Water Group Ions Research Articles

BepiColombo observations of cold oxygen and carbon ions in the flank of the induced magnetosphere of Venus

On 10 August 2021, the Mercury-bound BepiColombo spacecraft performed its second fly-by of Venus and provided a short-lived observation of its induced magnetosphere. Here we report results recorded by the Mass Spectrum Analyzer on board Mio, which reveal the presence of cold O+ and C+ with an average total flux of ~4 ± 1 × 104 cm−2 s−1 at a distance of about six planetary radii in a region that has never been explored before. The ratio of escaping C+ to O+ is at most 0.31 ± 0.2, implying that, in addition to atomic O+ ions, CO group ions or water group ions may be a source of the observed O+. Simultaneous magnetometer observations suggest that these planetary ions were in the magnetosheath flank in the vicinity of the magnetic pileup boundary downstream. These results have important implications regarding the evolution of Venus’s atmosphere and, in particular, the evolution of water on the surface of the planet.

Reactive scattering of water group ions on ice surfaces with relevance to Saturn's icy moons

Collisions of hyperthermal water group ions with ices are known to occur in astrophysical environments, for example, solar wind-comet interactions and moon-magnetosphere interactions (at Saturn and Jupiter), yet their effect on surface and gaseous composition is little understood. Here, we report molecular dynamics simulations of hyperthermal water-group molecules impacting water-ice surfaces using the ReaxFF reactive force fields to describe bond breaking and forming processes dynamically during the reactive collision events. Simulation conditions are chosen for relevance to Saturn's moons Dione and Rhea. At impact velocities of 10–35 kms−1 (8 to 102 eV for O), we find that water group projectiles undergo Eley-Rideal reactions with the ice surface to form O2, HO2, and H2O2. At a 45o angle of incidence the OO bonded products tend to embed within a few molecular layers of the surface. Smaller angles of incidence lead to formation of some molecular oxygen that promptly rebounds from the surface with ~3 eV kinetic energy. Impact velocity, angle of incidence, and hydrogenation of the impactor strongly influence O2 formation, with the yield peaking at 19% for atomic oxygen impactors at 20 kms−1. Notably, this reaction is relatively temperature independent and proceeds in hydrogen rich pristine ice, where radiolysis is hindered. This mechanism for O2 production from low energy impacts may contribute to O2 exospheres at Saturn and Jupiter's icy moons, where hyperthermal ion fluxes are large.

Zebra Stripe Patterns in Energetic Ion Spectra at Saturn

AbstractMagnetospheric convective electric fields that could radially transport particles adiabatically, may leave their imprints in the form of energy‐time dispersed patterns termed ”zebra stripes”. Inspired by comprehensive studies on zebra stripes in Earth's electron radiation belts, similar electron signatures were found and explained in Saturn's inner magnetosphere. Using Cassini Magnetosphere Imaging Instrument observations, we demonstrate that the zebra‐stripes in proton and water group ion spectrograms at the energy range from 30 keV to ∼1 MeV, which coexist with electron zebra‐stripes, can be traced to the same electric field, even though the associated radial flows were originally thought to be too slow to disturb the fast azimuthal energetic ion drift. The simultaneous observations of zebra stripes in electrons and ions further establish the importance of the planet's noon‐midnight electric field as a regulator of radiation belt dynamics.

Compositional Variation of the Dayside Martian Ionosphere: Inference from Photochemical Equilibrium Computations

Abstract The Martian ionosphere plays a crucial role in driving the interactions of the planet with solar photons and solar wind particles. The variations of the dayside Martian ionosphere with several controlling factors, including the solar extreme ultraviolet radiation, the background atmosphere, and the underlying thermal structure, have been the topic of extensive research in terms of electron distribution. In contrast, how the ionospheric composition varies has not been systematically investigated, a topic that is attempted in this study based on photochemical equilibrium computations performed at 100–200 km altitude, including a large number of reactions. Our calculations reveal the following compositional variations as natural outcomes of the ionospheric chemistry on Mars. (1) With increasing solar irradiance, the proportions of the majority of nonterminal ions are enhanced at the expense of reduced proportions of terminal ions, including O 2 + , HCO+, NO+, and H3O+. (2) At high electron temperatures, the proportion of NO+ is modestly reduced, whereas the proportions of the other species are nearly unaffected. (3) The response of the ionospheric composition to the upper atmospheric composition is complicated, showing the strong negative response of many trace ions to ambient CO2, O, and CO, as well as the strong positive response of protonated ions to H2, nitrogen-bearing ions to N and N2, water-group ions to H2O, and HO 2 + to O2. As an application of the model results, the recent ion measurements made on board the Mars Atmosphere and Volatile Evolution are used to provide hints about the realistic composition of the Martian upper atmosphere.

