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The electromagnetic instabilities driven by the pickup ions in the outer heliosheath play a crucial role in interpreting the energetic neutral atom (ENA) ribbon observed by the Interstellar Boundary EXplorer (IBEX). Previous studies on the stability of the outer heliosheath pickup ions focused on ring-like velocity distributions with pickup angles equal or close to 90°. In this study, we investigate the instabilities driven by a ring-beam velocity distribution of the pickup ions in the outer heliosheath using linear instability analysis. The modes propagating along (both parallel and antiparallel to) the background magnetic field are examined, and the emphasis is on how the instabilities change with the pickup angle, α, varying from zero to 90°. The parallel thermal spread of the pickup ions is chosen to make the plasma parameters lie in the Alfvén cyclotron ‘stability gap’ to exclude the well-studied Alfvén cyclotron instability. Our linear instability analysis reveals that the unstable modes all have left helicity but may propagate parallel or antiparallel to the background magnetic field. At small α, the parallel-propagating modes are unstable in two separate wavenumber (and frequency) ranges. The two unstable ranges merge into one when α increases to 82°. In addition, the antiparallel-propagating modes become unstable when α is above 5°, but the parallel-propagating modes have larger growth rates until α becomes larger than αc = 82.8°. The growth rates of the parallel-propagating modes quickly drop below zero when α goes beyond αc, while the antiparallel-propagating modes remain unstable until α approaches 90°.

The study of multiscale pickup ion phase-mixing in the lunar plasma wake with a hybrid model is the main subject of our investigation in this paper. Photoionization and charge exchange of protons with the lunar exosphere are the ionization processes included in our model. The computational model includes the self-consistent dynamics of the light ( H + or H 2 + and He + ), and heavy ( Na + ) pickup ions. The electrons are considered as a fluid. The lunar interior is considered as a weakly conducting body. In this paper we considered for the first time the cumulative effect of heavy neutrals in the lunar exosphere (e.g., Al, Ar), an effect which was simulated with one species of Na + but with a tenfold increase in total production rates. We find that various species produce various types of plasma tail in the lunar plasma wake. Specifically, Na + and He + pickup ions form a cycloid-like tail, whereas the H + or H 2 + pickup ions form a tail with a high density core and saw-like periodic structures in the flank region. The length of these structures varies from 1.5 R M to 3.3 R M depending on the value of gyroradius for H + or H 2 + pickup ions. The light pickup ions produce more symmetrical jump in the density and magnetic field at the Mach cone which is mainly controlled by the conductivity of the interior, an effect previously unappreciated. Although other pickup ion species had little effect on the nature of the interaction of the Moon with the solar wind, the global structure of the lunar tail in these simulations appeared quite different when the H 2 + production rate was high.

Linear dispersion theory is used to study the Alfven-cyclotron, mirror and ion Bernstein instabilities driven by a tenuous (1%) warm proton ring velocity distribution with a ring speed, vr, varying between 2vA and 10vA, where vA is the Alfven speed. Relatively cool background protons and electrons are assumed. The modeled ring velocity distributions are unstable to both the Alfven-cyclotron and ion Bernstein instabilities whose maximum growth rates are roughly a linear function of the ring speed. The mirror mode, which has real frequency ωr=0, becomes the fastest growing mode for sufficiently large vr/vA. The mirror and Bernstein instabilities have maximum growth at propagation oblique to the background magnetic field and become more field-aligned with an increasing ring speed. Considering its largest growth rate, the mirror mode, in addition to the Alfven-cyclotron mode, can cause pitch angle diffusion of the ring protons when the ring speed becomes sufficiently large. Moreover, because the parallel phase speed, v∥ph, becomes sufficiently small relative to vr, the low-frequency Bernstein waves can also aid the pitch angle scattering of the ring protons for large vr. Potential implications of including these two instabilities at oblique propagation on heliospheric pickup ion dynamics are discussed.

Acceleration of pickup ions (PUIs) in an environment surrounded by a pair of corotating interaction regions (CIRs) was investigated by numerical simulations using a hybrid code. Energetic particles associated with CIRs have been considered to be a result of the acceleration at their shock boundaries, but recent observations identified the ion flux peaks in the sub-MeV to MeV energy range in the rarefaction region, where two separate CIRs were likely connected by the magnetic field. Our simulation results confirmed these observational features. As the accelerated PUIs repeatedly bounce back and forth along the field lines between the reverse shock of the first CIR and the forward shock of the second one, the energetic population is accumulated in the rarefaction region. It was also verified that PUI acceleration in the dual CIR system had two different stages. First, because PUIs have large gyroradii, multiple shock crossing is possible for several tens of gyroperiods, and there is an energy gain in the component parallel to the magnetic field via shock drift acceleration. Second, as the field rarefaction evolves and the radial magnetic field becomes dominant, Fermi-type reflection takes place at the shock. The converging nature of two shocks results in a net energy gain. The PUI energy acquired through these processes is close to 0.5 MeV, which may be large enough for further acceleration, possibly resulting in the source of anomalous cosmic rays.

