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The nature of the interaction of Io with the Io torus has undergone an important paradigm shift due to the many results of the Galileo flyby and the reinterpretation of the Voyager data. The main obstacle to the flowing plasma appears to be mass loading. Only a small fraction of the flux tubes that are flowing toward Io's cross section at infinity join up with the Io magnetic field and flow slowly across Io's polar caps. The mass pickup on these flux tubes is heated much less than on the tubes that flow near to but around Io. Both Voyager and Galileo data show this deceleration and deflection. The slowed plasma in the wake is accelerated up to corotational velocities by 6 R Io downstream. This picture is much different than the original picture of Io as a unipolar inductor in which an electric potential drop of 500 kV appeared across Io. In fact, the potential drop is only about 50 kV. The current flowing along magnetic field lines in the vicinity of Io, however, is much greater than heretofore believed. The original theories did not take into account the size of the mass-loading region or the closure currents associated with the bent field lines. Thus they assumed that the currents would be limited by the Alfven conductance and the size of Io. The field-aligned current system appears to be much larger than previously proposed and shifted downstream. This downstream shift has important consequences for the possible size of the Io intrinsic field. It may be larger than originally proposed. The location and source of these current systems does not affect their capability of generating the potential drops along the magnetic field that were postulated to be responsible for radio emissions. The low potential drop across Io evidenced by the small fraction of the wake stream lines that intersect Io and the low pickup ion temperature produces cold, slow flowing flux tubes that can interchange with the hot torus tubes just inside the Io orbit, producing the cold plasma torus.

What's Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959 , published online 15 October), Fuselier et al. (p. 962 , published online 15 October), Funsten et al. (p. 964 , published online 15 October), and Möbius et al. (p. 969 , published online 15 October) present data taken by NASA's Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966 , published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971 , published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Sun's travel through the galaxy as was previously thought.

Multiscale hybrid kinetic simulations of low-{beta}{sub p} supercritical shocks demonstrate that pickup ions may be strongly accelerated by shock surfing, also known as multiply reflected ion acceleration. {copyright} {ital 1999} {ital The American Physical Society}

The time-dependent cosmic ray modulation due to solar activity and global merged interaction regions is studied with a self-consistent model of the interaction of a time-varying solar wind, including pickup ions, with cosmic rays. After a study of the spatial evolution of global merged interaction regions, their different effects on local as well as global modulation of both galactic and anomalous cosmic rays are analyzed. It is found that the long-term modulation cannot be explained alone with successive step decreases in the cosmic ray intensities caused by global merged interaction regions, but that an additional long-term global variation in spatial diffusion is required. The analysis for a solar wind with pickup ions reveals that there occurs a significant upstream expansion of global merged interaction regions accompanied by a decrease of their peak density enhancement. Both effects lead to an immediate recovery of the cosmic ray intensities after a step decrease. Furthermore, the calculated response of the solar wind termination shock to the passage of global merged interaction regions shows minor changes in the structure and position so that the shock remains in a gas-dynamic (pickup ion) dominated state; that is, it is never cosmic ray dominated. This investigation also shows that the decay of global merged interaction regions is particularly fast during their interaction with the termination shock and in the region beyond, so they probably cannot be interpreted as the downstream modulation barriers discussed in the literature.

We have performed a number of numerical experiments to study the interaction of interstellar pickup protons and helium ions with quasi-parallel stationary (bow) shocks and interplanetary traveling shocks. The collisionless shocks are modeled by the hybrid simulation method which treats the ions as macroparticles and the electrons as a massless background fluid. Solar wind protons, alpha particles, pickup H+ and He+ are included self-consistently. The pickup ion distributions are modeled as spherical shells in velocity space convected with the solar wind. Pickup ions are rather efficiently reflected at both types of shocks. The reflection coefficient, measured as the ratio of incident to reflected ions, is more than one order of magnitude larger than the reflection coefficients for either solar wind protons or alpha particles, and is of the order of 30%. We have found that at high Mach number shocks pickup ions can reach many tens to over hundred times the plasma ramming energy in essentially one shock encounter. The reflected pickup ion distributions have a pancake-type shape in velocity space.

Pickup hydrogen and helium distributions are measured in compressive corotating regions at high heliographic latitudes. The enhancements can be explained using a pickup ion transport model which includes compression but requires a long scattering mean free path. The long mean free path is generally observed for pickup ions at high latitudes, but it is not explained by current theories of wave-particle interaction. The feet that the mean free path is not reduced in the high-latitude compressed events suggests that the long mean free path is not isolated to highly static solar wind conditions. It also has important implications for wave-particle interaction theories since the turbulence within the compressed events is expected to be qualitatively different from the turbulence in static noncompressed solar wind.

A self-consistent time-dependent cosmic ray-hydrodynamic model is used in spherical symmetry to solve the standard cosmic ray transport equation for combined anomalous and galactic proton transport, and the one-fluid equations for the solar wind/pickup ion fluid including the dynamic effects of cosmic rays. The model calculates the evolution of a single global merged interaction region including its interaction with the heliospheric termination shock, and the consequent effect on cosmic ray modulation. A series of such regions is also simulated to study their combined contribution on the cosmic-ray modulation cycle.

A mechanism for the acceleration of pickup ions by repeated reflection from the electrostatic cross-shock potential of a quasi-perpendicular shock was proposed independently by Zank et al. [1996b] and Lee et al. [1996]. The acceleration mechanism, known variously as Multiply Reflected Ion (MRI) acceleration or shock surfing, was studied by these authors in the limit of an idealized shock, which possessed neither fine-scale structure (distinct foot, ramp, or overshoot) nor pickup ion scattering turbulence. Here the acceleration of pickup ions at cometary and interplanetary shocks and at the termination shock is studied in the test particle limit on the basis of a particle-mesh simulation. All simulations assume a shell distribution for the pickup ions (either pickup protons or helium), and the dynamics of pickup ions propagating in a fixed electromagnetic field profile are investigated. The effect of a shock foot, ramp and overshoot on the acceleration of pickup ions at perpendicular and oblique shocks is described. The acceleration of pickup ions in the presence of strong turbulence inside the foreshock region is also addressed. For quasi-perpendicular shocks with structure, we obtained the following results. First, the accelerated H + and He + spectrum is a very hard power law E -k , k = 0.92 -1.2, which is much harder than that predicted by diffusive shock acceleration. Also, as θ bn , the angle between the upstream magnetic field and shock normal, decreases from 90°, the accelerated pickup ion spectrum flattens until MRI acceleration ceases. Second, the fine structure of the shock is found to reduce slightly the maximum energy gain for an accelerated pickup ion compared to that gained at an unstructured shock. Third, a flat turbulence spectrum is found to lead to an increase in the maximum energy for transmitted pickup ions, whereas a power law turbulence spectrum leads to a modest reduction in the maximum pickup ion energy gain. However, the basic MRI acceleration mechanism continues to operate in the presence of turbulent magnetic fluctuations and the characteristic hard spectra are preserved. Fourth, MRI acceleration is found to work only for those shocks for which θ bn > 60° - 70°. The results from the test particle simulations described here are also used to interpret particle acceleration in self-consistent hybrid simulations.