Charge Exchange Ion Losses in Saturn’s Magnetosphere

AbstractWhile various source and loss processes have been proposed for ions in Saturn’s magnetosphere, it is not yet well understood what role they play in different regions. In this study, we use a physical model of charge exchange to predict how proton and water group ion intensity profiles evolve over time and compare the results to MIMI/CHEMS measurements collected during the Cassini mission. First, we divide the CHEMS data into inbound and outbound half‐orbit segments, and create intensity profiles for 3–220 keV H+ and W+ ions between 5 and 15 Saturn radii, then using the inbound half‐orbits as initial conditions, we find qualitative similarities between measured and predicted outbound intensity profiles. This result is important because it provides strong evidence that charge exchange is the dominant loss process for these species in this region. The observed rate of charge exchange also presents information on the density of Saturn’s neutral torus. We suggest that data‐model discrepancies in the water group ions may be an indication of a significant presence of ions with the water group mass that are multiply charged.

Analytical study of electromagnetic ion cyclotron wave for ring distribution with AC electric field in Saturn magnetosphere

Characteristics of electromagnetic ion cyclotron waves in Saturn inner magnetosphere has been investigated here in close proximity of equatorial plane at approximately mid of the E ring near 5Rs by using data sets released by Voyager and Cassini spacecraft. Previous encounter of these spacecraft with Saturn exposed the strong signature of EMIC wave and harmonics believed to be originated within 7.5 Rs due to dominant O+ ions and water group ions. Maxwellian ring distribution for pickup ions contributing ring around planet is opted to find unperturbed particle density. Therefore, investigating here growth of obliquely propagating EMIC wave undergoing wave particle mechanism by opting kinetic approach to establish mathematical dispersion model in view of different parameters and extended to comparative study for parallel propagation. Parametric analysis inferred that waves propagating in the oblique direction grows more as compare to parallel propagating waves for the extended value of temperature anisotropy and the angle of propagation with respect to the ambient magnetic field B (at equator) and second harmonic of ion cyclotron waves indicating that the modified ion cyclotron wave shifts the wave spectra to higher side with increase in bandwidth. Study can be carried out to analyze other instabilities to explore magnetospheric dynamics at different radial distances.

Saturn's Nightside Ring Current During Cassini's Grand Finale

AbstractDuring Cassini's Grand Finale proximal orbits, the spacecraft traversed the nightside magnetotail to ∼21 Saturn radii. Clear signatures of Saturn's equatorial current sheet are observed in the magnetic field data. An axisymmetric model of the ring current is fitted to these data, amended to take into account the tilt of the current layer by solar wind forcing, its teardrop‐shaped nature and the magnetotail and magnetopause fringing fields. Variations in ring current parameters are examined in relation to external driving of the magnetosphere by the solar wind and internal driving by the two planetary period oscillations (PPOs), and compared with previous dawn and dayside observations. We find that the relative phasing of the PPOs determines the ring current's response to solar wind conditions. During solar wind compressions when the PPOs are in antiphase, a thick partial ring current is formed on the nightside, dominated by hot plasma injected by tail reconnection. This partial ring current should close partly via magnetopause currents and possibly via field‐aligned currents into the ionosphere. However, during solar wind compressions when the PPOs are in phase, this partial ring current is not detected. During solar wind rarefactions an equatorial “magnetodisc” configuration is observed in the dayside/dawn/nightside regions, with similar total currents flowing at these local times. During very quiet intervals of prolonged solar wind rarefaction, a thin current sheet with an enhanced current density is formed, indicative of a ring current dominated by cool, dense, Enceladus water group ions.

