Water Group Ions Research Articles

Preliminary results on Saturn's inner plasmasphere as observed by Cassini: Comparison with Voyager

We present an analysis of Saturn's inner plasmasphere as observed by the Cassini Plasma Spectrometer (CAPS) experiment during Cassini's initial entry into Saturn's magnetosphere when the spacecraft was inserted into orbit around Saturn. The ion fluxes are divided into two sub‐groups: protons and water group ions. We present the relative amounts of these two groups and the first estimates of their fluid parameters: ion density, flow velocity and temperature. We also compare this data with electron plasma measurements. Within the plasmasphere and inside of Enceladus' orbit, water group ions are about a factor of ∼10 greater than protons in number with number densities exceeding 40 cm−3. Within this inner region the spacecraft acquires a negative potential so that the electron density is underestimated. The electron and proton temperatures, which could not be measured in this region by Voyager, are T ∼ 2 eV at L ∼ 3. Also, within this inner region the protons, because of a negative spacecraft potential, appear to be super‐corotating. By enforcing the condition that protons and water group ions are co‐moving we may be able to acquire an independent estimate of the spacecraft potential relative to that estimated when comparing ion‐electron measurements. Using our estimates of plasma properties, we estimate the importance of the rotating plasma on the stress balance equation for the inner magnetosphere and corresponding portion of the ring current.

Cassini observations of the thermal plasma in the vicinity of Saturn's main rings and the F and G rings

The ion mass spectrometer on Cassini detected enhanced ion flux near Saturn's main rings that is consistent with the presence of atomic and molecular oxygen ions in the thermal plasma. The ring “atmosphere” and “ionosphere” are likely produced by UV photosputtering of the icy rings and subsequent photoionization of O2. The identification of the O+ and O2+ ions is made using time‐of‐flight analysis and densities and temperatures are derived from the ion counting data. The ion temperatures over the main rings are a minimum near synchronous orbit and increase with radial distance from Saturn as expected from ion pick up in Saturn's magnetic field. The O2+ temperatures provide an estimate of the neutral O2 temperature over the main rings. The ion mass spectrometer also detected significant O2+ outside of the main rings, near the F ring. It is concluded that between the F and G rings, the heavy ion population most likely consists of an admixture of O2+ and water group ions O+, OH+, and H2O+.

Instabilities of Alfvén and magnetosonic waves in dusty cometary plasmas with an ion ring beam

The effects of charged dust grains on the instabilities of electromagnetic waves in the solar wind plasma due to ring-beam cometary ion velocity distributions are considered. The waves are assumed to propagate parallel to the solar wind magnetic field. Collisionless cold plasma theory is used to derive the dispersion relations of linear waves in the solar wind, for a delta-function ring-beam velocity distribution of cometary water-group ions. The resonant and nonresonant instabilities are considered, and it is shown that the dust has its greatest effect on the nonresonant firehose instability at very long wavelength. The dust can cause an unstable dust-free firehose mode to be stable, and vice versa. It is also shown that as the frequency approaches the cometary ion cyclotron frequency, there is an upper limit to the wave number for the unstable firehose mode. For the left-hand polarized wave, the presence of dust increases this upper wave number limit, until the firehose mode merges with the resonant mode. The right-hand polarized firehose mode also has an upper wave number limit, which is almost independent of the presence of dust. It is shown that, although there is a high frequency resonant instability of the normal whistler mode induced by the ring beam, there is no corresponding instability of the dust-induced whistler-like mode at low frequencies.