On the basis of recent observations that pickup ions are inefficiently scattered through 90° pitch angle, a pickup ion transport model has been developed which assumes that pickup ion distributions are uniform in the so-called forward and reverse hemispheres, containing ions with pitch angles less than 90° and greater than 90°, respectively. A simple, fast numerical scheme is then described which solves the transport equations. Several examples are solved and comparisons are performed between the numerical solutions and previously used analytical solutions. Results from the model are then compared to pickup ion spectra obtained with the solar wind ion composition spectrometer on the Ulysses satellite.

Studies of Alfven turbulence in the solar wind require the use of magnetic field and particle data. However, particle data is subject to a source of noise not present in magnetic field data, i.e. statistical noise because the bulk parameters of the plasma are calculated from a finite number of counts. The derivation of bulk parameters is unavoidably affected by Poisson statistics which add variance to the real value. The magnitude of the effects of Poisson noise on the frequency spectrum of Alfven waves in the solar wind has been calculated by numerical simulation for the case of cometary pickup ion turbulence observed at comet Halley. The results show that Poisson noise is “white” and flattens the frequency spectrum at high frequencies, where its power spectral density exceeds the solar wind turbulence. If a power law is fitted to the spectrum, the power law index can be reduced significantly even if the flattening is not obvious in the spectrum, and the effect can be significant even when the particle count rates seem high. In Elsasser variable studies the influence of Poisson noise is seen as a tendency to equality of the two spectra at high frequencies since each contains uncorrelated noise. The simulations show that Poisson noise effects must be taken into account in the interpretation of Alfven wave observations. A concrete result is that the difference between the spectral indices obtained separately from magnetic and particle data at comet Halley can be explained quantitatively by Poisson noise.

We present a simple model of anisotropic pickup ion distributions on a flux tube containing both a radial segment and segments oriented perpendicular to the radial direction. The flux tube is carried away from the Sun by a constant speed solar wind. We model the pitch-angle scattering of these ions by a hemispherical approximation, which assumes that the scattering is instantaneous except through the pitch angle of 90°, where the scattering is finite. The model includes a radially dependent ionization of pickup ions, with the newly created ions concentrated in the sunward hemisphere within the radial segment but distributed equally between the hemispheres in the perpendicular segments. The pickup ions stream along the magnetic field and decelerate in the expanding solar wind, but we assume that the radial segment is short enough that the effects of adiabatic focusing may be neglected. For weak scattering through 90°, the distributions exhibit nonlocal properties due to ion streaming along the field from regions where conditions are different than observed locally. We find that this model can yield a depletion of antisunward ions in the radial segment of the flux tube, such as has been observed, but the streaming of ions into the radial region limits the size of this effect unless the radial segment is very long.

Possible signatures of interstellar pickup hydrogen have been identified in data from the Pioneer 10 and 11 Ames plasma analyzers at locations out to beyond the orbit of Saturn, and at a wide range of longitudes in the heliosphere, both upstream and downstream in the interstellar wind. These signatures provide an extension of pickup ion observations out to beyond 15 AU heliocentric distance, including near where the maximum pickup hydrogen flux is expected. A depletion of interstellar pickup hydrogen downstream from the Sun in the interstellar flow may be indicated. These signatures are consistent with Ulysses pickup ion measurements.

The radial and latitudinal dependence of the cosmic ray diffusion tensor is investigated on the basis of a recently developed model of magnetohydrodynamic turbulence in the expanding solar wind [Zank et al., 1996a,b; Matthaeus et al., 1996]. In the ecliptic plane, decaying magnetohydrodynamic turbulence is assumed to be replenished in situ by turbulence generated through the interaction of streams (both shear and compressional effects) and by the creation of pickup ions. In the polar region, at least during solar minimum, stream interaction driven turbulence is neglected and only pickup ion driven turbulence is included. To model the perpendicular and drift elements of the cosmic ray diffusion tensor, we employ both a quasi-linear theory (QLT) and a newly developed nonperturbative theory (NPT) to describe the field line wandering which drives perpendicular transport. A resonant quasi-linear description is applied to the parallel component. For the QLT approach, we find that in the solar wind ecliptic plane (1) the radial diffusive length scale or mean free path (mfp) is very nearly constant until some 10 AU, after which it experiences some variation with increasing heliocentric distance; (2) the radial mfp is dominated at all radial distances by the component parallel to the mean magnetic field and the perpendicular component is completely unimportant; (3) the length scale associated with the drift component of the cosmic ray diffusion tensor is only comparable to the radial mfp beyond ∼ 10 AU; and (4) the rigidity P dependence of the radial mfp within 10 – 20 AU is weak and proportional to P1/3, but in the far outer heliosphere it is proportional to P2. For the QLT model in the polar region of the solar wind, we find that the radial cosmic ray mfp is much greater than the corresponding mfp in the ecliptic region, consistent with observed mfps for pickup ions reported by Gloeckler et al. [1995]. The polar models are, however, preliminary and assume vanishing cross-helicity. The polar radial mfp is dominated by the parallel component, and drift length scales are never comparable to the radial mfp in the high polar latitudes. By using instead a nonperturbative model for the perpendicular and drift components of the cosmic ray diffusion tensor, it was found that the mfps for these coefficients could be significantly larger than their QLT counterparts. The increased perpendicular mfp was found to be important in the radial mfp only beyond ∼ 20 AU, which remains dominated by the parallel diffusion within this distance. Within the ecliptic, the nonperturbative model yields a radial mfp for cosmic rays that is almost constant with heliocentric distance. Similar order of magnitude differences between the radial mfps in the ecliptic and polar regions of the solar wind are found with the nonperturbative models.