Constraining the Positive Ion Composition in Saturn’s Lower Ionosphere with the Effective Recombination Coefficient

Abstract The present study combines Radio and Plasma Wave Science/Langmuir Probe and Ion and Neutral Mass Spectrometer data from Cassini’s last four orbits into Saturn’s lower ionosphere to constrain the effective recombination coefficient α 300 from measured number densities and electron temperatures at a reference electron temperature of 300 K. Previous studies have shown an influx of ring material causes a state of electron depletion due to grain charging, which will subsequently affect the ionospheric chemistry. The requirement to take grain charging into account limits the derivation of α 300 to upper limits. Assuming photochemical equilibrium and using an established method to calculate the electron production rate, we derive upper limits for α 300 of ≲ 3 × 10−7 cm3 s−1 for altitudes below 2000 km. This suggests that Saturn’s ionospheric positive ions are dominated by species with low recombination rate coefficients like HCO+. An ionosphere dominated by water group ions or complex hydrocarbons, as previously suggested, is incompatible with this result, as these species have recombination rate coefficients > 5 × 10−7 cm3 s−1 at an electron temperature of 300 K.

Revisiting magnetospheric CAPS TOF data post Cassini

The Cassini Plasma Spectrometer has a Time-of-Flight sensor capable of separating out the major magnetospheric ions at Saturn of protons, ions with a mass to charge of 2 amu/q, and water group ions (O+, OH+, H2O+ and H3O+), yet until this study a survey of their relative proportions in density with respect to radial distance has not been carried out. Most past work had used a standard code for the Time-of-Flight analysis, however we find that this code had a few features that resulted in it underestimating the relative proportion of water group ions and overestimating those of the light ions, which has consequences for past studies that relied on this code. Those features are addressed in this study to find that the water group ions dominate the equatorial magnetosphere out to ≈40RS, further than previously assumed. The ions of mass to charge 2 amu/q (assumed to be H2+) are the third most abundant ion group (after protons and the water group), with less than 12% of the total ion density. In addition, the presence of an H2+ distribution is found that appears to have Titan as the source, since H2+ ions can dominate the near Titan environment.

The Composition of ~96 keV W+ in Saturn's Magnetosphere

AbstractEnceladus produces plumes of water vapor and ice particles whose gaseous H2O molecules dissociate into OH and O. The gas ejecta form a large toroidal planetary cloud colocated with and partially sourced by a similarly large torus of ice grain ejecta. Gas torus neutrals are ionized by charge exchange, solar ultraviolet, and electron impacts, producing the thermal water group ions, W+ (O+, OH+, H2O+, and H3O+), which become energized in Saturn's magnetosphere. We study the components of energetic (~96 keV) W+ using Cassini Charge‐Energy‐Mass Spectrometer (CHEMS) data from 78 near‐equatorial main ring current passes (dipole L = 7–16, ±10° in latitude) in 2004–2010. W+ fractional abundances are ~53% O+, ~22% OH+, ~22% H2O+, and ~3% H3O+, when averaged over L = 7–16, resulting in a mean water group mass of 16.7 amu. This large energetic O+ fractional abundance, over twice that observed for thermal O+ at L < 10, qualitatively agrees with the broad atomic O cloud observed by Cassini and predicted by some models. Fractional abundance ratios for O+/W+, OH+/W+, and H2O+/W+ vary little over L = 7–21. Our observation of energetic H2O+/W+ ≈ 22% out to L ~ 21 suggests that neutral H2O spreads throughout the magnetosphere rather than being confined to a narrow torus centered on Enceladus' orbit. Although nearly constant at <2% for L > 13, H3O+/W+ tends to increase significantly to ~4–5% at L ~ 7–8.