We investigate a generation mechanism of O+ ion beams observed above the Martian bow shock by analyzing ion velocity distribution functions (VDFs) measured by the Suprathermal and Thermal Ion Composition instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. In the solar wind near Mars, MAVEN often observes energetic O+ ion beams (similar to 10 keV or higher). Accompanied with the O+ ion beam events, we sometimes observe characteristic ion VDFs in the magnetosheath: a partial ring distribution. The partial ring distribution corresponds to pickup ions with a finite initial velocity (i.e., not newborn pickup ions), and its phase space density is much smaller than that of local pickup O+ ions of the magnetosheath. Thus, the partial ring distribution is most likely produced by the reflection of pickup O+ ions precipitating from the upstream solar wind below the bow shock. After being injected into the magnetosheath from the solar wind, the precipitating O+ ions are subject to the significantly enhanced magnetic field in this region and start to gyrate around the guiding center of the plasma frame in the magnetosheath. Consequently, a part of precipitating O+ ions are reflected back to the solar wind, generating O+ beams in the solar wind. The beams direct quasi-sunward near the subsolar region but have large angle with respect to the sunward direction at high solar zenith angles (>50 degrees). The reflected O+ beams are accelerated by the convection electric field of the solar wind and may escape Mars.

Non-thermal pickup ions (PUIs) are created in the solar wind (SW) by charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It has long been theorized, but not directly observed, that PUIs should be preferentially heated at quasi-perpendicular shocks compared to thermal SWIs. We present in situ observations of interstellar hydrogen (H+) PUIs at an interplanetary shock by the New Horizons' Solar Wind Around Pluto (SWAP) instrument at ~34 au from the Sun. At this shock, H+ PUIs are only a few percent of the total proton density but contain most of the internal particle pressure. A gradual reduction in SW flow speed and simultaneous heating of H+ SWIs is observed ahead of the shock, suggesting an upstream energetic particle pressure gradient. H+ SWIs lose ~85% of their energy flux across the shock and H+ PUIs are preferentially heated. Moreover, a PUI tail is observed downstream of the shock, such that the energy flux of all H+ PUIs is approximately six times that of H+ SWIs. We find that H+ PUIs, including their suprathermal tail, contain almost half of the total downstream energy flux in the shock frame., 11 pages, 4 figures, accepted for publication in Physical Review Letters

We use Interstellar Boundary Explorer (IBEX) measurements of energetic neutral atoms (ENAs) to constrain the proton (mostly pickup ion, PUI) distribution in the heliotail. In our previous study, we solved the Parker transport equation and found that the velocity diffusion coefficient D(v) for PUIs is approximately D(v) ~ 1.1 × 10⁻⁸ km² s⁻³ (v/v₀)¹̇͘͘·³, assuming the initial proton distribution processed by the termination shock (TS), fp,₀, is a kappa distribution with kappa index κp,₀ = 1.63. In this study, we test different forms for f p,₀. We find that if f p,₀ is kappa-distributed and D(v) = D₀(v/v₀)¹̇͘͘·³, any kappa index in the range 1.5 < κp,₀ < 10 is consistent with IBEX data if D₀ ~ 0.8–1.3 × 10⁻⁸ km² s⁻³. While the case where D(v) ∝ v¹̇͘͘·³ yields ENA fluxes that appear to best reproduce IBEX data for any κp,₀, it is possible for D(v) to scale close to ~v²′³ or ~v² within our uncertainties by changing D₀. We also show that an upstream PUI filled-shell distribution that is heated by a quasi-stationary TS, generating a downstream filled-shell with large cutoff speed, yields an excess of ENAs >2 keV compared to IBEX. However, using a fully kinetic particle-in-cell simulation to process a PUI filled-shell across the TS yields ENA spectra consistent with IBEX, reinforcing the significance of self-consistent, preferential PUI heating and diffusion at the TS. Interestingly, an upstream PUI distribution inferred from the particle-in-cell simulation to reproduce Voyager 2 observations of the nose-ward TS is inconsistent with IBEX observations from the heliotail, suggesting differences in the upstream PUI distribution or TS properties.

The interaction between the solar wind and the interstellar medium represents a collision between two plasma flows, resulting in a heliosphere with an extended tail. While the solar wind is mostly ionized material from the corona, the interstellar medium is only partially ionized. The ion and neutral populations are coupled through charge-exchange collisions that operate on length scales of tens to hundreds of astronomical units. About half the interstellar hydrogen flows into the heliosphere where it may charge-exchange with solar wind protons. This process gives rise to a nonthermal proton, known as a pickup ion, which joins the plasma. In this paper we investigate the effects of approximating the total ion distribution of the subsonic solar wind as a generalized Lorentzian, or κ distribution, using an MHD neutral code. We illustrate the effect different values of the κ parameter have on both the structure of the heliosphere and the energetic neutral atom flux at 1 AU. We find that using a κ distribution in our simulations yields levels of energetic neutral atom flux that are within a factor of about 2 or 3 over the IBEX-Hi range of energies from 0.5 to 6 keV. While the presence of a suprathermal tail in the proton distribution leads to the production of high-energy neutrals, the sharp decline in the charge-exchange cross section around 10 keV mitigates the enhanced transfer of energy from the ions to the neutrals that might otherwise be expected.

