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[1] A particle-in-cell code is used to examine kinetic properties of pickup ions at the heliospheric termination shock. The code treats the pickup ions self-consistently as a third component. The simulations are one-dimensional in spatial variations. We use a relative pickup ion density of 30% and two different values of the magnetic field–shock normal angle: ΘBn = 90° and ΘBn = 87°. The oblique shock is chosen in order to allow for wave vectors parallel to the magnetic field due to instabilities in the foot. In addition, a run is presented with a 60% relative pickup ion density to investigate a pickup ion–dominated shock. Upstream of the ramp is an extended foot due to reflected pickup ions. In this foot the magnetic field continuously increases, and the bulk speed of the pickup ions as well as the bulk speed of the solar wind ions decrease and reach at the magnetic field ramp the downstream value. The positive bulk velocity of the pickup ions in the extended foot perpendicular to the magnetic field and to the shock normal causes an electric field in the shock normal direction. This leads to a large increase of the shock potential well upstream of the magnetic field ramp. The maximum value of the potential is ∼0.35 the shock ram energy and is by a factor 5 larger than expected for a weak shock without pickup ions. Pickup ion reflection at the shock is almost 100%; part of the pickup ions are essentially specularly reflected by the magnetic field force term of the Lorentz force in the overshoot and part of the pickup ions are reflected in the extended foot due to a combination of the magnetic force term and the cross-shock potential. In the 30% pickup ion case, about 90% of the total thermal energy gain in the shock is gained by pickup ions, a little under 10% by the solar wind ions. The thermal energy gain by pickup ions increases as the pickup ion relative density increases. The pickup ion temperature increases continuously from the upstream edge of the extended foot to the shock ramp and then stays constant through the overshoot and downstream.

[1] The Solar Wind Ion Detectors (SWIDs) on the Chang'E-1 spacecraft, while orbiting the Moon, occasionally observed two continuous flux peaks with energies not exceeding 8 and 4 times that of the prevailing solar wind proton energy. These form parallel curves (PCs) with an energy ratio of 2 in the energy-time spectrogram. The fluxes of the two curves are comparable, around 10−5 ∼ 10− 4 of the solar wind flux. The pitch angle distribution of PC particles is concentrated around 90°. The velocity space distribution of PC particles shows distinct double-ring feature, suggesting the existence of a pickup ion species with m/q = 2. Pickup ions from local interstellar medium, the inner sources and the lunar exosphere are investigated. We conclude that this observation may be the first in situ evidence for H2+ ions in the lunar exosphere, thus providing new insights on the evolution and fate of solar wind hydrogen in the solar system.

[1] Long-term observations of proton cyclotron waves in the upstream region of Venus raise the question of under which general solar wind conditions these waves are generated and maintained. The waves are characterized by their occurrence at the local proton cyclotron frequency and left-hand polarization, both in the spacecraft frame. Magnetometer data of the Venus Express spacecraft for two Venus years of observations are analyzed before, during, and after the occurrence of these waves. The configuration of the upstream magnetic field and the solar wind velocity is investigated, to study if the waves are generated from a ring distribution of pickup ions in velocity space or from a parallel pickup ion beam, i.e., for quasi-parallel conditions of solar wind velocity and magnetic field when the solar wind motional electric field is weak. It is found that stable and mainly quasi-parallel magnetic field conditions for up to ∼20 min prior to wave observation are present, enabling sufficient ion pickup and wave growth to obtain observable waves in the magnetometer data. Persistent waves occur mainly under quasi-parallel conditions. This is in agreement with linear theory, which predicts efficient wave growth for instabilities driven by field-aligned planetary ion beams, already for low pickup ion density. The occurrence of highly coherent waves at 4 RV upstream toward the Sun implies that planetary neutral hydrogen is initially picked up at least 5 RV toward the Sun from a sufficiently dense Venus hydrogen exosphere.

The cometary ion pickup process along the sun-comet line at Comet Halley is investigated using a quasi-linear diffusion model including both pitch angle and energy diffusion, adiabatic compression, and convective motion with the solar wind flow. The model results are compared with energetic ion distributions observed by instruments on board the Giotto spacecraft. The observed power spectrum index of magnetic turbulence (gamma) is 2-2.5. The present simulation shows that when gamma was 2, the calculated proton distributions were much more isotropic than the observed ones. The numerical solutions of the quasi-linear diffusion equations show that the isotropization of the pickup ion distribution, particularly at the pickup velocity, is not complete even close to the bow shock. Given the observed turbulence level, quasi-linear theory yields pickup ion energy distributions that agree with the observed ones quite well and easily produces energetic ions with energies up to hundreds of keV.

