements of the Europa moon-magnetosphere system with respect to variable upstream magnetic field and flux or density distributions of plasma and energetic ions, electrons, and neutral atoms. This capability is critical for improving the interpretation of the existing Europa flyby measurements from the Galileo orbiter mission, and for planning flyby and orbital measurements (including the surface and atmospheric compositions) for future missions. The simulations are based on recent models of the atmosphere of Europa ( [Cassidy et al., 2007] and [Shematovich et al., 2005]). In contrast to previous approaches with MHD simulations, the hybrid model allows us to fully take into account the finite gyroradius effect and electron pressure, and to correctly estimate the ion velocity distribution and the fluxes along the magnetic field (assuming an initial Maxwellian velocity distribution for upstream background ions). Non-thermal distributions of upstream plasma will be addressed in future work. Photoionization, electron-impact ionization, charge exchange and collisions between the ions and neutrals are also included in our model. We consider two models for background plasma: (a) with O⁺⁺ ions; (b) with O⁺⁺ and S⁺⁺ ions. The majority of O₂ atmosphere is thermal with an extended cold population (Cassidy et al., 2007). A few first simulations already include an induced magnetic dipole; however, several important effects of induced magnetic fields arising from oceanic shell conductivity will be addressed in later work.     

The effect of spacecraft radiation sources on electron moments from the Cassini CAPS electron spectrometer

Data from the Cassini plasma spectrometer (CAPS) electron spectrometer (ELS) have been found to be contaminated with an energy-independent background count rate which has been associated with radiation sources on Cassini. In this paper we describe this background radiation and quantitatively assess its impact on numerically integrated electron moments. The general properties of such a background and its effects on numerical moments are derived. The properties of the ELS background are described and a model for the background presented. A model to generate synthetic ELS spectra is presented and used to evaluate the density and temperature of pure noise and then extended to include ambient distributions. It is shown that the presence of noise produces a saturation of the electron density and temperature at quasi-constant values when the instrument is at background, but that these noise level moments are dependent on the floating spacecraft potential and the orientation of the ELS instrument with respect to the spacecraft. When the ambient distribution has a poor signal-to-noise ratio (SNR) the noise determines the density and temperature; however, as the SNR increases (increasing primarily with density) the density and temperature tend to those of the ambient distribution. It is also shown that these noise effects produce highly artificial density-temperature inverse correlations. A method to subtract this noise is presented and shown to correct for the presence of the noise. Simulated error estimates for the density and temperature are also presented. The analysis described in this paper not only applies to weak background noise, but also to more significant penetrating backgrounds such as those in radiation belt regions of planetary magnetospheres.