Interstellar neutrals penetrating into the inner heliosphere are ionized by photoionization, charge exchange with solar wind ions, and electron impact ionization. These processes comprise the first step in the evolution of interstellar pickup ion (PUI) distributions. Typically, PUI distributions have been described in terms of velocity distribution functions that cool adiabatically under solar wind expansion, with a cooling index of 3/2. Recently, the cooling index has been determined experimentally in observations of He PUI distributions with Advanced Composition Explorer (ACE)/Solar Wind Ion Composition Spectrometer and found to vary substantially over the solar cycle. The experimental determination of the cooling index depends on the knowledge of the ionization rates and their spatial variation. Usually, ionization rates increase with 1/r2 as neutral particles approach the Sun, which is not exactly true for electron impact ionization, because the electron temperature increases with decreasing distance from the Sun due to the complexity of its distributions and different radial gradients in temperature. This different dependence on distance may become important in the study of the evolution of PUI distributions and is suspected as one of the potential reasons for the observed variation of the cooling index. Therefore, we investigate in this paper the impact of electron ionization on the variability of the cooling index. We find that the deviation of the electron ionization rate from the canonical 1/r2 behavior of other ionization processes plays only a minor role.

The microstructure of the heliospheric termination shock and the accompanied local acceleration processes of both ions and electrons are investigated by utilizing one-dimensional full particle-in-cell simulations for a variety of parameters. The relative pickup ion density is assumed to be 20–30%. The magnetic field and the shock potential profiles exhibit significant differences, since the former mostly reflects the dynamics of solar wind ions, whereas the latter is mainly sustained by the bulk motion of the reflected pickup ions in the extended foot. The discrepancy between the magnetic field profile and the potential profile increases with Alfven Mach number. Most of the downstream thermal energy is gained by the pickup ions, while some heating of the solar wind ions and electrons occurs through the modified two-stream instability excited in the extended foot. Self-reformation can occur when the relative pickup ion density is 20% but is blurred when it becomes as large as 30%. Reformation is also suppressed if the local solar wind ion temperature in the extended foot is high, which can either be due to heating by the modified two-stream instability or is already determined by the solar wind temperature far upstream. In all runs presented in this study no evidence for shock surfing acceleration of pickup ions could be found. Nonthermal particle acceleration occurs for oblique shocks. Electron (pickup ion) shock drift acceleration is evidenced when the shock angle is below 80° (60°).

In this report we discuss the global plasma environment of the TA flyby from the perspective of 3D hybrid modeling. In our model the background, pickup, and ionospheric ions are considered as particles, whereas the electrons are described as a fluid. Inhomogeneous photoionization, electron-impact ionization and charge exchange are included in our model. We also take into account the collisions between the ions and neutrals. Our modeling shows that mass loading of the background plasma ( H + , O + ) by pickup ions H 2 + , CH 4 + and N 2 + differs from the T9 encounter simulations when O + ions are not introduced into the background plasma. In our hybrid modeling we use Chamberlain profiles for the atmospheric components. We also include a simple ionosphere model with average mass M =28 amu ions that were generated inside the ionosphere. Titan׳s interior is considered as a weakly conducting body. Special attention has been paid to comparing the simulated pickup ion density distribution with CAPS-ELS and with RPWS LP observations by the Cassini–Huygens spacecraft along the TA trajectory. Our modeling shows an asymmetry of the ion density distribution and the magnetic field, including the formation of Alfven wing-like structures.

In this report we discuss the self-consistent dynamics of pickup ions in the solar wind flow around the lunar-like object. In our model the solar wind and pickup ions are considered as a particles, whereas the electrons are described as a fluid. Inhomogeneous photoionization, electron-impact ionization and charge exchange are included in our model. The Moon will be chosen as a basic object for our modeling. The current modeling shows that mass loading by pickup ions H + , H 2 + , He + , and Na + may be very important in the global dynamics of the solar wind around the Moon. In our hybrid modeling we use exponential profiles for the exospheric components. The Moon is considered as a weakly conducting body. Special attention will be paid to comparing the modeling pickup ion velocity distribution with ARTEMIS observations. Our modeling shows an asymmetry of the Mach cone due to mass loading, the upstream flow density distribution and the magnetic field. The pickup ions form an asymmetrical plasma tails that may disturb the lunar plasma wake.