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

[1] The Los Alamos hybrid simulation code is used to examine kinetic properties of pickup ions at the heliospheric termination shock and in the downstream heliosheath. All simulations are one-dimensional in spatial variations, represent the electrons as a zero-mass fluid, and address only perpendicular shocks. Interpretation of measurements from the IBEX and Voyager spacecraft depend sensitively on the properties of the suprathermal ions downstream of the termination shock, so this research addresses three topics concerning such ions. First, a careful examination of pickup ion trajectories shows that their initial acceleration does not require specular reflection at the shock, as is sometimes assumed, but is the consequence of gyromotion by selected ions at the shock. The primary factor in this energy gain is a gyro-phase-dependent interaction with the motional electric field upstream of, and the magnetic field at, the shock. Second, shock simulations are carried out in which the upstream pickup ions are assumed to have four different types of velocity distributions. The downstream ion perpendicular velocity distributions f(v⊥) are similar in each of the runs and may be approximately characterized as a thermal Maxwellian and a suprathermal distribution. The only significant difference among the four downstream distributions is in the tails of the suprathermal component. Third, simulations are carried out for three different upstream Mach numbers; the results show that faster solar wind flows lead to increased fluxes of ions in the tails of the suprathermal component and are generally consistent with energetic neutral atom observations by the IBEX spacecraft.

[1] Extended pickup ion distributions in the heliosphere have been found to originate either from interstellar neutral gas or from the so-called inner source. Because of the motion of the Sun through the interstellar medium, interstellar neutral gas flows through the inner heliosphere and for some of the species is focused on the downwind side by the gravitation of the Sun. This gravitational focusing cone, which is absent for inner source ions, has been identified for He with UV backscattering and pickup ion observations and is used to diagnose the interstellar parameters. Using the large geometric factor of the Plasma and Suprathermal Ion Composition instrument on the Solar Terrestrial Relations Observatory mission, we have studied heavy pickup ions in the mass-per-charge range 12–20 during 2007 and 2008. We have clearly identified C+, N+, O+, the H2O+ group, and Ne+ among the heavy pickup ions. Out of these, Ne+ shows clear enhancements during two consecutive focusing cone passages, as evidenced by concurrent He+ enhancements during these periods, whereas all the other species are evenly distributed. This is the first observation of the interstellar focusing cone for Ne, whose survival probability as neutrals is only slightly lower than that of He and substantially higher than for other elements.

[1] This study examines the detailed distribution of interstellar pickup hydrogen ions measured by the Surface Wave Process Program instrument on the New Horizons spacecraft en route to Pluto. The observations cover an 8 h interval on 15 October 2008 when the craft was at ∼11.3 AU, constituting the first detailed pickup ion measurements in the outer heliosphere, beyond ∼8 AU. We show the pickup ion cutoff to be roughly twice the solar wind speed, as expected; however, we also measure both a significant suprathermal tail above this “cutoff” and a pickup ion asymmetry between inward and outward ions of ∼4:1, very similar to that observed on Ulysses at

Whether the phenomenology governing MHD turbulence is Kolmogorov or Iroshnikov-Kraichnan (IK) remains an open question, theoretically as well as observationally. The ion heating profile observed in the solar wind provides a quantitative, if indirect, observational constraint on the relevant phenomenology. Recently, a solar wind heating model based on Kolmogorov spectral scaling has produced reasonably good agreement with observations, provided the effect of turbulence generation due to pickup ions is included in the model. Without including the pickup ion contributions, the Kolmogorov scaling predicts a proton temperature profile that decays too rapidly beyond a radial distance of 15 AU. In the present study, we alter the heating model by applying an energy cascade rate based on IK scaling, and show that the model yields higher proton temperatures, within the range of observations, with or without the inclusion of the effect due to pickup ions. Furthermore, the turbulence correlation length based on IK scaling seems to follow the trend of observations better.

In this paper we focus on hybrid simulations, which were performed in order to support data analysis and interpretation of Voyager data and most recent IBEX observations. Two‐dimensional multi‐species hybrid simulations for the termination shock have been performed in which we used almost realistic plasma parameters. In these simulations we included self‐consistently solar wind protons and pickup protons. We investigated the change of the turbulence around and at the shock front as well as spatial and temporal evolution of the solar wind proton distribution under the impact of pickup protons. The results of this study show that pickup ions have a significant impact. Pickup ions mediate the shock turbulence, ion reflection, thermalization, and also slow down incoming solar wind. Furthermore, these simulations also show that gyrating solar wind and pickup ions may be a significant source for Energetic Neutral Atoms (ENAs).