Enhanced echoes from the 430 MHz radar at Arecibo were observed during burns of the space shuttle orbital maneuver subsystem (OMS) engines near 317 km altitude. Similar radar signatures of enhanced backscatter were also obtained by the Millstone Hill radar observing the plume of a Centaur engine burning in the ionosphere. A theoretical model of incoherent scatter is presented to explain the radar backscatter observations. The theory considers molecular ion beams generated in the exhaust plume as a result of charge exchange between the ambient O+ ions and the high-speed exhaust molecules (primarily H2O). The field-aligned gyromotion of the pickup ions affects the radio wave scattering from the random thermal fluctuations of electron density. Numerical calculations are carried out for plasmas modified by the space shuttle or Centaur engines, and reasonable agreement with observations is found for the total scattered power. Incoherent backscatter spectra respond to characteristics of the exhaust plume such as vector flow velocity, temperature, and composition. The nonequilibrium velocity distributions for the ions in the pickup ion plume are similar to the distributions found in strongly convecting auroral region ionospheres. The incoherent scatter from the plume ions can be used to validate techniques used to study naturally disturbed plasmas. The predictions of our radar scatter calculations will be tested in future experiments using the space shuttle OMS engines over incoherent scatter radars located at equatorial latitudes and midlatitudes.

Observations of interstellar pickup He + by the Suprathermal Energy Ionic Charge Analyzer (SULEICA) instrument on the AMPTE/IRM spacecraft show significant reductions in the antisunward flux of these particles in the solar wind frame when the interplanetary magnetic field is directed radially. During such times, which can be as short as the 20-min time resolution of our instrument, the typically sharp cutoff in the pickup ion distribution at spacecraft-frame energy E c = 2 M V sw 2 , where M is the helium mass and V SW is the solar wind speed, is replaced by a smooth decline to background flux levels over several energy steps. We interpret these reductions as the result of inhibited pitch angle scattering into the observable antisunward region of phase space, implying a strong sunward anisotropy during these times. Thus we unambiguously identify the appearance of such anisotropies with the radial orientation of the magnetic field. We analyze the observations in terms of a steady two-stream model of the ion distribution and use this model to estimate an effective λ, the scattering mean free path of the ions along the field. We obtain values of λ which range from 0.16 to 0.76 AU and discuss the significance of these results.

International audience; Sputtering of the Martian atmosphere by O+ pickup ions has been proposed as a potentially important process in the early evolution of the Martian atmosphere. In preparation for the MAVEN mission, we performed a study using a Monte Carlo model coupled to a molecular dynamic calculation to investigate the cascade sputtering effects in the region of the Martian exobase. Pickup ion fluxes based on test particle simulations in an MHD model for three different solar wind conditions are used to examine the local and global sputtering efficiencies. The resultant sputtering escape rate is 2 × 1024 s− 1 at nominal solar wind condition, and can be enhanced about 50 times when both the IMF strength and the solar wind pressure increase. It is found that when the IMF strength becomes stronger, both the pickup ion precipitation energies and the resultant sputtering efficiencies increase. The related escape flux, hot component, and atmospheric energy deposition deduced from the MAVEN measurements may reveal clues about the prominent enhanced sputtering effects. Significant hemispheric asymmetries can be observed related to the solar wind electric fields. The shielding by the crustal fields and the recycling onto the night-side due to different magnetic field draping features can also lead to regional variations of sputtering efficiencies. The results suggest that disturbed or enhanced solar wind conditions provide the best prospects for detecting sputtering effects for MAVEN mission.

We have revisited the series of observations recently used to infer a temporal variation of the interstellar helium flow over the last forty years. Concerning the recent IBEX-Lo direct detection of Helium neutrals, there are two types of precise and unambiguous measurements which do not rely on the exact response of the instrument: the count rate maxima as a function of the spin angle, which determines the ecliptic latitude of the flow, and the count rate maxima as a function of IBEX longitude, which determines a tight relationship between the ecliptic longitude of the flow and its velocity far from the Sun. These measurements provide parameters (and couples of parameters in the second case) remarkably similar to the canonical, old values. In contrast, the preferential choice of a lower velocity and higher longitude reported before from IBEX data is based only on the count rate variation (at each spin phase maximum) as a function of the satellite longitude, when drifting across the region of high fluxes. We have examined the consequences of dead time counting effects, and conclude that their inclusion at a realistic level is sufficient to reconcile the data with the old parameters, calling for further investigations. We discuss the analyses of the STEREO pickup ion (PUI) data and argue that the statistical method that has been preferred to infer the neutral flow longitude (instead of the more direct method based on the PUI maximum flux directions), is not appropriate. Moreover, transport effects may have been significant at the very weak solar activity level of 2007-2009, in which case the longitudes of the PUI maxima are only upper limits on the flow longitude. Finally, we found that the use of some flow longitude determinations based on UV glow data are not adequate. At variance with recent conclusions we find no evidence for a temporal variability of the interstellar helium flow., Comment: 8 pages, 3 figures, accepted for publication in Astronomy and Astrophysics

During the Galileo-Io flyby, nearly field-aligned, left-hand circularly polarized ion cyclotron waves were observed in a band near the sulfur-dioxide ion gyrofrequency. We have performed a warm plasma dispersion analysis using nominal Io torus composition ratios, pickup ion “ring”-type velocity-space distributions, and a thermal background plasma of typical torus temperature. Analysis shows that the SO2+ wave is dominant, particularly as the ring begins to broaden. The observed spectral peak indicates that the ring distribution of pickup SO2+ is highly unstable and generates waves, but there is unlikely to be sufficient time for ions to fully thermalize before dissociation occurs. Assuming wave-particle scattering of ions towards a “bispherical” shell-type distribution, a free energy analysis and comparison with observed wave amplitudes suggests that the ions are not scattered far from the ring and/or that the SO2+ composition ratio in the Io torus falls off considerably from Io wake values.