In this report we discuss the self-consistent dynamics of pickup ions in the solar wind flow around the lunar-like object. In our model the solar wind and pickup ions are considered as a particles, whereas the electrons are described as a fluid. Inhomogeneous photoionization, electron-impact ionization and charge exchange are included in our model. The Moon will be chosen as a basic object for our modeling. The current modeling shows that mass loading by pickup ions Na + and He + may be very important in the global dynamics of the solar wind around the Moon. In our hybrid modeling we use exponential profiles for the exospheric components. The Moon is considered as a weakly conducting body. Special attention will be paid to comparing the modeling pickup ion velocity distribution with ARTEMIS observations. Our modeling shows an asymmetry of the Mach cone due to mass loading, the upstream flow density distribution and the magnetic field. The pickup ions form an asymmetrical plasma tails that may disturb the lunar plasma wake.

Global dynamics of ionized and neutral gases in the environment of Titan plays an important role in the interaction of Saturn s magnetosphere with Titan. Several hybrid simulations of this problem have already been done (Brecht et al., 2000; Kallio et al., 2004; Modolo et al., 2007a; Simon et al., 2007a, 2007b; Modolo and Chanteur, 2008). Observational data from CAPS for the T9 encounter (Sittler et al., 2009) indicates an absence of O(+) heavy ions in the upstream that change the models of interaction which were discussed in current publications (Kallio et al., 2004; Modolo et al., 2007a; Simon et al., 2007a, 2007b; Ma et al., 2007; Szego et al., 2007). Further analysis of the CAPS data shows very low density or even an absence of H(+) ions in upstream. In this paper we discuss two models of the interaction of Saturn s magnetosphere with Titan: (A) high density of H(+) ions in the upstream flow (0.1/cu cm), and (B) low density of H(+) ions in the upstream flow (0.02/cu cm). The hybrid model employs a fluid description for electrons and neutrals, whereas a particle approach is used for ions. We also take into account charge-exchange and photoionization processes and solve self-consistently for electric and magnetic fields. The model atmosphere includes exospheric H(+), H(2+), N(2+)and CH(4+) pickup ion production as well as an immobile background ionosphere and a shell distribution for active ionospheric ions (M(sub i)=28 amu). The hybrid model allows us to account for the realistic anisotropic ion velocity distribution that cannot be done in fluid simulations with isotropic temperatures. Our simulation shows an asymmetry of the ion density distribution and the magnetic field, including the formation of Alfven wing-like structures. The results of the ion dynamics in Titan s environment are compared with Cassini T9 encounter data (CAPS).

In this report we discuss the ion velocity distribution dynamics from the 3D hybrid simulation. In our model the background, pickup, and ionospheric ions are considered as a particles, whereas the electrons are described as a fluid. Inhomogeneous photoionization, electron-impact ionization and charge exchange are included in our model. We also take into account the collisions between the ions and neutrals. The current simulation shows that mass loading by pickup ions H(+); H2(+), CH4(+) and N2(+) is stronger than in the previous simulations when O+ ions are introduced into the background plasma. In our hybrid simulations we use Chamberlain profiles for the atmospheric components. We also include a simple ionosphere model with average mass M = 28 amu ions that were generated inside the ionosphere. The moon is considered as a weakly conducting body. Special attention will be paid to comparing the simulated pickup ion velocity distribution with CAPS T9 observations. Our simulation shows an asymmetry of the ion density distribution and the magnetic field, including the formation of the Alfve n wing-like structures. The simulation also shows that the ring-like velocity distribution for pickup ions relaxes to a Maxwellian core and a shell-like halo.

First results from NASA's Interplanetary Boundary Explorer mission showed an unexpected "ribbon" of enhanced energetic neutral atom (ENA) flux spanning most of the sky. One explanation put forward suggests that the ribbon may be produced by secondary ENAs originating from pickup ions (PUIs) in the outer heliosheath (OHS). These PUIs are generated when primary ENAs born in the solar wind and inner heliosheath cross the heliopause and charge exchange in the nearby interstellar medium. One of the core assumptions underpinning this theory is that the newly born PUI ring is relatively stable with respect to wave generation, so that it can undergo charge exchange before becoming isotropized. We test this assumption using a linear kinetic theory and hybrid simulations of a low-density PUI ring interacting with instability-generated waves in a warm plasma of the OHS. It is shown that a broadband spectrum of waves is excited as a result of the cyclotron instability that efficiently scatters the ring ions. We also show that the ambient fluctuations in the OHS are unlikely to produce a measurable degree of resonant scattering of PUIs because their intensity is too low compared with the waves excited by the instability.

Energetic particles such as pickup ions, solar energetic particles, or cosmic rays play an important role in determining shock structure. Cosmic-ray modified shocks were discussed by Axford et al. [2]. Jokipii and Williams [8] considered the effect of cosmic ray viscosity on the structure of cold thermal gas shocks mediated by cosmic rays. In the present paper, we consider a background thermal gas of arbitrary temperature to extend their work. The Zank et al. [7] model is used to determine the shock structure when energetic particle collisionless heat flux and viscosity is included.