Inner source pickup ions are thought to originate from the interaction of solar wind ions with interplanetary dust grains in the inner heliosphere. Processes which produce inner source pickup ions, and which have been considered so far are implantation of solar wind on grains and subsequent desorption, charge exchange of solar wind ions during transit through submicron dust grains, sputtering and backscattering of ions. A large fraction if not all of the dust crossing the sphere of the Earth’s orbit must end up as pickup ions as is evidenced from the comparable order of magnitude of dust flux inward and pickup ion flux outward at 1 AU. This suggests that the ultimate fate for a large fraction of small interplanetary dust particles after evaporation or sputtering is conversion into pickup ions. Sputtering becomes particularly efficient when dust particles—after fragmentation by collisions with each other—have diminished to sizes comparable to the range of solar wind ions in dust material. The sputter produ...

Maps of energetic neutral atoms (ENA) of the heliosphere from Cassini [1] have been constructed spanning the energy range ∼5≤E≤55 keV, and show a “Belt” in the sky of ∼100° FWHM. Similarly, maps >6 keV have been obtained by the IBEX mission [2] and show a “Ribbon” that is narrower than the Belt and inclined to it in both ecliptic latitude (∼25°) and longitude (∼30°). The overlap in energy between Voyager ions [3] and Cassini ENA intensities (averaged over the ENA line of sight) enables us to deduce ion fluxes in the heliosheath, thus providing a continuous spectrum 5≤E≤4000 keV. These measurements are then used to estimate the local partial pressure over this energy range (∼0.1 pPa), suggesting β>25 [4] and the thickness of the heliosheath (∼50 AU). Using a simulated PUI distribution [5], we estimate the E

Fast supernova remnant shocks in partially neutral gas should produce a population of pickup ions from neutrals that pass through the shock. In favorable cases, these pickup ions may be detectable as a departure from a Gaussian profile in the Hα line. This paper discusses models of the profiles, along with the Hα profile of a shock in Tycho’s Supernova Remnant. The distortion of that profile could be interpreted as an indication of a pickup ion population, but several other interpretations are possible.

During its first year of operations the IBEX satellite discovered a nearly circular ribbon of enhanced fluxes of energetic neutral atoms (ENAs) with energies from 0.2 keV to ∼3 keV. One recent model proposes that the ribbon consists of secondary ENAs produced in the outer heliosheath by charge exchange of PUIs from heliosheath and solar wind ENAs with interstellar neutral hydrogen. This mechanism relies on a long lifespan (>1 year) of the PUI partial shell, something that is difficult to justify in view of the expected instability of such distributions. We analyze stability properties of PUI ring‐beams produced from solar wind ENAs using a simple linear Vlasov theory and more sophisticated hybrid simulations that trace nonlinear interactions of ions with the self‐excited waves. Dispersion analyses produce exponential growth rates of resonant waves of the order of the ion gyrofrequency. We show that many kinds of PUI rings (narrow, broad, and wide) are intrinsically unstable, and that scattering isotropizes them on timescales of days.

International audience; We present initial results from the first community-wide effort to compare global plasma interaction model results for Mars. Seven modeling groups participated in this activity, using MHD, multi-fluid, and hybrid assumptions in their simulations. Moderate solar wind and solar EUV conditions were chosen, and the conditions were implemented in the models and run to steady state. Model output was compared in three ways to determine how pressure was partitioned and conserved in each model, the location and asymmetry of plasma boundaries and pathways for planetary ion escape, and the total escape flux of planetary oxygen ions. The two participating MHD models provided similar results, while the five sets of multi-fluid and hybrid results were different in many ways. All hybrid results, however, showed two main channels for oxygen ion escape (a pickup ion 'plume' in the hemisphere toward which the solar wind convection electric field is directed, and a channel in the opposite hemisphere of the central magnetotail), while the MHD models showed one (a roughly symmetric channel in the central magnetotail). Most models showed a transition from an upstream region dominated by plasma dynamic pressure to a magnetosheath region dominated by thermal pressure to a low altitude region dominated by magnetic pressure. However, calculated escape rates for a single ion species varied by roughly an order of magnitude for similar input conditions, suggesting that the uncertainties in both the current and integrated escape over martian history as determined by models are large. These uncertainties are in addition to those associated with the evolution of the Sun, the martian dynamo, and the early atmosphere, highlighting the challenges we face in constructing Mars' past using models.