Observation of interplanetary particles in a corotating interaction region and of energetic water group ions from comet Grigg-Skjellerup

During the flyby of the Giotto S/C with comet Grigg-Skjellerup (G-S) (9–11 July 1992) energetic particles ( E H 2O > 60 keV, E p > 30 keV, E α > 3.5 MeV), magnetic fields and plasma ions were measured by three different detectors on board Giotto. The flyby period was characterized by high magnetic field magnitudes (> 12 nt) and an enhanced background flux caused most likely by the reverse shock of a corotating interaction region (CIR). Within the CIR three different particle acceleration events were detected: 1. 1. Proton and α-particle acceleration ( E p > 4.5 MeV, E α > 3.5 MeV) caused by a sudden increase of the magnetic field combined with a change in direction. 2. 2. Proton and H 2O-ion ( E p > 220 eV, E H 2O > 260 keV) acceleration caused by a directional discontinuity of the magnetic field. 3. 3. Acceleration of water group ions in the foreshock and bow shock regions of comet G-S associated with a further increase of the magnetic field magnitude. Particle fluxes, magnetic field measurements, pitch angles and energy spectra are used to discuss the particle acceleration processes such as shock drift mechanism, inductive acceleration, Fermi acceleration and adiabatic compression. The results are compared with those obtained during the Giotto-Halley encounter.

Characteristic boundary transitions in energetic particle data (60– ≥ 260 keV) recorded at comets P/Grigg-Skjellerup and P/Halley by the EPONA instrument on Giotto

An overview of published magnetic field, low energy electron and low to intermediate energy ion data recorded on board Giotto during an encounter with P/Grigg-Skjellerup (G-S) on 10 July, 1992 is presented and these measurements compared with particle data secured by the onboard energetic particles experiment EPONA ( E = 60 to ≥260 keV). It is shown that, in general, the same cometary boundaries (Bow Wave, Bow Shock, Mystery Boundary Transitions (MBTs)) were sensed in energetic particles as by the other experiments. Elevated fluxes of accelerated particles, interpreted to be ions of species M = 16–18, were identified in the G-S cometosheath. It is inferred that heavy cometary ions of species M = 28–33 were present in the innermost region of the comet (as viewed in the sunward direction). Energetic particles recorded by EPONA on Giotto at P/Halley are considered relative to other particles and fields measurements obtained onboard. Evidence based on pitch angle data indicates that, inbound, the MBTs at G-S and at Halley, were spatially associated with changes from trapped to “flowing” distributions of water group ions, suggesting that mirroring of these particles was taking place during encounter in the magnetic field piled up around each comet. Downstream of the MBTs, outbound, flowing distributions were, in each case, identified. Evidence of MBTs in energetic particle data are, herein, reported for the first time. The roles of various mechanisms in accelerating cometary ions in association with Bow Wave/Bow Shock boundaries at G-S and at Halley are inferred from comparisons between experimental and theoretical spectra pertinent to each comet. Similarities and differences between the signatures of characteristic boundaries recorded in each case are discussed in the light of basic differences between the comets themselves, and between the ambient interplanetary circumstances pertaining during each cometary encounter.

Compressive character of low frequency waves driven by newborn ions at comet Grigg-Skjellerup

The investigation by Giotto of the environment of comet P/Grigg-Skjellerup has provided a large set of observations on cometary low-frequency waves for a weakly active comet, after the observations at comets P/Giacobini-Zinner and P/Halley. A large amount of waves are observed at frequencies close to the local water group ion cyclotron frequencies on the magnetic field and the plasma density in a wide region around the comet. Cross-analysis of the electron parameters with the magnetic field reveals the existence of a compressive wave component even at large distances from the nucleus. The inferred characteristics of the observed wave modes are quantitatively reproduced from a theoretical work studying the electromagnetic instabilities driven by the distribution of the newborn ions.

Cometary ion pressure anisotropies at comets Halley and Grigg‐Skjellerup

Cometary water group ions are the dominant population controlling the interaction of comets with the solar wind. The distribution function of this population has important effects on the overall structure and boundaries in the comet‐solar wind interaction and on the plasma wave activity produced. In this paper we present new results on the behavior of the water group ions measured by Giotto at comets Halley and Grigg‐Skjellerup. The pressures perpendicular and parallel to the magnetic field are presented and the implications explored. We show that during the Halley inbound pass, the firehose instability may excite the observed waves with low wavenumber. Also, we find that the energy density of energetic cometary ions is greater than the observed difference between the energy released from the ring‐like distribution and the energy in the pickup‐excited waves. This indicates that another energy source must be contributing to the acceleration, probably related to the deceleration of the solar wind in the comet frame.