Pickup ions are expected to play a fundamental role in determining the global structure of the heliosphere since they dominate the internal energy of the outer solar wind beyond the ionization cavity. The dynamical effect of pickup ions on the propagation characteristics of interplanetary shocks is addressed here. It is found that (1) forward and reverse shocks in a heliosphere mediated by pickup ions have higher speeds than expected of shocks in an adiabatic solar wind without pickup ions; (2) identical initial conditions evolve differently in a pickup ion mediated heliosphere compared to an adiabatic heliosphere, producing more complicated velocity, density and temperature structure, and often with multiple forward-reverse shock pairs; (3) pickup ions act to weaken and dissipate shocks in the outer heliosphere although the velocity jumps can remain large; (4) density fluctuations in the outer pickup ion mediated heliosphere do not correlate with velocity fluctuations as well as they do in the inner heliosphere, and (5) the amplitudes of density fluctuations in a pickup ion mediated heliosphere are considerably smaller than those found in an adiabatic heliosphere without pickup ions.

We present an analytical model for the distribution function of interstellar pickup ions in a radially directed heliospheric magnetic field which naturally results in anisotropic particle distributions. The model includes the effects of convection and spatial transport in the solar wind, adiabatic deceleration in the radial flow, adiabatic focusing in the radial field, and pitch angle scattering toward isotropy in the frame of the solar wind. The pitch angle scattering is approximated by the hemispherical assumption: we take the scattering to be very efficient within each pitch angle range μ ≷ 0 (where μ is the cosine of the pitch angle) but inhibited between the hemispheres separated by μ = 0. The analytical solution is obtained for the case where the scattering rate across μ = 0 scales as the particle speed divided by the radial position of the fluid parcel. The model distribution functions can be used to interpret recent observations of anisotropic pickup ions.

During 1993 and 1994 the solar coronal holes and the heliospheric current sheet were relatively stationary, and recurrent streams and interaction regions with periods of the order of the solar rotation period were present within 5 AU. One expects that during 1994 Voyager 2 (V2) (located at ≈43 AU and at ≈12°S latitude, in the sector zone) would have observed some evolutionary form of corotating streams and interaction regions. The “sector zone” is the latitude band in which a spacecraft observes both positive and negative sectors [Burlaga and Ness, 1996]. We present the observations of the magnetic field strength (B), the speed (V), density (N), and proton temperature (T) made by V2-94, and for reference we also discuss the observations made by V2 at ≈14 AU during 1983 (V2-83) a solar cycle earlier. Correlated, quasiperiodic variations in B and N with a period of ≈26 days (corotating merged interaction regions) were observed at ≈14 AU but not at ≈43 AU. The speed and temperature profiles were irregular at ≈14 AU but quasiperiodic at ≈43 AU. An ƒ−2 spectrum of the magnetic field strength B (indicating the dominance of shocks) was observed at ≈14 AU, but an ƒ−5/3 spectrum (indicating the dominance of Kolmogorov turbulence) was observed at ≈43 AU in the range (2.7 × 10−6 to 2.3 × 10−5)Hz. An ƒ−2.5 spectrum of the speed fluctuations was observed at ≈14 AU, but an ƒ−2 spectrum was observed at ≈43 AU in the range (8.8 × 10−7 to 2.3 × 10−5)Hz. We suggest the hypothesis that the qualitative differences between the observations at ≈14 AU and ≈43 AU represent a change in the state of the solar wind as it moves between these two positions. This change involves a transition from a quasiperiodic (ordered) state in B and N at ≈14 AU to a disordered state at ≈43 AU and from an aperiodic state in V and T at ≈14 AU to a quasiperiodic state at ≈43 AU. The standard MHD models for the radial evolution of corotating streams and interaction regions have not predicted such a transition. Our results suggest that it would be fruitful to develop a new MHD model of such flows, which should include (1) three-dimensional effects, (2) the intermediate-scale fluctuations, and (3) the interstellar pickup ion pressure.

The interaction of the solar wind with the neutral environment of comets takes place by means of a number of microscopic processes, Photoionization of the neutral particles by solar UV radiation allows a relatively simple analytical treatment and has been well described in theoretical models. However, it is well known that the charge exchange of protons with heavy cometary molecules is an even more efficient physical mechanism. At the time of the observed spacecraft encounters with the comets Halley and Giacobini-Zinner the charge exchange rate was of the same order as the photoionization rate. We develop an analytical model of the interaction of the supersonic solar wind with a cometary environment to include charge exchange, and we discuss the relative importance of photoionization and charge exchange in slowing down the plasma flow. It is shown that the main effect of charge exchange near comets can be accounted for by a simple increase in the total effective ionization rate and a corresponding rescaling of the interaction pattern. As a result of the small proton to pickup ion mass ratio at comets, the dynamical differences in the pickup process by photoionization and charge exchange are small. For realistic solar wind and cometary environment parameters the rescaling factor for the interaction pattern is quite large because charge exchange is twice as efficient as photoionization. However, as a result of the nonmonotonic dependence of the distance to the sonic point on the ionization rate, the shock position (for M = 2) is only shifted by a factor 1.14. The corresponding correction to the neutral density is shown to be small, less than 7%, and is found to be almost constant at large distances from the comet.

According to one possible picture of the Titan atmosphere interaction with Saturn's magnetosphere, newborn exospheric pickup ions may exhibit a marked asymmetry with respect to the corotation wake due to their large gyroradii in the local ∼5 nT magnetic field. These finite gyroradius effects should persist until the ions are scattered by wave-particle interactions (a scale length that is presently unknown). To the extent that simple test-particle-like motion is maintained, the combination of the small (on the scale of most ion gyroradii) size of Titan and the small, steady local magnetic field at ∼20 Saturn Radii provides a natural ion mass spectrometer configuration. The resulting spatial dispersion of the pickup ion trajectories can be used for both analyzing the composition of the Titan exosphere and determining the ionization rates of its various constituents. Possible evidence of this ion mass spectrometer effect may be visible as a magnetic field perturbation in the Voyager 1 Titan flyby data.

Pickup ions measured deep inside the heliosphere open a new way to determine the absolute atomic density of a number of elements and isotopes in the local interstellar cloud (LIC). We derive the atomic abundance of hydrogen and the two isotopes of helium from the velocity and spatial distributions of interstellar pickup protons and ionized helium measured with the Solar Wind Ion Composition Spectrometer (SWICS) on the Ulysses spacecraft between ~2 and ~5 AU. The atomic hydrogen density near the termination shock derived from interstellar pickup ion measurements is 0.115±0.025 cm-3, and the atomic H/He ratio from these observations is found to be 7.7 ± 1.3 in the outer heliosphere. Comparing this value with the standard universal H/He ratio of ~10 we conclude that filtration of hydrogen is small and that the ionization fraction of hydrogen in the LIC is low.