[1] The Los Alamos hybrid simulation code is used to examine heating and the partition of dissipation energy at the perpendicular heliospheric termination shock in the presence of pickup ions. The simulations are one-dimensional in space but three-dimensional in field and velocity components, and are carried out for a range of values of pickup ion relative density. Results from the simulations show that because the solar wind ions are relatively cold upstream, the temperature of these ions is raised by a relatively larger factor than the temperature of the pickup ions. An analytic model for energy partition is developed on the basis of the Rankine-Hugoniot relations and a polytropic energy equation. The polytropic index γ used in the Rankine-Hugoniot relations is varied to improve agreement between the model and the simulations concerning the fraction of downstream heating in the pickup ions as well as the compression ratio at the shock. When the pickup ion density is less than 20%, the polytropic index is about 5/3, whereas for pickup ion densities greater than 20%, the polytropic index tends toward 2.2, suggesting a fundamental change in the character of the shock, as seen in the simulations, when the pickup ion density is large. The model and the simulations both indicate for the upstream parameters chosen for Voyager 2 conditions that the pickup ion density is about 25% and the pickup ions gain the larger share (approximately 90%) of the downstream thermal pressure, consistent with Voyager 2 observations near the shock.

Observations by the Voyager 2 spacecraft of the structure of the heliospheric termination shock revealed a quasi‐perpendicular structure that possessed many of the characteristics that are familiar to us from perpendicular shocks observed in the inner heliosphere. However, a surprise was the relative lack of heating experienced by thermal solar wind ions (and electrons), leading to the suggestion that the pickup ions were primarily heated at the heliospheric termination shock. Because the Voyager plasma instrument cannot measure the interstellar pickup ions directly, their behavior at the termination shock could not be gauged directly. Nonetheless, it had already been predicted [16] that the primary dissipation mechanism for the quasi‐perpendicular termination shock would be the reflection of pickup ions rather than solar wind ions; this because of the shell‐like structure of the pickup ion distribution function. We discuss the effect of pickup ions including their trajectories within the structure of the shock observed by Voyager 2. A model for the shock structure is constructed on the basis of preferentially reflected pickup ions, solutions are discussed, and we estimate the degree of heating for the various components.

Most of Titan's atmospheric organic and nitrogen chemistry, aerosol formation, and atmospheric loss are driven from external energy sources such as Solar UV, Saturn's magnetosphere, solar wind and galactic cosmic rays. The Solar UV tends to dominate the energy input at lower altitudes~1,200 km but which can extend down to ~400 km, while the plasma interaction from Saturn's magnetosphere, Saturn's magnetosheath or solar wind are more important at higher altitudes ~1,400 km, but the heavy ion plasma (O+) ~5 keV and energetic ions (H+) ~ 30 keV or higher from Saturn's magnetosphere can penetrate below 950 km. Cosmic rays with energies >1 GeV can penetrate much deeper into Titan's atmosphere with most of its energy deposited ~70 km altitude. Haze layers are observed in scattered solar photons starting at 510 km, but aerosols are broadly distributed and measured in extinction from 1,000 km downward, diffusively separated to 400 km. The induced magnetic field from Titan's interaction with the external plasma can be very complex and will tend to channel the flow of energy into Titan's upper atmosphere. Cassini observations combined with advanced hybrid simulations of the plasma interaction with Titan's upper atmosphere show significant changes in the character of the interaction with Saturn local time at Titan's orbit where the magnetosphere displays large and systematic changes with local time. The external solar wind can also drive sub-storms within the magnetosphere which can then modify the magnetospheric interaction with Titan. Another important parameter is solar zenith angle (SZA) with respect to the co-rotation direction of the magnetospheric flow which is referred to as the solar incidence-ram angle. Titan's interaction can contribute to atmospheric loss via pickup ion loss, scavenging of Titan's ionospheric plasma, loss of ionospheric plasma down its induced magnetotail via an ionospheric wind, and non-thermal loss of the atmosphere via heating and sputtering induced by the bombardment of magnetospheric keV ions and electrons. This energy input evidently drives the large positive and negative ions observed below ~1,100 km altitude with ion masses exceeding 10,000 Da. We refer to these ions as seed particles for the aerosols observed below 1,000 km altitude. These seed particles can be formed, for example, from the polymerization of acetylene (C2H2) and benzene (C6H6) molecules in Titan's upper atmosphere to form polycyclic aromatic hydrocarbons (PAH) and/or fullerenes (C60). In the case of fullerenes, which are hollow spherical carbon shells, magnetospheric keV O+ ions can become trapped inside the fullerenes and eventually find themselves inside the aerosols as free oxygen. The aerosols are then expected to fall to Titan's surface as polymerized hydrocarbons with trapped free oxygen where unknown surface chemistry can take place.