Interstellar atoms penetrate deep into the heliosphere after passing through the heliospheric interface—the region of the interaction of the solar wind with the interstellar medium. The heliospheric interface serves as a filter for the interstellar atoms of hydrogen and oxygen, and, to a lesser extent, nitrogen, due to their coupling with interstellar and heliospheric plasmas by charge exchange and electron impact ionization. The filtration has great importance for the determination of local interstellar abundances of these elements, which becomes now possible due to measurements of interstellar pickup by Ulysses and ACE, and anomalous cosmic rays by Voyagers, Ulysses, ACE, SAMPEX and Wind. The filtration of the different elements depends on the level of their coupling with the plasma in the interaction region. The recent studies of the filtration of the interstellar atoms in the heliospheric interface region is reviewed in this paper. The dependence of the filtration on the local interstellar proton and H atom number densities is discussed and the roles of the charge exchange and electron impact ionization on the filtration are evaluated. The influence of electron temperature in the inner heliosheath on the filtration process is discussed as well. Using the filtration coefficients obtained from the modeling and SWICS/Ulysses pickup ion measurements, the local interstellar abundances of the considered elements are determined.

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

We present new and definitive results of Cassini plasma spectrometer (CAPS) data acquired during passage through Saturn's inner plasmasphere by the Cassini spacecraft during the approach phase of the Saturn orbit insertion period. This analysis extends the original analysis of Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32, L14S07, doi:10.1029/2005GL022653 ] to L∼10 along with also providing a more comprehensive study of the interrelationship of the various fluid parameters. Coincidence data are sub-divided into protons and water group ions. Our revised analysis uses an improved convergence algorithm which provides a more definitive and independent estimate of the spacecraft potential ΦSC for which we enforce the protons and water group ions to co-move with each other. This has allowed us to include spacecraft charging corrections to our fluid parameter estimations and allow accurate estimations of fluctuations in the fluid parameters for future correlative studies. In the appendix we describe the ion moments algorithm, and minor corrections introduced by not weighting the moments with sinθ term in Sittler et al. [2005] (Correction offset by revisions to instruments geometric factor). Estimates of the spacecraft potential and revised proton densities are presented. Our total ion densities are in close agreement with the electron densities reported by Moncuquet et al. [2005. Quasi-thermal noise spectroscopy in the inner magnetosphere of Saturn with Cassini/RPWS: electron temperatures and density. Geophys. Res. Lett. 32, L20S02, doi:10.1029/2005GL022508 ] who used upper hybrid resonance (UHR) emission lines observed by the radio and plasma wave science (RPWS) instrument. We show a positive correlation between proton temperature and water group ion temperature. The proton and thermal electron temperatures track each with both having a positive radial gradient. These results are consistent with pickup ion energization via Saturn's rotational electric field. We see evidence for an anti-correlation between radial flow velocity VR and azimuthal velocity Vφ, which is consistent with the magnetosphere tending to conserve angular momentum. Evidence for MHD waves is also present. We show clear evidence for outward transport of the plasma via flux tube interchange motions with the radial velocity of the flow showing positive radial gradient with V R ∼ 0.12 ( L / 4 ) 5.5 km / s functional dependence for 4 D LL ∼ D 0 L 11 for fixed stochastic time step δt). Previous models with centrifugal transport have used D LL ∼ D 0 L 3 dependence. The radial transport seems to begin at Enceladus’ L shell, L∼4, where we also see a minimum in the W+ ion temperature T W ∼ 35 eV . For the first time, we are measuring the actual flux tube interchange motions in the magnetosphere and how it varies with radial distance. These observations can be used as a constraint with regard to future transport models for Saturn's magnetosphere. Finally, we evaluate the thermodynamic properties of the plasma, which are all consistent with the pickup process being the dominant energy source for the plasma.

A two-dimensional kinetic model calculation for the water group species (H2O, H2, O2, OH, O, H) in Europa's atmosphere is undertaken to determine its basic compositional structure, gas escape rates, and velocity distribution information to initialize neutral cloud model calculations for the most important gas tori. The dominant atmospheric species is O2 at low altitudes and H2 at higher altitudes with average day–night column densities of 4.5 × 10 14 and 7.7 × 10 13 cm −2 , respectively. H2 forms the most important gas torus with an escape rate of ∼ 2 × 10 27 s −1 followed by O with an escape rate of ∼ 5 × 10 26 s −1 , created primarily as exothermic O products from O2 dissociation by magnetospheric electrons. The circumplanetary distributions of H2 and O are highly peaked about the satellite location and asymmetrically distributed near Europa's orbit about Jupiter, have substantial forward clouds extending radially inward to Io's orbit, and have spatially integrated cloud populations of 4.2 × 10 33 molecules for H2 and 4.0 × 10 32 atoms for O that are larger than their corresponding populations in Europa's local atmosphere by a factor of ∼200 and ∼1000, respectively. The cloud population for H2 is a factor of ∼3 times larger than that for the combined cloud population of Io's O and S neutral clouds and provides the dominant neutral population beyond the so-called ramp region at 7.4–7.8 R J in the plasma torus. The calculated brightness of Europa's O cloud on the sky plane is very dim at the sub-Rayleigh level. The H2 and O tori provide a new source of europagenic molecular and atomic pickup ions for the thermal plasma and introduce a neutral barrier in which new plasma sinks are created for the cooler iogenic plasma as it is transported radially outward and in which new sinks are created to alter the population and pitch angle distribution of the energetic plasma as it is transported radially inward. The europagenic instantaneous pickup ion rates are peaked at Europa's orbit, dominate the iogenic pickup ion rates beyond the ramp region, and introduce new secondary plasma source peaks in the solution of the plasma transport problem. The H2 torus is identified as the unknown Europa gas torus that creates both the observed loss of energetic H+ ions at Europa's orbit and the corresponding measured ENA production rate for H.