Oxygen isotope fractionation between gypsum and its formation waters: Implications for past chemistry of the Kawah Ijen volcanic lake, Indonesia

Abstract Gypsum (CaSO4·2H2O) provides an opportunity to obtain information from both the oxygen isotopic composition of the water and sulfate of its formation waters, where these components are commonly sourced from different reservoirs (e.g., meteoric vs. magmatic). Here, we present δ18O values for gypsum and parent spring waters fed by the Kawah Ijen crater lake in East Java, Indonesia, and from these natural samples derive gypsum-fluid oxygen isotope fractionation factors for water and sulfate group ions of 1.0027 ± 0.0003‰ and 0.999 ± 0.001‰, respectively. Applying these fractionation factors to a growth-zoned gypsum stalactite that records formation waters from 1980 to 2008 during a period of passive degassing, and gypsum cement extracted from the 1817 eruption tephra fall deposit, shows that these fluids were in water-sulfate oxygen isotopic equilibrium. However, the 1817 fluid was >5‰ lighter. This indicates that the 1817 pre-eruption lake was markedly different, and had either persisted for a much shorter duration or was more directly connected to the underlying magmatic-hydrothermal system. This exploratory study highlights the potential of gypsum to provide a historical record of both the δ18Owater and δ18Osulfate of its parental waters, and provides insights into the processes acting on volcanic crater lakes or any other environment that precipitates gypsum.

Distribution in Saturn's Inner Magnetosphere From 2.4 to 10 RS: A Diffusive Equilibrium Model

AbstractElectron density measurements have been obtained by the Cassini Radio and Plasma Wave Science (RPWS) instrument covering the period from 30 June 2004 to 19 April 2017, spanning latitudes up to ~30° and L values from 2.4 to 10. Near the F ring, electron densities are derived from RPWS measurements of electron plasma oscillations at high latitudes and from the Langmuir Probe (RPWS/LP) sweep data at low latitudes. The electron density measurements from the ring‐grazing orbits, beginning in December 2016, have made it possible to extend the work of a previous diffusive equilibrium model to include the distribution of the ring plasma. Beyond the ring‐grazing orbits, the densities are derived from RPWS measurements of the upper hybrid resonance frequency. These density measurements are used to anchor the fit of an expanded diffusive equilibrium density model for a two‐species plasma consisting of water group and hydrogen ions in Saturn's inner magnetosphere. Density contour plots for the two ion species and the electrons are presented. The distribution of the derived plasma densities is consistent with two primary sources, the Enceladus plume and the extended ring atmosphere. There is also an indication of a weaker plasma source at Dione. In the region just outside the A ring and in the region including the Enceladus orbit, the diffusive equilibrium model also shows the expansion of lighter ions and electrons to higher latitudes along the magnetic field lines with evidence of a weaker plasma expansion between the orbits of Tethys and Dione.

Global Maps of Energetic Ions in Saturn's Magnetosphere

AbstractThe Magnetospheric Imaging Instrument/Charge Energy Mass Spectrometer onboard the Cassini spacecraft measured suprathermal H+ and water group (W+) ions during the 13‐year mission from 2004 to 2017. This paper generates statistical maps of pulse height analyzed fluxes for 3.24–3.34 keV H+ and 8.79–9.05 keV W+ from the start of day 2 in 2005 through the end of day 366 in 2016. The fluxes of both H+ and W+ are more intense on the nightside than on the dayside. The dayside magnetodisk thickness of H+ is greater than on the nightside, while the day and night thicknesses are comparable for W+. On both dayside and nightside, fluxes concentrate near the equator and also along the magnetic field lines near and just outside the Rhea L shell (L ≈ 8.75RS, 1RS = 60,268 km). The flux peaks map to the ionosphere well equatorward of the aurora. This intensity peak is near the boundary between where interchange events occur and where tail collapse (reconnection) events occur. The intensity variations can be represented by lognormal distributions in range r for each latitude λ, and the lognormal coefficients can then be fitted in latitude, thus allowing construction of simple models of the H+ and W+ spatial distributions. The models can then be used to estimate the fluxes of low energy suprathermal ions from ~5 to ~21RS on the dayside and ~5 to ~30RS on the nightside in both r and λ, or alternately in ρ and z.