The purpose of this paper is to demonstrate that solar cycle variations of the cross helicity of the Alfvenic turbulence can be the cause of the solar modulation of the galactic cosmic rays and its charge dependence if combined with nonvanishing magnetic helicity values of the turbulence. It is argued that the ratio of forward propagating Alfven waves originating on the Sun to the total intensity is high at solar maxima and low at solar minima, whereas the intensity of backward moving waves probably originating by the pickup ion instability stays constant on time scales long compared to the 11-year periodicity. Our explanation fulfills the observational constraints that the average solar wind speed stays constant and that the total level of magnetic fluctuation power remains constant within 25% during the solar cycle.

[1] The evolution of density fluctuations throughout the solar wind is investigated as the basis of a newly developed theory of nearly incompressible hydrodynamics for an inhomogeneous flow. The model is explored using two-dimensional numerical simulations. The lowest-order density fluctuations (absent in the original homogeneous nearly incompressible theory) obey a passive scalar evolution equation with an additional source term that results from coupling to the large-scale inhomogeneous mean density gradient. The importance of this source term is explored, and we estimate analytically an upper bound for the maximum possible effect of a source term for nearly incompressible flows with an inhomogeneous background (static and spherically symmetric). For typical solar wind parameters, we show that this effect is rather weak beyond 0.1 AU and that the density fluctuations can be described sufficiently accurately as a pure passive scalar. Our simulations identify the sensitive dependence of density fluctuation evolution on typical initial length-scale ratio of scalar (density) and velocity fields, an effect known from the theory of passive scalar decay and experimentally measured in grid-generated turbulence. It has long been thought that the variance in the density fluctuations (δρ)2 should decay in the manner analogous to the mean background density, implying that with heliocentric distance (δρ)2 ∝ R−4 throughout the heliosphere. Analysis of plasma data obtained by the Voyager spacecraft by Bellamy et al. (2005) showed that the density fluctuations decay much more slowly than R−4 and the decay rate exhibits a flattening between 20–30 AU and a possible rise afterward. A possible mechanism to reduce the decay rate within 30 AU was suggested to be turbulence driven by stream-stream interactions followed by more dominant pickup ion interactions beyond 30 AU. Here we show that the variance in the density fluctuations as described by the inhomogeneous nearly incompressible theory evolves essentially independently of the mean density background with a decay rate that reduces and possibly levels off with increasing heliocentric distance.

[1] The one-dimensional hybrid simulation technique using massless fluid electrons and kinetic ions has been shown to successfully reproduce the growth of electromagnetic plasma waves generated by ion populations with a ring velocity distributions (T ⊥ > T ∥ ). Such populations are found at Jupiter and Saturn, where newborn ions are picked up into corotating plasma flows that are nearly perpendicular to the background magnetic field. In previous simulation work, which focused on pickup ion generated waves near lo, we considered all the pickup ions to be present in the simulation at the start of the run (initial value). For a more realistic treatment of ion pickup, we now modify the simulation to include continuous ion injection over time. There are several important differences between the initial value and the injection simulations, namely that when ions are injected, the generated waves eventually reach a quasi-steady level, which is proportional to the injection rate. The results indicate that less than 20% of the pickup ion energy resides in the waves at any given time. Indeed, for certain conditions, the ion cyclotron waves observed at lo may represent only a few percent of the initial pickup ion energy. We carry out simulations varying the plasma and pickup conditions, including the background plasma density and temperature, pickup rate, and multiple pickup ion species to see how the wave amplitudes are affected. We also briefly look at the oblique propagation of the waves to estimate group velocities.

The Balmer line profiles of nonradiative supernova remnant shocks provide the means to measure the post-shock proton velocity distribution. While most analyses assume a Maxwellian velocity distribution, this is unlikely to be correct. In particular, neutral atoms that pass through the shock and become ionized downstream form a nonthermal distribution similar to that of pickup ions in the solar wind. We predict the H alpha line profiles from the combination of pickup protons and the ordinary shocked protons, and we consider the extent to which this distribution could affect the shock parameters derived from H alpha profiles. The Maxwellian assumption could lead to an underestimate of shock speed by up to about 15%. The isotropization of the pickup ion population generates wave energy, and we find that for the most favorable parameters this energy could significantly heat the thermal particles. Sufficiently accurate profiles could constrain the strength and direction of the magnetic field in the shocked plasma, and we discuss the distortions from a Gaussian profile to be expected in Tycho's supernova remnant., 13 pages, 6 figures