The Ultraviolet Coronograph on board SOHO (UVCS) has observed the 58.4 nm glow of the interplanetary He focusing cone at regular intervals since 1996. The intensity decrease with time already observed during the first two years (Michels et al. 2002) has dramatically amplified during the solar activity increase. Intensities seem to reach a plateau in 2001. Using a model of the cone emission which takes into account both photoionization and electron impact ionization of neutral helium we show that the photoionization increase alone cannot explain the observed intensity drop. Data can be fitted if at minimum activity the electron impact ionization rate is the solar cycle average rate predicted by Rucinski & Fahr (1989), and if this rate is increased by a factor of about 3.5 between 1996 and 2001. Assuming the Rucinski and Fahr radial dependence, such high electron impact rates create averaged He + pickup ion (PUI) fluxes which may reach 50% of the fluxes of ions born after photoionization, or 35% of PUI total fluxes, as far as 1 AU from the Sun. In slow and dense solar wind enhancements, in particular in the presence of strong suprathermal tails, PUIs from electron impact could be the dominant species. This could explain a fraction of the observed correlation between He + and H + pickups and anticorrelation of He + fluxes with solar wind velocity.

In this Letter we report on our study of the filtration of interstellar atoms of hydrogen, oxygen and nitrogen in the interaction region between the solar wind and the local interstellar medium. The filtration has great importance for the deter- mination of local interstellar abundances of these elements, which becomes now possible due to measurements of interstellar pickup ions by Ulysses and ACE, and anomalous cosmic rays by Voyagers, Ulysses, ACE, SAMPEX and Wind. The filtra- tion of the different elements depends on the level of their coupling with the plasma in the interaction region. We study the dependence of the filtration on the local interstellar proton and H atom number densities and evaluate the effects of charge exchange and electron impact ionization on the filtration. We explore the influence of electron temperature in the inner he- liosheath on the filtration process. Using our filtration coefficients and SWICS/Ulysses pickup ion measurements we conclude nOI,LIC = (7.8 ± 1.3) × 10 −5 cm −3 and nNI,LIC = (1.1 ± 0.2) × 10 −5 cm −3 .

The regions in space plasmas where mass loading occurs are rich environments for the generation of low frequency waves. We discuss wave generation in the interaction regions of Io and Europa with Jupiter's magnetosphere. We contrast processes in these regions with wave generation near comets. Mass loading occurs when neutrals from Io exosphere are ionized and picked up into the plasma torus. Pickup ions have ring distributions able to generate waves. Galileo data showed the existence of ion cyclotron waves in the Io torus with frequencies near the gyrofrequencies of SO 2 + , SO + , and S + ions. Wave location suggests a fan-shaped region of ion pickup extending downstream from Io. In addition, mirror mode waves have been observed in Io's wake. At Europa the mass loaded into the magnetosphere is less than from Io. Galileo observed transverse waves in the wake with frequencies near the gyrofrequencies of O 2 + , and Cl + ions. Wave characteristics indicate that they are ion cyclotron waves generated by pickup ions. Wave power suggests that the pickup ion rate is larger when Europa is near the center of the jovian current sheet than when the moon is outside of it. The dominant mode near these satellites depends on pickup ion composition. Wave properties are not uniform indicating that the spatial distribution of pickup ions is not homogeneous. Wave generation near comets occurs because cometary material is ionized and picked up into the solar wind. The nature of the interaction region and of pickup ions is determined by solar wind properties and the gas production rate of the comet. In contrast to moon–satellite interactions, in this case pickup ions can have a significant parallel drift velocity and more wave modes are unstable. Waves near comets include, H 2 O + ion cyclotron waves, magnetosonic/whistler waves, and mirror mode.