Internal Versus External Sources of Plasma at Saturn: Overview From Magnetospheric Imaging Investigation/Charge‐Energy‐Mass Spectrometer Data

AbstractPlasma composition observations provide a useful mechanism for investigating plasma sources and subsequent evolution within planetary magnetospheres. While He++ is the second most abundant ion species in the solar wind, there are no known sources of He++ ions within the magnetosphere of Saturn, allowing He++ to serve as a tracer of solar wind ions within the Kronian magnetosphere. Meanwhile, water group ions (W+, consisting of O+, OH+, H2O+, and H3O+) and H2+, known to originate within the magnetosphere of Saturn, serve as a tracer of internally ionized plasma. In this paper, we investigate the relative abundances and properties of energetic (32–220 keV) ion species originating from sources within the magnetosphere and from the solar wind. Solar wind‐originating ions are observed to have significant relative abundance (up to ~0.05) in the midnight, dawn, and noon local time quadrants at high radial distances (~40, ~45, and ~20 RS, respectively). Several possible entry processes, such as reconnection and Kelvin‐Helmholtz instabilities, are outlined in this paper as well as a discussion of subsequent transport.

Distribution of water-group ion cyclotron waves in Saturn\u2019s magnetosphere

The water-group ion cyclotron waves (ICWs) in Saturn’s magnetosphere were studied using the magnetic field data provided by the MAG magnetometer on board the Cassini satellite. The period from January 2005 to December 2009, when the Cassini radial distance is smaller than 8 RS, was used. ICWs were identified by their left-hand circularly polarized magnetic perturbations and wave frequencies near the water-group ion gyrofrequencies. We obtained the spatial distribution of ICW amplitude and found that the source region of ICWs is mostly located in the low-latitude region, near the equator and inside the 6 RS radial distance. However, it can extend beyond 7 RS in the midnight region. In general, the wave amplitude is peaked slightly away from the equator, for all local time sectors in both the Northern and Southern Hemispheres. By assuming that the water-group ions are composed of pickup ions and background thermal ions, we obtained the local instability condition of the ICWs and estimated their growth rate along the field lines. If the wave amplitude is correlated with the growth rate, the observed latitudinal dependence of the wave amplitude can be well explained by the local stability analysis. Also, latitudinal location of the peak amplitude is found to depend on the local time. This implies a local time dependence for the water-group ion parallel temperature T|, as determined from the theoretical calculations. Graphical abstractFigure 7. The upper panel shows the projected orbit of Cassini in the cylindrical R–Z plane across the L-shell from the Northern Hemisphere to the Southern Hemisphere. The red line denotes Cassini’s orbit and the blue lines denote the L-shell field lines going through points A and B, respectively. The lower panel shows the power spectrum of delta B_{phi }, the blue dotted line shows the local W+ gyrofrequency (flocal), and the black line is the W+ gyrofrequency at the equator (feq) of the dipole L-shell of the Cassini orbit, which moves from A to B

Survey of thermal plasma ions in Saturn's magnetosphere utilizing a forward model

AbstractThe Cassini Plasma Spectrometer instrument gathered thermal ion data at Saturn from 2004 to 2012, predominantly observing water group ions and protons. Plasma parameters, with uncertainties, for those two ion species are derived using a forward model of anisotropic convected Maxwellians moving at a shared velocity. The resulting data set is filtered by various selection criteria to produce a survey of plasma parameters derived within 10° of the equator at radial distances of 5.5 to 30 RS (1 RS = Saturn's radius). The previous 2008 work used a simpler method and had just 150 records over 5 orbits; this comprehensive survey has 9736 records over all 9 years. We present the results of this survey and compare them with a previous survey derived from numerical moments, highlighting the differences between the reported densities and temperatures from the two methods. Radial profiles of the plasma parameters in the inner and middle magnetospheres out to ≈22RS are stable year by year, but variable at distances larger than 23 RS near the magnetopause. New results include proton densities increasing in the near magnetopause region, suggestive of plasma mixing; evidence for the global electric field in Saturn's inner magnetosphere extends out to ≈15RS; no evidence for supercorotating plasma nor the middle magnetosphere “plasma cam” feature is present; the thermal plasma β is found to exceed unity at equatorial distances greater than 15 RS.

Diamagnetic depression observations at Saturn's magnetospheric cusp by the Cassini spacecraft

AbstractThe magnetospheric cusp is a region where shocked solar wind plasma can enter a planetary magnetosphere, after magnetic reconnection has occurred at the dayside magnetopause or in the lobes. The dense plasma that enters the high‐latitude magnetosphere creates diamagnetic effects whereby a depression is observed in the magnetic field. We present observations of the cusp events at Saturn's magnetosphere where these diamagnetic depressions are found. The data are subtracted from a magnetic field model, and the calculated magnetic pressure deficits are compared to the particle pressures. A high plasma pressure layer in the magnetosphere adjacent to the cusp is discovered to also depress the magnetic field, outside of the cusp. This layer is observed to contain energetic He++ (up to ∼100 keV) from the solar wind as well as heavy water group ions (W+) originating from the moon Enceladus. We also find a modest correlation of diamagnetic depression strength to solar wind dynamic pressure and velocity; however, unlike at Earth, there is no correlation found with He++ counts.