[1] We report a newly created highly parallelized global test particle model for resolving the pickup oxygen ion distribution around Mars. The background magnetic and convection electric fields are calculated using a three-dimensional multispecies magnetohydrodynamic model, which includes the effect of the Martian crustal magnetic field. In addition to photo-ionization, charge exchange collisions and solar wind electron impact ionization are included for the pickup ion generation. The most novel feature of our model is that more than one billion test particles are launched in the simulation domain in total. This corresponds to a profound enhancement by at least 3 orders of magnitude in the total number, compared to all existing test particle models. This substantial improvement enables an unprecedented examination of the pickup ion flux distribution in velocity space, which is not achievable in previous simulation studies due to the insufficient statistics arising from the limited number of test particles. Using the velocity space distribution of pickup O + ions as a tool, the Mars-solar wind interaction can be investigated in a unique way. It is shown that the velocity space distribution is highly non-Maxwellian, exhibiting non-gyrotropic and non-symmetric distributions, including many beam-like features. In the tail region, pickup ions have a prominent outflowing component in the whole energy range. The energy examination of particles traveling across the tail region shows that the acceleration highly depends on the source region where the particles originate. The strong convection electric field in the magnetosheath region is favorable to the pickup ion acceleration.

We discuss some theoretical aspects of pickup ion acceleration at the solar wind termination shock and beyond in the heliosheath. Our model of anisotropic transport permits a wide range of acceleration mechanisms, including (a) adiabatic compression by the shock of PUI suprathermal tails, (b) standard diffusive acceleration at the shock, (c) mirroring by the magnetic field kink at a quasi‐perpendicular shock, and (d) momentum diffusion in the solar wind and the heliosheath. It is shown that momentum diffusion in the heliosheath is an efficient particle accelerator at low energies. We also investigate the processing of a pre‐accelerated pickup ion spectra by the termination shock and analyze the long‐term averaged anisotropies of the combined distributions near a shock whose obliquity varies with time.

Context. Interstellar and inner-source pickup ions (PUIs) are produced by the ionization of neutral atoms that originate either outside or inside the heliosphere. Just after ionization, the singly charged ions are picked up by the magnetized solar wind plasma and develop strong anisotropic toroidal features in their velocity distribution functions (VDF). As the plasma parcel moves outwards with the solar wind, the pickup ion VDF gets more and more affected by resonant wave-particle interactions, changing heliospheric conditions, and plasma drifts, which lead to a gradual isotropization of the pickup ion VDF. Past investigations of the pickup ion torus distribution were limited to He+ pickup ions at 1 astronomical unit (AU).Aims. The aim of this study is to quantify the state of anisotropy of the He+ , C+ , N+ , O+ , and Ne+ pickup ion VDF at 1 AU. Changes between the state of anisotropy between PUIs of different mass-per-charges can be used to estimate the significance of resonant wave-particle interactions for the isotropization of their VDF, and to investigate the numerous simplifications that are generally made for the description of the phase-space transport of PUIs.Methods. Pulse height analysis data by the PLAsma and SupraThermal Ion Composition instrument (PLASTIC) on board the Solar Terrestrial RElations Observatory Ahead (STEREO A) is used to obtain velocity-spectra of He+ , C+ , N+ , O+ , and Ne+ relative to the solar wind, f (w sw ). The w sw -spectra are sorted by two different configurations of the local magnetic field – one in which the torus distribution lies within the instrument’s aperture, φ ⊥ , and one in which the torus distribution lies exclusively outside the instrument’s field of view, φ ∥ . The ratio of the PUI spectra between φ ⊥ and φ ∥ is used to determine the degree of anisotropy of the PUI VDF.Results. The data shows that the formation of a torus distribution at 1 AU is significantly more prominent for O+ (and N+ ) than for He+ (and Ne+ ). This cannot be explained by resonant wave-particle interactions as the sole mechanism for the isotropization of the PUI VDF. The anisotropy of the O+ VDF compared to He+ is highly fluctuating but consistently higher over an observation period of six years and therefore unlikely to be related to either specific heliospheric conditions or solar activity variations. To our surprise, we also found a clear signature of a C+ torus distribution at 1 AU very similar to the one of He+ , although as an inner-source PUI, C+ should have a considerably different spectral and spatial injection pattern than interstellar PUIs.

Chemical thrusters are widely used in spacecraft for attitude control and orbital manoeuvres. They create an exhaust plume of neutral gas which produces ions via photoionization and charge exchange. Measurements of local plasma properties will be affected by perturbations caused by the coupling between the newborn ions and the plasma. A model of neutral expansion has been used in conjunction with a fully three-dimensional hybrid code to study the evolution and ionization over time of the neutral cloud produced by the firing of a mono-propellant hydrazine thruster as well as the interactions of the resulting ion cloud with the ambient solar wind. Results are presented which show that the plasma in the region near to the spacecraft will be perturbed for an extended period of time with the formation of an interaction region around the spacecraft, a moderate amplitude density bow wave bounding the interaction region and evidence of an instability at the forefront of the interaction region which causes clumps ...