Data from the Shuttle Potential and Return Electron Experiment (SPREE) flown as part of the Tethered Satellite System (TSS 1) are used to determine the detailed characteristics of beam arc distributions of pickup ions due to molecules outgassed or ejected from the shuttle. These ion distributions are only detected near the plane perpendicular to the magnetic field direction in an angular range of ±45° about the minimum angle to ram. Their flux is largest when the angle between this plane and the shuttle ram direction is smallest. Generally, ion spectra peak in the range 19 to 25 eV at the minimum angle between the perpendicular plane and the ram direction. The peak energy decreases smoothly as this angle increases. Weak fluxes are measured above the peak, to energies as high as 150 eV. Within the SPREE energy range, two-dimensional distribution functions of beam arc ions in the perpendicular plane have teardrop shapes, symmetric about the minimum angle to shuttle ram with deep minima in the centers. Variations in the peak energies of differential number fluxes agree with collisionless trajectory analysis, assuming that the ions are H2O+ and allowing for different initial velocities before charge exchange. The lowest densities for beam arc ions occur during periods of purely residual outgassing from the shuttle. Ion densities increase by a factor of 5 during waste water dumps. The highest densities occur during operation of the flash evaporator system when the pickup ion densities in daylight can exceed 105 ions cm−3, about 30% of the estimated ambient plasma density. We also present a nonlinear numerical analysis to study the stability of beam arc generated plasma waves and explain electrostatic spectra measured during previous shuttle flights.

The pressure of interstellar pickup protons in the distant heliosphere can be determined by analyzing pressure-balanced structures, observed on a scale of a few hundredths of an AU. This paper extends the earlier work of L. F. Burlaga et al. (Journal of Geophysical Research, 99, 21,511, 1994) by analyzing pressure-balanced structures observed by Voyager 2 from 39.3 to 40.6 AU in 1993 and from 42.6 to 43.2 AU during 1994. The pickup proton temperature is high in the region of the distant heliosphere that we considered: (5.4 ± 0.1) × 106 K at 39–41 AU and (6.0 ± 0.4) × 106 K at ≈43 AU. The density of the pickup protons is (1.6 ± 0.3) × 10−4 cm−3 at 39–41 AU and (1.2 ± 0.2) × 10−4 cm−3 at ≈43 AU. The ratio of the pickup proton density to the solar wind proton density (Ni/N) is small, only 0.03 ± 0.01 during both 1993 and 1994. Nevertheless, the pickup proton pressures are relatively high because of their high temperatures. The pickup proton pressure was (11 ± 2) × 10−14 erg cm−3 at 39–41 AU and (9 ± 1) × 10−14 erg cm−3 at 43 AU. There is a possible decrease in Pi with increasing distance from the Sun. The pickup proton pressure is an order of magnitude greater than the solar wind proton pressure: Pi/Pswp = 10 ± 2 at 39–41 AU and 8 ± 2 at 43 AU. Our results support the hypothesis of Burlaga et al. that the pickup proton pressure is more important than the solar wind thermal pressure in the dynamics of the distant heliosphere. The ratio of the pickup ion pressure to the magnetic pressure is Pi/(B2/8π) = 1.7 ± 0.3 at 39–41 AU and 1.7 ± 0.72 at ≈43 AU. These results are compared with a model.

Over the last 10 years, the experimental basis for the study of the local interstellar medium has been substantially enhanced by the direct detection of interstellar pickup ions and of interstellar neutral helium within the heliosphere. Pickup ions can be studied for a wide range of interstellar species. However, currently the accuracy of the method to determine the parameters of the interstellar medium, namely neutral density, temperature and relative velocity, is hampered by two problems: (1) In most cases the crucial ionization rates are not available from simultaneous measurements and (2) the transport of the pickup ions in the interplanetary medium substantially modifies the measured spatial distribution of the ions. In this study we will discuss how the enhanced capabilities of the instrumentation on SOHO and Cluster in combination with ongoing efforts to model the pickup ion distributions will lead to a significant improvement over the coming years.

We investigate the properties of a bispherical shell distribution of pickup ions when dispersion of the resonant waves is taken into account. We also extend the method of Johnstone et al. [1991] and Huddleston and Johnstone [1992] for determining the spectra of the quasi-linear self-generated waves to include dispersion. Our specific results assume that all waves propagate along the average magnetic field, and that the waves satisfy the cold electron-proton dispersion relation. The major effect of including dispersion in the analysis is that the particles in a bispherical distribution retain more of their initial energy than in the nondispersive treatment, so the resulting wave spectra are less intense than predicted when dispersion is neglected. The dispersive analysis also encounters the well-known gap in the cyclotron resonance with fast-mode waves which is absent in the nondispersive picture. We present detailed results for pickup protons in the two extreme configurations of magnetic field perpendicular to, and parallel to, the solar wind flow for a range of values of VA/VSW. We find that when pickup protons are scattered to a bispherical distribution with VA/VSW = 0.1, typical of conditions in the solar wind, the discrepancy in the total energy density of the self-generated waves between the dispersive and nondispersive results amounts to 18% in the perpendicular configuration. This discrepancy falls between 9% and 73% in the parallel configuration, depending on the extent of the particle transport through the resonance gap. These discrepancies are substantially larger for larger values of VA/VSW. These results indicate that dispersive effects can be important in the correct modeling of the quasi-linear resonant wave-particle interaction between pickup ions and self-generated waves in the solar wind.

Energization at a quasi-perpendicular shock is described for ions which approach the shock with a speed much less than that of the incoming plasma. These ions may be trapped between the shock electrostatic potential and the upstream Lorentz force and accelerated by “surfing” along the shock surface, before eventually escaping the shock into the upstream or downstream plasma. The process is described in detail, extending previous work on the mechanism, and energy gains are calculated. It is pointed out that pickup ions in the solar wind are ideally configured, so that a reasonable fraction of the ions can be accelerated by this mechanism at cometary bow shocks, the solar wind termination shock, and interplanetary traveling shocks. The mechanism may provide the required “injection” or preacceleration at quasi-perpendicular shocks for subsequent diffusive shock acceleration.