Three-dimensional calculations are presented for the circumplanetary nature of the iogenic plasma source (pickup ions produced by electron and charge exchange processes in the plasma torus) created by O and S gases located above Io's exobase in its corona and escaping extended neutral clouds (designated as the “Outer Region”). These calculations are undertaken using neutral cloud models for O and S with realistic incomplete collisional cascade source velocity distributions and rates at Io's exobase and realistic spacetime loss processes in the plasma torus. The resulting spatial distributions for O and S about Jupiter are highly peaked at Io but extend at much lower density levels all about the planet, particularly within Io's orbit where they may play a role in the pitch angle scattering and energy loss of radially inward diffusing energetic electrons for the synchrotron radiation belts of Jupiter, in producing bite-outs in the energy distribution of energetic heavy ions near Io's orbit, and in providing a charge exchange source for energetic neutral atoms (ENAs) detected both near and far from Jupiter. For the iogenic plasma source created by these neutrals, two-dimensional distributions produced by integrating the three-dimensional information along the magnetic field lines are presented for the instantaneous values of the pickup ion rates, the total- and net-mass loading rates, the mass-per-unit-magnetic-flux source rate, the pickup conductivity, the pickup radial current, and the pickup ion power (or energy rate). On the circumplanetary spatial scale, the instantaneous iogenic plasma source is highly peaked about Io's position on its orbit around Jupiter. The degree of orbital asymmetry and its physical origin are discussed, and overall spatially integrated rates are presented. The spatially integrated net-mass loading rate is 154 kg s −1 and the total (electron impact and charge exchange) mass loading rate is 275 kg s −1 . Rough minimum estimates are made for the spatially integrated total-mass loading rate created by the “Inner Region” (spatial region below Io's exobase) and are at least ∼1 to 2.5 times larger than that for the Outer Region. Implications of the iogenic plasma source created by the Outer Region and the Inner Region are discussed.

Theoretical analyses of the global heliospheric pickup ion population generally involve the solution of forms of the Boltzmann or transport equation. Like their cosmic-ray counterparts, these approaches often presume a simplified geometry for the interplanetary magnetic field and that diffusive processes dominate the already picked up particles' behavior. At the opposite extreme lies the test particle picture of pickup ions, wherein their motion is tracked from the birth of the ions through their outward travel in the Parker spiral magnetic field. This approach has the advantage that it can include details of the source distribution, the initial pickup by the solar wind convection electric field at the site of ion production, and the combined effects of the radially evolving electric and magnetic fields experienced by the ions. Here test particles representing O+ pickup ions are used to examine the influences of the source characteristics, including the heliocentric radius of their birth and the velocities of their parent neutrals. The consequences for ion populations from interstellar, interplanetary dust, and Jovian sources are considered. The results suggest some potentially observable features of general pickup ion distributions that could aid in the interpretation of their different sources.

Maintaining the stability of pickup ions in the outer heliosheath is a critical element for the secondary energetic neutral atom (ENA) mechanism, a theory put forth to explain the nearly annular band of ENA emission observed by the Interstellar Boundary EXplorer. A recent study showed that a pickup ion ring can remain stable to the Alfven/ion cyclotron (AC) instability at propagation parallel to the background magnetic field when the parallel thermal spread of the ring is comparable to that of a background population. This study investigates the potential role that the mirror or ion Bernstein (IB) instabilities can play in the stability of pickup ions when conditions are such that the AC instability is suppressed. Linear Vlasov theory predicts relatively fast mirror and IB instability growth even though AC instability growth is suppressed. For a few such cases, two-dimensional hybrid and macroscopic quasi-linear simulations are carried out to examine how the unstable mirror and IB modes evolve and affect the pickup ion ring beyond the linear theory picture. For the parameters used, the mirror mode dominates initially and leads to a rapid parallel heating of the pickup ions in excess of the parallel temperature of the background protons. The heated pickup ions subsequently trigger onset of the AC mode, which grows sufficiently large to be the dominant pitch angle scattering agent after the mirror mode has decayed away. The present results indicate that the pickup ion stability needed may not be guaranteed once the mirror and IB instabilities are taken into account.

A large number of numerical experiments has been performed in order to study the interaction of interstellar pickup protons and helium ions with quasiparallel collisionless shocks. The shocks are modeled by a one-dimensional hybrid simulation method which treats the ions as macroparticles and the electrons as a massless fluid. Solar wind alpha particles and pickup protons are included self-consistently. In addition, the particle splitting method is used for the solar wind ions so that the distribution function can be followed over more than 10 orders of magnitude. A large part of the pickup ion distribution is reflected; the reflection efficiency is very high, and can reach in cases where the pickup ion density is low as much as 50%–60%. The reflection efficiency is almost independent of magnetic field–shock normal angle. This indicates that magnetic mirroring is unimportant and does not lead to larger reflection efficiencies. The reflection efficiency of pickup protons rapidly decreases when the pickup i...

The effects of Alfven cyclotron waves generated due to the formation in the outer heliosphere of pickup ions on the transport coefficients of low-energy electrons is investigated here. To this end, parallel mean free path (MFP) expressions are derived from quasilinear theory, employing the damping model of dynamical turbulence. These are then used as inputs for existing expressions for the perpendicular MFP and turbulence-reduced drift coefficient. Using outputs generated by a two-component turbulence transport model, the resulting diffusion coefficients are compared with those derived using a more typically assumed turbulence spectral form, which neglects the effects of pickup ion-generated waves. It is found that the inclusion of pickup ion effects greatly leads to considerable reductions in the parallel and perpendicular MFPs of 1–10 MeV electrons beyond ~10 au, which are argued to have significant consequences for studies of the transport of these particles.