Structure and dynamics of inner cometary plasmas

The inner cometary coma is a weakly ionized plasma, and its structure and dynamics are governed mainly by ionization due to solar radiation and solar wind electrons and losses due to the radiative processes, recombination, and transport. At comet Halley a narrow ion density depletion region was observed by spacecraft as well as ground‐based instruments and has been linked to the dynamics of the plasma and radiation. A model of the cometary plasma consisting of water group ions, bulk electrons, and energetic electrons produced mainly by photoionization is presented. The dominant losses in the inner coma are the radiation from the excitation of rotational and vibrational levels of water molecules and the recombination of the plasma. The electron energy losses due to these processes peak near 4000 K, and at temperatures higher than this value a localized cooling leads to further cooling arising from increased radiation loss and consequently to a thermal instability. The resulting increase in recombination leads to an ion density depletion, and the estimates for this depletion at comet Halley agree with the observations. This instability is sensitive to the plasma conditions and the transport processes, that is, diffusion and thermal conductivity. There is no direct measurement of the electron temperature in this range, and the electron temperature profile from MHD simulations has been used to develop a model of the inner cometary plasma, which yields the localization of the thermal instability and, hence, the observed ion density depletion region. The resulting electron temperature profile is also consistent with that obtained from the temperature dependence of the electron‐ion recombination rate.

D/H and 18O/16O ratio in the hydronium ion and in neutral water from in situ ion measurements in comet Halley

The D/H ratio and the 18O/16O ratio in hydronium ions in the gas coma of comet Halley have been determined from ion data obtained by the high intensity sensor (HIS) of the ion mass spectrometer (IMS) on board the spacecraft Giotto in 1986. The HIS instrument measured reliable data for the water group ions over a large distance range from the nucleus. We could therefore minimize the statistical errors. This large range also allowed us to study the systematic influence of formaldehyde on the D/H ratio in the hydronium ion and to deduce from the hydronium ion a D/H ratio for the water molecule. The measured 18O/16O ratio of (1.93±0.12) × 10−3 is compatible with the telluric value of 2.06 × 10−3. The D/H ratio for the hydronium ion measured is D/H = 3.08−0.53+0.38 × 10−4. We show that since the water chemistry is not significantly different for deuterated and nondeuterated water molecules and ions and since the contribution from other deuterated molecules to the deuterium abundance in the hydronium ion is small, the D/H value of the neutral water molecule is not significantly different from the value determined for the ion, although the uncertainty is somewhat larger. An upper limit for the deuterium abundance in formaldehyde in comet Halley can be derived from the D/H ratio in the hydronium ion as a function of distance from the nucleus. From our results we conclude that the D/H ratio in formaldehyde is certainly smaller than 2% and probably smaller than 0.5% and that the enrichment of the deuterium in the hydronium ion due to formaldehyde is lower than 5%. The D/H ratio in water in comet Halley is therefore significantly higher than the telluric sea water value, whereas the deuterium enrichment in formaldehyde is not as high as, for example, that observed in the Orion compact ridge.