Pickup ions in a ring velocity distribution are unstable to several kinetic plasma instabilities. At large heliocentric distances where the overall plasma β (ratio of kinetic to magnetic energy) is dominated by the energy density of interstellar pickup ions and pickup is perpendicular to the interplanetary magnetic field, the dominant of these is the Alfven ion cyclotron instability (AIC). We demonstrate by hybrid particle simulation that, for conditions where the solar wind β is low, AIC driven by the pickup ions couples to the solar wind. The result is perpendicular heating, leading to an anisotropic solar wind distribution. This process may contribute to enhanced solar wind temperatures at large heliocentric distances and may allow for indirect measurement of interstellar pickup ions.

A new mechanism for the acceleration of pickup ions by repeated reflections from the electrostatic cross shock potential of a quasi-perpendicular shock is presented. The acceleration mechanism, multiply reflected ion (MRI) acceleration, offers a resolution to the issue of injecting pickup ions into an efficient particle energization scheme, and the injection efficiency for pickup ions is found to be inversely proportional to ion mass and proportional to charge. By studying the particle energy gain in the motional electric field (where a steady shock frame is assumed) the energized pickup ion spectrum can be computed. Extremely hard power law spectra (E −1.5 , for example) emerge from the upstream pickup ion distribution. The maximum energy that a reflected pickup ion can gain is found to be proportional to the square of the product of the Alfven speed and (r−1), where r is the shock compression ratio. For solar wind conditions at either interplanetary shocks or the termination shock the upper energy limit is typically in excess of 0.5 MeV. It is suggested here that MRI acceleration provides an efficient mechanism for injecting low-energy pickup ions into a subsequent acceleration process such as diffusive Fermi acceleration. Such a two-step acceleration scheme alleviates many of the difficulties which plague ion energization models at perpendicular shocks. The structure of a quasi-perpendicular shock modified by shock reflection of pickup ions is discussed in general terms. By way of application we present a detailed study of the MRI acceleration mechanism at the termination shock for a wide range of parameters and discuss the implications for the anomalous cosmic ray component. The acceleration of pickup ions by an interplanetary traveling shock is also discussed, and the observations made by Ulysses [Gloeckler et al., 1994] are addressed. The puzzling aspects of the Gloeckler et al. [1994] observations appear to be explained quite naturally by shock energization based on repeated pickup ion reflections. Observational tests of MRI acceleration may be possible by using pickup He + at either the terrestrial or Jovian bow shock or by using cometary ions at a cometary bow shock.

In this thesis, interstellar pickup ions at 1 astronomical unit are analysed with the PLAsma and SupraThermal Ion Composition instrument (PLASTIC) on board the Solar Terrestrial Relations Observatory (STEREO). Interstellar pickup ions originate from the neutral component of the Local InterStellar Medium (LISM), which enters the heliosphere with a velocity of 25 km/s as a result of its relative motion with respect to our Sun. This neutral wind is gradually ionized by charge exchange with the solar wind, by solar ultraviolet radiation, or more rarely, by electron impact and thus acts as the seed population for interstellar pickup ions. The freshly created ions are 'picked up' by the interplanetary magnetic eld that is frozen into the solar wind and rapidly acquire speeds equaling the solar wind speed in a solar wind frame of reference. More importantly, these pickup ions carry information on the LISM and provide an excellent opportunity to study the LISM in-situ. In dieser Arbeit werden Pickup-Ionen bei einer astronomischen Einheit mit dem Plasma and Suprathermal Ion Composition Instrument (PLASTIC) an Bord des Solar Terrestrial Relations Observatory (STEREO) untersucht. Interstellare Pickup-Ionen entstammen der neutralen Komponente des interstellaren Mediums, welches auf Grund seiner Relativbewegung zu unserer Sonne die Heliosphäre mit einer Geschwindigkeit von 25 km/s durchfliesst. Dieser neutrale Wind wird dabei sukzessive durch Ladungsaustausch mit dem Sonnenwind, ultra-violetter Strahlung der Sonne oder aber Elektronenstossionisation ionisiert und stellt somit die Ursprungspopulation der interstellaren Pickup-Ionen dar. Die dabei entstandenen Ionen werden vom dem im Sonnenwind eingefrorenen Magnetfeld mitgetragen und erreichen dabei Geschwindigkeiten von bis zu einer Sonnenwindgeschwindigkeit im Bezugssytem des Sonnenwindes. Entscheidend für diese Arbeit ist zudem, dass trotz des Ionisationsprozesses der Pickup-Ionen einige der Charakteristiken des LISMs erhalten bleiben und somit eine hervorragende Möglichkeit bieten das LISM eingehender zu studieren.

A gasdynamic model of the magnetosheath is extended into the tail region by incorporating a cometary tail field produced by mass loading. By means of test particle simulations, a picture of the martian pickup ion wake is described and contrasted with particle measurements of the Phobos 2 automatic space plasma experiment with a rotating analyzer (ASPERA). It is shown that the convection electric field alone is not sufficient to explain the observations. If magnetic shear stresses of the draped field are taken into account, the flux of low-energy oxygen ions close to the central wake of Mars (a persistent observational feature in the particle data) is reproduced. Simulation results suggest that the low-energy ion observations (E ≤ 350 eV) of the ASPERA particle instrument are due to particles picked up in a source region at lower altitude close to the terminator plane at low latitudes, while moderate energy ions are created at high areographic latitudes, depending on the orientation of the transverse interplanetary magnetic field component. High-energy ions (E ≥ 2 keV) usually originate from more distant regions in the magnetosheath or in the solar wind above the dayside of Mars.

The distribution function of interstellar pickup hydrogen is measured with the SWICS instrument on Ulysses at high heliographic latitudes, over a broad range of velocities, including those below that of the solar wind. It is found that these few MV rigidity protons exhibit a pronounced inward radial streaming in the frame of the solar wind, suggesting that neither instabilities generated by the pickup ions nor ambient turbulence is effective in scattering and isotropizing these particles. These results have implications for theories for the instabilities associated with pickup ions and for the propagation of low rigidity particles in the solar wind.