The Interstellar Boundary EXplorer (IBEX) ribbon is an intense energetic neutral atom (ENA) emission feature encircling the sky, spanning energies ≤0.5–6 keV. The ribbon may be produced by the “secondary ENA” mechanism, where ENAs emitted from a source plasma population inside the heliosphere propagate outside the heliopause, undergo two charge-exchange events, and become secondary ENAs that may be directed back toward Earth and detected by IBEX. In this scenario, the source plasma population is governed by the interaction of the solar wind (SW) with the interstellar medium and is thus sensitive to the global SW properties. Moreover, this scenario predicts that the distance to the source of secondary ENAs depends on the ENA energy and SW speed, which in turn may affect the shape of the ribbon. In this paper, we use a computational model of the heliosphere with simplified SW boundary conditions to analyze the influence of ENA energy and SW speed, independent of time and latitude, on the global spatial and geometric properties of the ribbon. We find a strong dependence of the simulated ribbon energy spectrum and spatial symmetry on SW speed and ENA energy, and only a slight dependence on ribbon geometry. Our results suggest a significant number of primary ENAs from the inner heliosheath may contribute to the pickup ion source population outside the heliopause, depending on the ENA energy and SW speed. The lack of variation in the simulated ribbon center as a function of ENA energy and SW speed, in contrast to the observations, implies that the asymmetry of the SW plays an important role in determining the position of the ribbon. Comparisons to the IBEX data also signify the ribbon’s dependence on the properties of the local interstellar medium, particularly the interstellar magnetic field.

By reevaluating a 13-month stretch of Ulysses SWICS H pickup ion measurements near 5 AU close to the ecliptic right after the previous solar minimum, this paper presents a determination of the neutral interstellar H density at the solar wind termination shock and implications for the density and ionization degree of hydrogen in the LIC. The density of neutral interstellar hydrogen at the termination shock was determined from the local pickup ion production rate as obtained close to the cut-off in the distribution function at aphelion of Ulysses. As shown in an analytical treatment for the upwind axis and through kinetic modeling of the pickup ion production rate at the observer location, with variations in the ionization rate, radiation pressure, and the modeling of the particle behavior, this analysis turns out to be very robust against uncertainties in these parameters and the modeling. Analysis using current heliospheric parameters yields the H density at the termination shock equal to $0.087\pm0.022$ cm$^{-3}$, including observational and modeling uncertainties., Re-edited version, density revised downward due to data re-processing, accepted by A&A

[1] Spatial variations of the neutral source gases of pickup ions are known to affect the velocity distributions of the ions. An expression for the phase space density of pickup ions is derived from the Vlasov equation with a delta function ion source in velocity space that explicitly accounts for the spatial variation of the neutral gas. The background plasma velocity is uniform and perpendicular to a constant ambient magnetic field, which together produce a uniform motional electric field. The neutral source density is one dimensional, varying exponentially along the flow axis with a fixed scale height. The solutions apply to the limiting case of a weak interaction with negligible mass loading. The resulting ring distribution is applied to two examples approximating these criteria; i.e., pickup ions formed in the solar wind interaction with the Earth's Moon and the interaction of Saturn's rotating magnetosphere with Titan. A fundamental parameter appears in the resulting phase space density expression; namely, α = rg/H, the ratio of the gyroradius to the scale height. When α ≪ 1, the interaction is fluid-like with all orbit phases of pickup ion cycloidal motion present at an observation point. If α ≫ 1, the pickup ions appear as ion beams, where the phase space distribution peaks over a small velocity range at an observation site in the source region and the ions will have only executed the beginning phases of their cycloidal motion. The principal contribution to the ions in the velocity peak derives from ions born over a neutral scale height upstream from the observation site. The pickup ion phase space density expression, constrained by spacecraft plasma ion spectrometer and magnetometer measurements, can be used to estimate the neutral source densities of a planetary body's exosphere and its composition. Potential applications are the lunar surface composition, Venus' atmospheric interaction with the solar wind, the interaction of the Galilean moons with Jupiter's magnetosphere, Saturn's icy moons interacting with its magnetosphere, and Titan's interaction with Saturn's magnetosphere.