Pickup proton cyclotron turbulence at comet P/Halley

Low‐frequency electromagnetic turbulence near the proton gyrofrequency observed far upstream of comet P/Halley can be excited by a beam instability driven by relative streaming between cometary protons, solar wind protons, and water group ions. For a given solar wind velocity the growth rates peak at a certain optimum frequency shift (from the exact proton gyrofrequency), and the wavelengths involved can be deduced in a self‐consistent way from the dispersion law. Only under ideal conditions when all parameters remain constant would the mode corresponding to the optimum frequency shift grow fastest and might it be possible to observe a nearly constant frequency mode. However, if the solar wind parameters were not constant, then a mode that was in resonance earlier would no longer remain so, and some other mode with a slightly different frequency shift might start to grow fastest, leading to a mixing of many modes. Thus only rarely would one be able to observe a single mode near the proton gyrofrequency, exactly as happens in the observations. Our self‐consistent approach yields resonant instabilities with left‐handed polarizations in the spacecraft frame.

Low‐frequency instabilities and the resulting velocity distributions of pickup ions at comet Halley

The interaction between the solar wind and newborn cometary ions is studied using a new analytical theory as well as one‐ and two‐dimensional hybrid simulations. Using the observed parameters upstream of the comet Halley, a detailed study of wave excitation and the resulting particle distributions is presented. Linear theory as well as simulations show that a variety of modes such as the fast magnetosonic mode, high frequency whistlers and obliquely propagating Alfvén ion cyclotron waves can be excited. However, parallel propagating waves are found to be dominant in the wave spectrum and to control the scattering of the pickup ions. Several features of the observed distributions of pickup protons are explained. In particular, it is shown that the observed asymmetric pitch angle distribution for the pickup protons is due to the small saturation amplitude of the waves for the given parameters. Water group associated waves can lead to energy diffusion and further pitch angle scattering of protons. This effect is most likely to be important in the vicinity of the bow shock of comet Halley where the density of water group ions becomes comparable to that of protons. It is shown that the observed increase in the radius of the proton velocity shell just outside the bow shock can be due to water group waves. The nearly isotropic proton pitch angle distribution observed by Neugebauer et al. [1989] just outside the bow shock may, however, be related to the presence of a rotational discontinuity which has been identified in the magnetic field data. Just outside the bow shock, simulations show that parallel propagating water group waves can steepen with attached whistler wave packets. The steepening process at parallel propagation is a transient effect, in an important contrast to the case of steepening at oblique angles. The smaller beam densities at comet Halley appears to be the main reason not only why waves at comet Halley have smaller amplitudes but also why oblique, steepening magnetosonic waves have not been detected at comet Halley, whereas they have been seen at comet Giacobini‐Zinner.

Modeling of pickup ion distributions in the Halley cometosheath: Empirical limits on rates of ionization, diffusion, loss and creation of fast neutral atoms

The shape of the velocity distribution of water group ions observed by the Giotto ion mass spectrometer on its approach to comet Halley is modeled to derive empirical values for the rates of ionization, energy diffusion, and loss in the midcometosheath. The model includes the effect of rapid pitch angle scattering into a bispherical shell distribution as well as the effect of the magnetization of the plasma on the charge exchange loss rate. It is found that the average rate of ionization of cometary neutrals in this region of the cometosheath appears to be of the order of a factor 3 faster than the “standard” rates ∼1×10−6 s−1 that are generally assumed to model the observations in most regions of the comet environment. For the region of the coma studied in the present work (∼1−2×105 km from the nucleus), the inferred energy diffusion coefficient is D0 ≈ 0.002 to 0.005 km²s−3, which is generally lower than values used in other models. The empirically obtained loss rate appears to be about an order of magnitude greater than can be explained by charge exchange with the “standard” cross section of ∼2×10−15 cm². However such cross sections are not well known and for water group ion / water group neutral interactions, rates as high as 8×10−15 cm² have previously been suggested in the literature. Assuming the entire loss rate is due to charge exchange yields a rate of creation of fast neutral atoms of the order of ∼ 10−4 s−1 or higher, depending on the level of velocity diffusion. The fast neutrals may, in turn, be partly responsible for the higher‐than‐expected ionization rate.