A precision determination of the interstellar neutral (ISN) flow direction is important in several ways. As a cardinal axis of the heliosphere it has strong leverage on the plane subtended by the ISN velocity and interstellar magnetic field vector, which controls the shape of the heliosphere and its interaction with the interstellar medium. Continuing observations of the ISN flow through the heliosphere for several decades allow the search for potential temporal variations of the ISN flow and comparison with astronomical observations. Recent efforts to obtain a consistent ISN vector and temperature with Ulysses and IBEX neutral gas observations point to remaining uncertainties and potential systematic effects. In particular, IBEX measurements provide a very precise relation between ISN flow longitude and speed via the hyperbolic trajectory equation, but they contain larger uncertainties along the parameter tube defined by this relation. The pickup ion (PUI) cut-off variation with ecliptic longitude at 1 AU can provide a complementary determination of the ISN flow longitude with high precision. We compare STEREO PLASTIC and ACE SWICS observations with a simple analytical model of the cut-off. We perform a Pearson correlation analysis of the cut-off as a function of ecliptic longitude with its mirrored function and obtain the symmetry axis with a statistical uncertainty

Pickup ions are created when interstellar neutral atoms resonantly exchange charge with the solar wind (SW) ions, especially in the supersonic part of the wind, where they carry most of the plasma pressure. Here we present numerical simulation results of the 3D heliospheric interface treating pickup ions as a separate proton fluid. To satisfy the fundamental conservation laws, we solve the system of equations describing the flow of the mixture of electrons, thermal protons, and pickup ions. To find the density and pressure of pickup ions behind the termination shock, we employ simple boundary conditions that take into account the \emph{Voyager} observations that showed that the decrease in the kinetic energy of the mixture at the termination shock predominantly contributed to the increase in the pressure of pickup ions. We show that this model adequately describes the flow of the plasma mixture and results in a noticeable decrease in the heliosheath width., Comment: Accepted for publication in Journal of Physics, Conference Series

Voyager 2 and IMP 8 plasma data are used to look for the predicted slowdown of the solar wind with heliospheric distance. Decreases of roughly 7% in the radial velocity and of the same order in the flux are found if the Voyager 2 and IMP 8 velocities are normalized to agree in the inner heliosphere. This decrease is consistent with a pickup ion density equal to 8% of the total ion density, similar to predictions and other determinations of this density. Comparison with published model results allows us to infer an interstellar neutral density of 0.05 cm−3.

We study the physics of locally born interstellar pickup proton acceleration at the nearly perpendicular solar wind termination shock (SWTS) in the presence of a random magnetic field spiral angle using a focused transport model. Guided by Voyager 2 observations, the spiral angle is modeled with a q-Gaussian distribution. The spiral angle fluctuations, which are used to generate the perpendicular diffusion of pickup protons across the SWTS, play a key role in enabling efficient injection and rapid diffusive shock acceleration (DSA) when these particles follow field lines. Our simulations suggest that variation of both the shape (q-value) and the standard deviation (σ-value) of the q-Gaussian distribution significantly affect the injection speed, pitch-angle anisotropy, radial distribution, and the efficiency of the DSA of pickup protons at the SWTS. For example, increasing q and especially reducing σ enhances the DSA rate.

We present results from hybrid simulations (kinetic ion/fluid electron) of the interaction of interstellar pickup ions with collisionless shocks. Since cross-field transport is unphysically suppressed in the one-dimensional geometry used here, an ad hoc scattering algorithm is used to model this effect. This is a necessary step to accelerate the pickup ions from their initial low energies at quasi-perpendicular shocks to the high energies which are often observed associated with traveling interplanetary shocks by Ulysses.

Suprathermal energetic particles, such as solar energetic particles (SEPs) in the inner heliosphere and pickup ions (PUIs) in the outer heliosphere and the very local interstellar medium, often form a thermodynamically dominant component in their various environments. In the supersonic solar wind beyond > 10 AU, in the inner heliosheath (IHS), and in the very local interstellar medium (VLISM), PUIs do not equilibrate collisionally with the background plasma. Similarly, SEPs do not equilibrate collisionally with the background solar wind in the inner heliosphere. In the absence of equilibration between plasma components, a separate coupled plasma description for the energetic particles is necessary. Using a collisionless Chapman-Enskog expansion, we derive a closed system of multi-component equations for a plasma comprised of thermal protons and electrons, and suprathermal particles (SEPs, PUIs). The energetic particles contribute an isotropic scalar pressure to leading order, a collisionless heat flux at the next order, and a collisionless stress tensor at the second-order. The collisionless heat conduction and viscosity in the multi-fluid description results from a nonisotropic energetic particle distribution. A simpler single-fluid MHD-like system of equations with distinct equations of state for both the background plasma and the suprathermal particles is derived. We note briefly potential pitfalls that can emerge in the numerical modeling of collisionless plasma flows that contain a dynamically important energetic particle component.

[1] Observations of interstellar pickup ions inside the heliosphere provide an indirect method to access information on the surrounding interstellar medium. The so-called pickup ion focusing cone and pickup ion crescent, which show an imprint of the related longitudinal distribution of interstellar neutrals in form of two overabundances on the down- and upwind side of the sun, are both believed to be aligned along the inflow vector of the interstellar medium. By finding their longitudinal positions, we can give an accurate value for the inflow direction λISM of interstellar matter. For that we performed an epoch analysis of interstellar pickup ions measured by the PLAsma and SupraThermal Ion Composition instrument (PLASTIC) on the Solar TErrestrial RElations Observatory mission (STEREO) and were able to reveal in situ the longitudinal distribution of interstellar He+, O+, and Ne+ pickup ions in the ecliptic plane at 1 AU. The previously accepted values for the inflow direction of interstellar matter in ecliptic longitude, as obtained with Ulysses/GAS (λ = 75.4° ± 0.5°), Prognoz 6 (λ = 74.5° ± 1°), and ACE/SWICS (λ = 74.43° ± 0.33°), are currently debated, especially in view of recent results from the Interstellar Boundary Explorer (IBEX) mission that show an inflow direction of interstellar neutral helium of λ = 79° + 3.0°(−3.5°). Four years of data collected with PLASTIC aboard STEREO A provided statistics sufficient not only to obtain values for the inflow direction of interstellar helium (λCone = 77.4° ± 1.9° and λCrescent = 80.4° ± 5.4°, deduced from an analysis of the He+ focusing cone and crescent, respectively) but also to derive values for the inflow direction of interstellar neon (λCone = 77.4° ± 5.0° and λCrescent = 79.7° ± 2.6°) and oxygen (λCrescent = 78.9° ± 3.1°). Although our values for He+, O+, and Ne+ are consistent with results from ACE, Ulysses, and Prognoz 6, considering the statistical and systematic uncertainties (except λNe,Crescent), they are systematically larger than the previously accepted values of 74.99 ± 0.55° and show a better agreement with the values from IBEX.