The solar wind in the outer heliosphere is fundamentally different from that in the inner heliosphere, with the influence from the local interstellar medium becoming significant. Observational evidence of pickup ions has accumulated as Voyager 2 continues its journey to the interstellar medium. Specifically, the nonadiabatic solar wind proton temperature profile, the slowdown of the solar wind, and an increased pressure associated with pickup ions are the manifestations of pickup ions in the outer heliosphere. In order to understand the solar wind in the outer heliosphere, we extend the three-fluid model first proposed by Isenberg [1986] to include the energy sharing between the solar wind protons and pickup ions. For simplicity, we introduce a parameter, the energy partition ratio e, to represent the division of the total energy provided by the pickup process between the solar wind protons and pickup ions. We find that only a small percentage of this total thermal energy is needed to heat the solar wind proton to produce the observed temperature profile. As expected, the higher the interstellar neutral hydrogen density, the smaller the percentage. The energy partition ratio has little effect on the slowdown of the solar wind and the pickup ion density distribution in the outer heliosphere, which is primarily controlled by the interstellar neutral hydrogen density. As far as the slowdown is concerned, a neutral hydrogen density of nH∞ = 0.08 cm−3 at the termination shock gives the best match to the observations [Wang et al., 2000]. In this case, the pickup ion density at 40 AU is also of the order of that estimated by Burlaga et al. [1996], and the energy partition ratio e = 0.05 gives the best fit to the observed proton temperature profile.

In this work, we discuss the properties of the ion cyclotron waves observed by the Galileo spacecraft in the Io torus on December 7, 1995. These waves are generated by anisotropies in the pickup ion distributions present in the region. To zeroth order the waves grow at the sulfur dioxide gyrofrequency, are left-hand polarized and propagate along the magnetic field. A more detailed study reveals that in some regions the waves grow over a finite bandwidth, with frequencies near the SO + and S + gyrofrequencies, propagate at an angle to the field, and are elliptically polarized. We perform a kinetic dispersion analysis and show that while the parallel propagating SO 2 + cyclotron mode has the largest growth, oblique cyclotron waves can also grow with a significant rate in the multicomponent plasma near Io. We find that decreasing the sulfur dioxide pickup ion density, and including a SO + pickup component can lead to instabilities generated by the sulfur monoxide and sulfur components in the plasma, and to the observed bandwidth of frequencies. Therefore, it is possible that in addition to the waves generated by SO 2 + pickup ions, in some regions Galileo observed ion cyclotron waves generated by anisotropic distributions of SO + and S + ions. When Voyager 1 flew by the torus in 1979 the spacecraft detected the Alfven wing but no cyclotron waves were observed in the data. We evaluate the group and phase velocities of ion cyclotron waves and find that both are smaller than the Alfven velocity. Then it is possible that the cyclotron waves can get carried away before they reach Voyager.

We explore the possibility of remote, from 1 AU, study of the heliopause by an observer outside the geocorona. We argue that the heliopause, a boundary that separates the solar wind and the galactic plasma of the local interstellar medium (LISM), can be imaged by detecting solar extreme ultraviolet (EUV) radiation reflected by interstellar ions. Such EUV imaging would map the heliopause and provide important insight into its three-dimensional structure and the LISM parameters as well. We consider heliopause mapping in the oxygen O+ ion resonance line (83.4 nm); imaging in the helium He+ ion line (30.4 nm) will be considered in a future article. We show that the expected heliopause brightness map at 83.4 nm is essentially different from that of the foreground glow of the solar wind O+ pickup ions. The interstellar plasma glow is brighter in the upwind (with respect to the interstellar wind) direction, while the pickup ion glow dominates in the downwind direction. The spectral characteristics of the radiation scattered by the LISM plasma and by the pickup ions are significantly different. The all-sky images at 83.4 nm are highly sensitive to the ionization state of the LISM and would allow one to probe the asymmetry of the interstellar magnetic field. We briefly discuss the experimental requirements to heliopause EUV mapping, which would require 3 orders of magnitude improvement in instrumentation sensitivity. This is a challenging but not impossible task.

Ion pickup processes invariably produce anisotropic ion distributions and hence can give rise to wave generation. This paper discusses the waves due to the Io pickup source in Jupiter's magnetosphere. In the near-Io torus, the Galileo spacecraft observed ion cyclotron waves near the sulfur dioxide gyrofrequency, which grow due to the absence of a thermalized background component of these molecular ions in a torus plasma of predominantly dissociated species. These wave observations allow limits on pickup source rates to be estimated. On the edge of the Io wake, mirror mode waves were seen where the pickup ion contribution to the pressure anisotropy is sufficient to overcome the instability threshold. The mirror mode can dominate over the individual ion cyclotron modes in a multi-species plasma. The mass density in the torus cannot increase indefinitely and the plasma must be radially transported through the Jovian magnetosphere. Wave processes may be associated with any ion distribution anisotropies that arise during their radial transport, and with unsteady reconnection processes that may facilitate their eventual loss from the magnetotail.

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}

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 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.

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.

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.

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.

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.

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.

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.

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