Giotto ion mass spectrometer measurements at comet P/Grigg‐Skjellerup

The Giotto ion mass spectrometer high‐intensity spectrometer (IMS‐HIS) measured fluxes of ions from about 260,000 km before (1008∶37 UT spacecraft time) to about 86,000 km after (1701∶33 UT spacecraft time) closest approach to comet P/Grigg‐Skjellerup during the encounter on July 10, 1992. Although the HIS sensor was not designed to measure protons, these ions were measured far from the comet. Close in to the comet, the ions observed were probably also protons, although heavier ions cannot be completely ruled out. Considerable temporal structure appears in the data, well‐correlated with the data of other instruments onboard, especially those of the magnetometer. In particular, the ion count rate correlates with the direction of the magnetic field. This strong modulation at the water group ion cyclotron period (∼90 s) inside the inbound bow wave indicates a very narrow ion pitch angle distribution. Hence at Grigg‐Skjellerup the ions appear to experience very little pitch angle scattering. This may result from strong compression in the rapidly increasing magnetic field.

Shock normal determination for multiple‐ion shocks

We have adapted the single‐ion Viñas and Scudder (1986) solution to the Rankine‐Hugoniot problem to a multiple‐on solution. Using this technique, we can calculate a shock normal direction, shock speed, best estimate of the upstream and downstream magnetic field and plasma asymptotic states, and θBn, the angle between the shock normal and the upstream magnetic field. We test the multi‐ion solution with a theoretical case but are restricted to a perpendicular shock in order to close the multi‐ion Rankine‐Hugoniot equations. For this test case both single‐ion and multi‐ion solutions are equally valid. We examine parameter regimes to look for differences between single‐ion and multi‐ion solutions of the R‐H equations, and we find that the largest differences occur for quasi‐parallel shocks, small values of solar wind speed, large values of heavy ion density, and very strong and very weak shocks. For both the inbound and outbound crossing of comet Halley we have a slow solar wind speed, small values of water group ions and fairly weak shocks. We examine both the quasi‐perpendicular inbound crossing and the quasi‐parallel outbound crossing at comet Halley. For the inbound crossing we have good resolution data and make an accurate determination. However, it is likely that the shock normal determination was influenced by the interplanetary disturbances flowing from the Sun that permeate the inbound crossing region and that force the normal to an antisunward direction. For the outbound case the FIS experiment is not providing data because it was damaged by cometary dust soon after closest approach. However, the results are more definitive: all of the normals are within 31° of each other, which makes the analysis essentially independent of interval. In both cases there are differences between the single‐ and multi‐ion solutions with the multi‐ion solution being the more reliable.

Velocity space diffusion and nongyrotropy of pickup water group ions at comet Grigg‐Skjellerup

The diffusion of water group cometary ions in velocity space at comet Grigg‐Skjellerup was measured during the Giotto spacecraft encounter. The evolution of the collapsed pitch angle and energy distributions during the inbound and outbound passes shows that the timescale for energy diffusion may be similar to that for pitch angle diffusion. Fully isotropic pitch angle distributions were never seen. Also the bulk parameters of the three‐dimensional distributions are examined. Transformation of these parameters into a field‐aligned solar wind frame allows us to test the gyrotropy of the distributions. The observations imply that there were deviations from gyrotropy throughout the encounter becoming most important near to closest approach.

Water group ion distributions in the midcometosheath of comet Halley

In the midcometosheath of comet Halley (1 × 105−2 × 105 km from the nucleus) the center‐of‐mass plasma frame is approximately the bulk flow velocity of the cometary ions, and the Alfvén wave speed is an appreciable fraction of the flow speed. Here, the peaks of the water group ion distributions observed by the Giotto ion mass spectrometer are at velocities consistently below the expected pickup speed. It is shown that this effect is consistent with the scattering of the new pickup ions onto a bispherical shell distribution. The model does not fit the data inside ∼1.2×105 km, however, possibly as a result of the growing importance of collisions or the presence of other processes such as scattering on obliquely propagating magnetosonic waves.