Plasma waves confined to the diamagnetic cavity of comet 67P/Churyumov–Gerasimenko

Ion acoustic waves were observed in the diamagnetic cavity of comet 67P/Churyumov– Gerasimenko by the Rosetta spacecraft on 2015 August 3, when the comet was 1.25 au from the Sun. Wave spectra recorded by the Langmuir probe (RPC-LAP), peak near 200 Hz, decrease for higher frequencies and reach the noise floor at approximately 1.5 kHz. These waves were observed only when the spacecraft was in the diamagnetic cavity or at its boundary, which is identified as a sharp drop in magnetic field magnitude, measured by RPC-MAG. The plasma, on both sides of the boundary, is dominated by a cold (a few hundred K) water group ion population, one cold (k B T e ∼ 0.1 eV) and one warm (k B T e ∼ 10 eV) electron population. The observations are interpreted in terms of current-driven ion acoustic waves, generated by currents that flow through bulges on the boundary of the diamagnetic cavity.

A 1D Model of Radial Ion Motion Interrupted by Ion–Neutral Interactionsin a Cometary Coma

Abstract Because ion–neutral reaction cross sections are energy dependent, the distance from a cometary nucleus within which ions remain collisionally coupled to the neutrals is dictated not only by the comet’s activity level but also by the electromagnetic fields in the coma. Here we present a 1D model simulating the outward radial motion of water group ions with radial acceleration by an ambipolar electric field interrupted primarily by charge transfer processes with H2O. We also discuss the impact of plasma waves. For a given electric field profile, the model calculates key parameters, including the total ion density, n I , the H3O+/H2O+ number density and flux ratios, R dens and R flux, and the mean ion drift speed, , as a function of cometocentric distance. We focus primarily on a coma roughly resembling that of the ESA Rosetta mission target comet 67P/Churyumov–Gerasimenko near its perihelion in 2015 August. In the presence of a weak ambipolar electric field in the radial direction the model results suggest that the neutral coma is not sufficiently dense to keep the mean ion flow speed close to that of the neutrals by the spacecraft location (∼200 km from the nucleus). In addition, for electric field profiles giving n I and within limits constrained by measurements, the R dens values are significantly higher than values typically observed. However, when including the ion motion in large-amplitude plasma waves in the model, results more compatible with observations are obtained. We suggest that the variable and often low H3O+/H2O+ number density ratios observed may reflect nonradial ion trajectories strongly influenced by electromagnetic forces and/or plasma instabilities, with energization of the ion population by plasma waves.

RPC observation of the development and evolution of plasma interaction boundaries at 67P/Churyumov-Gerasimenko

One of the primary objectives of the Rosetta Plasma Consortium, a suite of five plasma instruments on-board the Rosetta spacecraft, is to observe the formation and evolution of plasma interaction regions at the comet 67P/Churyumov-Gerasimenko (67P/CG). Observations made between 2015 April and 2016 February show that solar wind–cometary plasma interaction boundaries and regions formed around 2015 mid-April and lasted through early 2016 January. At least two regions were observed, separated by an ion-neutral collisionopause boundary. The inner region was located on the nucleus side of the boundary and was characterized by low-energy water-group ions, reduced magnetic field pileup and enhanced electron densities. The outer region was located outside of the boundary and was characterized by reduced electron densities, water-group ions that are accelerated to energies above 100 eV and enhanced magnetic field pileup compared to the inner region. The boundary discussed here is outside of the diamagnetic cavity and shows characteristics similar to observations made on-board the Giotto spacecraft in the ion pileup region at 1P/Halley. We find that the boundary is likely to be related to ion-neutral collisions and that its location is influenced by variability in the neutral density and the solar wind dynamic pressure.