ARTEMIS observes pickup ions around the Moon, at distances of up to 20,000 km from the surface. The observed ions form a plume with a narrow spatial and angular extent, generally seen in a single energy/angle bin of the ESA instrument. Though ARTEMIS has no mass resolution capability, we can utilize the analytically describable characteristics of pickup ion trajectories to constrain the possible ion masses that can reach the spacecraft at the observation location in the correct energy/angle bin. We find that most of the observations are consistent with a mass range of approx. 20-45 amu, with a smaller fraction consistent with higher masses, and very few consistent with masses below 15 amu. With the assumption that the highest fluxes of pickup ions come from near the surface, the observations favor mass ranges of approx. 20-24 and approx. 36-40 amu. Although many of the observations have properties consistent with a surface or near-surface release of ions, some do not, suggesting that at least some of the observed ions have an exospheric source. Of all the proposed sources for ions and neutrals about the Moon, the pickup ion flux measured by ARTEMIS correlates best with the solar wind proton flux, indicating that sputtering plays a key role in either directly producing ions from the surface, or producing neutrals that subsequently become ionized.

[1] Enhanced electromagnetic plasma waves generated by unstable pickup ion distributions have been detected by spacecraft in several planetary environments including those of Venus, Earth, Mars, comets, Jupiter and Saturn. These waves are important because they can be used as a diagnostic of the local plasma and the ion pickup conditions. Interpretation of these waves relies on understanding the source of free energy and how that free energy is reduced through instability growth. In this manuscript, we focus on the regime where the pickup velocity is sub-Alfvenic and systematically illustrate some basic differences in instability behavior by carrying out one-dimensional hybrid simulations of pickup ion-generated waves for varying pickup angle, α. Although several pickup ion-driven instabilities are possible in this regime, the dominant electromagnetic instability that is observed is the ion cyclotron ring instability, which generates waves in the spacecraft frame with frequencies near the pickup ion cyclotron frequency. To better understand the instability behavior as well as what wave properties a spacecraft would potentially observe, we carry out the simulations in the bulk plasma frame and in the planetary rest frame (i.e. spacecraft frame). The simulations presented here are not meant to reproduce any particular planetary environment but are intended to illustrate the general behavior in the sub-Alfvenic pickup velocity regime. We also review the previous observations and dispersion analyses of pickup ion-driven waves in the relevant planetary environments, and discuss them in terms of our simulation results.

[1] Observations upstream of Mars show strong and persistent plasma wave activity near the local proton cyclotron frequency. These waves are thought to be generated by unstable populations of newborn ions produced by ionization of the Martian hydrogen exosphere. It is generally believed that the amplitudes of the waves are related to the energy of the local pickup ion population, such that the observed spatial and temporal distribution of the wave amplitudes could be used as a diagnostic of local exospheric structure and loss rate. To understand the relationship between wave amplitudes and ion pickup rate, we carry out a parametric study using the 1D hybrid simulation (kinetic ions, fluid electrons) for varying pickup rates and pickup geometry for the Martian planetary environment. Our results indicate that, contrary to expectations, the observed local ion cyclotron wave amplitudes cannot be easily related to local ion pickup rates because the growth time of the instability is long compared to the transit time of the waves past Mars. Additionally, we find the amount of pickup ion energy lost to wave growth is not a constant over all injection rates and wave amplitudes are likely affected by propagation through regions of spatially non-uniform ion production. Because the waves in the upstream region are in a state of growth rather than saturated, the simulations suggest the pickup ion distributions are likely only partially scattered.

© 2011 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. Ion pickup is one of the fundamental processes at work throughout the solar system. The ionization of neutral gas provides fresh pickup ions which then interact with the plasma environment – e.g. the solar wind or a planetary magnetosphere. Initiallythe motion is acceleration along the electric field, but the presence of a magnetic field eventually causes a cycloid in real space and a ring in velocity space. Givenenoughtime, scattering may occur in pitch angle and in energy. If there is a variation in therate of pickup over the cycloid, nongyrotropic ion distributions can be seen. HereI review the ion pickup process and the observations at comets [Giacobini-Zinner (GZ), Halley, Grigg-Skjellerup (GS), and Borrelly], and at Mercury, Venus, Mars, in the SaturnandJupiter systems (particularly at Titan, Enceladus, in Saturn's inner magnetosphere and at Io), and at the Moon. Results from several space missions are reviewed (ICE, Giotto, DS1, Messenger, Venus Express, Mars Express, Cassini, Galileo, Kaguya, and Chandrayaan-1). I compare the process and timescales in the various contexts and examine the prospects for future missions including Rosetta, BepiColombo, and EJSM.

We outline the disagreement between the theoretical expectation of rapid isotropization for interstellar pickup ions in the solar wind and the observations of frequent anisotropy. This disagreement is related to the further problem of explaining the scarcity of the wave enhancements predicted to accompany the isotropization. We describe a computational investigation of the kinetic processes involved, and our plans to resolve these discrepancies. We present results of a preliminary calculation to illustrate the capabilities of this technique.

Pickup ions formed from ionized neutral exospheres in flowing plasmas have phase space distributions that reflect their source's spatial distributions. Phase space distributions of the ions are derived from the Vlasov equation with a delta function source using three.dimensional neutral exospheres. The ExB drift produced by plasma motion picks up the ions while the effects of magnetic field draping, mass loading, wave particle scattering, and Coulomb collisions near a planetary body are ignored. Previously, one.dimensional exospheres were treated, resulting in closed form pickup ion distributions that explicitly depend on the ratio rg/H, where rg is the ion gyroradius and H is the neutral scale height at the exobase. In general, the pickup ion distributions, based on three.dimensional neutral exospheres, cannot be written in closed form, but can be computed numerically. They continue to reflect their source's spatial distributions in an implicit way. These ion distributions and their moments are applied to several bodies, including He(+) and Na(+) at the Moon, H(+2) and CH(+4) at Titan, and H+ at Venus. The best places to use these distributions are upstream of the Moon's surface, the ionopause of Titan, and the bow shock of Venus.