Low‐frequency electromagnetic plasma waves at comet P/Grigg‐Skjellerup: Overview and spectral characteristics

Large‐amplitude electromagnetic plasma waves are one of the dominant features of the solar wind‐comet interaction. Wave characteristics strongly depend on parameters such as the solar wind flow and Alfvén velocities and the angle between flow and interplanetary magnetic field as well as the production rate. With respect to the latter the flyby of the spacecraft Giotto at comet P/Grigg‐Skjellerup provides a unique possibility to study such waves in further detail. Pickup ion‐related wave signatures have been observed up to a distance of 600,000 km from the nucleus. Peak spectral power in the spacecraft frame of reference occurs at frequencies mainly somewhat below the water group ion gyrofrequency. From this the waves are determined to be mainly left‐hand polarized waves, causing one‐sided pitch angle diffusion outbound. The wave activity strongly increases close to the comet; upstream it exhibits a quadratic dependence on the water group pickup ion free energy. Furthermore, a phenomenological study of the wave characteristics provides a unique description of the fine‐structure of the interaction region. Indications of steepened magnetosonic waves have been found in the outbound magnetosheath region.

Mirror modes and fast magnetoacoustic waves near the magnetic pileup boundary of comet P/Halley

Large‐amplitude ultralow‐frequency wave structures observed on both sides of the magnetic pileup boundary of comet P/Halley during the flyby of the Giotto spacecraft have been analyzed using suprathermal electron density and magnetic field observations. Upstream of the boundary, electron density and magnetic field magnitude variations are anticorrelated, while in the pileup region these quantities are clearly correlated. Both in front of and behind the pileup boundary the observed waves are quasi‐perpendicular wave structures as a minimum variance analysis shows. A detailed comparison of our observations in the prepileup region with theoretical and numerical results shows that the mirror mode mode waves may have been generated by a mirror instability driven by the pressure anisotropy of the ring‐type distributions of the heavy (water group) pickup cometary ions. A mean wavelength λ1 = 2071 km is found, which is about 4 times the water group ion thermal gyroradius. For the fast mode wave structures in the pileup region, analysis of wave propagation on both sides of the pileup boundary shows that mode conversion from the mirror mode to fast mode type waves at the boundary is not a possible source mechanism. However, we show that the fast mode waves may be interpreted as stationary wave structures in a multifluid plasma. The wavelength λ2 = 1141 km derived from the observations, when using the stationary wave hypothesis, is in good agreement with the theoretical expected wave length λtheo = (774 ± 506) km computed from available ion observations.

Mass loading and velocity diffusion models for heavy pickup ions at comet Grigg‐Skjellerup

We compare model predictions of cometary water group ion densities and the solar wind slow down with measurements made by the Giotto Johnstone plasma analyzer implanted ion sensor at the encounter with comet Grigg‐Skjellerup (G‐S) on July 10, 1992. The observed slope of the ion density profile on approach to the comet is unexpectedly steep. Possible explanations for this are discussed. We present also a preliminary investigation of the quasilinear velocity‐space diffusion of the implanted heavy ion population at G‐S using a transport equation including source, convection, adiabatic compression, and velocity diffusion terms. Resulting distributions are anisotropic, in agreement with observations. We consider theoretically the waves that may be generated by the diffusion process for the observed solar wind conditions. At initial ion injection, waves are generated at ω ∼ Ωi the ion gyrofrequency, and lower frequencies are predicted for diffusion toward a bispherical shell.

Pickup water group ions at comet Grigg‐Skjellerup

The density and velocity distribution of cometary water group ions was measured by the Giotto spacecraft in the regions upstream and downstream of the ‘bow shock’ at comet Grigg‐Skjellerup. The results show that the distributions of ions are ring‐like until quite close to the shock, the timescales for pitch angle and energy diffusion appear similar and the ion density follows a r−2 dependence.