Two ionospheric modification experiments were carried out as part of the NRL shuttle ionospheric modification with pulsed localized exhaust (SIMPLEX) program. The experiments used 10 second burns of the dual orbital maneuver subsystem (OMS) engines to produce the injection of high speed molecules in the ionosphere near 380 km altitude. Charge exchange between the high speed exhaust molecules and the ambient oxygen ions yield unstable molecular ion beams that disturb the natural state of the ionosphere. Radar scatter from the Millstone incoherent scatter radar in Massachusetts provided measurements of the ion velocity distributions and plasma turbulence that result from the ion beam interactions. Direct simulation Monte Carlo (DSMC) techniques have been used to provide the distribution of exhaust molecules up to a few hundred kilometers from the space shuttle engines. After ion-molecule charge exchange, these velocity distributions trigger ion-ion counter streaming instabilities. Prompt signatures of unstable, non-equilibrium ion distributions in the OMS engine plume are seen in ground radar data taken during the SIMPLEX III and IV experiments for the space shuttle flights STS-108 and STS-110, respectively. Strong backscatter in the UHF radar spectra is attributed to ion-acoustic waves driven by the pickup ion beams.

Context. The Rosetta spacecraft arrived at the comet 67P/Churyumov-Gerasimenko on August 6, 2014, which has made it possible to perform the first study of the solar wind interacting with the coma of a weakly outgassing comet.Aims. It is shown that the solar wind experiences large deflections (>45°) in the weak coma. The average ion velocity slows from the mass loading of newborn cometary ions, which also slows the interplanetary magnetic field (IMF) relative to the solar wind ions and subsequently creates a Lorentz force in the frame of the solar wind. The Lorentz force in the solar wind frame accelerates ions in the opposite direction of cometary pickup ion flow, and is necessary to conserve momentum.Methods. Data from the Ion and Electron Sensor are studied over several intervals of interest when significant solar wind deflection was observed. The deflections for protons and for He++ were compared with the flow of cometary pickup ions using the instrument’s frame of reference. We then fit the data with a three-dimensional Maxwellian, and rotated the flow vectors into the Comet Sun Equatorial coordinate system, and compared the flow to the spacecraft’s position and to the local IMF conditions.Results. Our observations show that the solar wind may be deflected in excess of 45° from the anti-sunward direction. Furthermore, the deflections change direction on a variable timescale. Solar wind protons are consistently more deflected than the He++ . The deflections are not ordered by the spacecraft’s position relative to the comet, but large changes in deflection are related to changes in the orthogonal IMF components.

Rosetta's unique trajectory is allowing exciting measurements of the development of cometary activity between ~3.6 and 1.2 AU for the first time. For a few months following Rosetta's arrival at comet 67P in August 2014, data from the Rosetta Plasma Consortium (RPC) particle instruments (the Ion and Electron Spectrometer (IES) and the Ion Composition Analyser (ICA)), have shown that the low activity cometary environment was initially dominated by the solar wind. This was expected in the early stages of the mission. In addition to the solar wind and related He+ populations, a low energy pickup ion population is seen intermittently in the early phase of the mission near the comet. The population is very time dependent, but at times reaches higher energy approaching the solar wind energy. During these intervals, ICA data indicate that the composition is mainly water group ions. The rising energy signatures of these ions observed at times indicate that they are in the early phases of the pickup process, initially accelerated by the electric field ('early phase pickup'). Here, we compare these exciting pickup ion measurements with Giotto measurements at the relatively weak (compared to Halley) comet Grigg-Skjellerup, where early phase pickup was seen including non-gyrotropic cometary ions, and with the AMPTE lithium and barium releases. Our results reveal some striking similarities with the AMPTE releases, particularly the rising energy signature related to early pickup, and a momentum balance between the pickup ions and the deflected solar wind. There is also evidence for momentum transfer between the pickup ions and the solar wind, with less velocity change seen in the solar wind alpha particles compared to the protons; this was also observed in an AMPTE lithium release. We discuss the effects of increasing activity observed between 3.6 to 1.8 AU, including the increasing dominance and energisation of pickup ions, increasing ionospheric effects and the decreasing effect of the solar wind.

It is generally accepted that the solar wind is significantly heated beyond ∼10 AU by the turbulent decay of pickup ion generated Alfvenic fluctuations. Here, we present a detailed and general calculation of the pickup ion ionization frequencies, and we evaluate these quantities within the solar wind termination shock along the stagnation line. For this supersonic solar wind region, inside of the solar wind termination shock, our results compare well with earlier estimates of these frequencies. When, in the future, turbulence transport models are extended into the heliosheath, the methodology outlined in this paper can be used to calculate ionization frequencies in this hot and dense plasma region where the resulting calculations become more complex.

[1] Detailed observations of the plasma environment of Comet 19P/Borrelly were made by the Plasma Experiment for Planetary Exploration (PEPE) during the Deep Space 1 (DS1) flyby on 22 September 2001. DS1 flew from north to south relative to the plane of the ecliptic on the sunward side of the comet and all plasma boundaries (i.e., first and last pickup ions, bow shock, decelerated solar wind) were observed to be shifted northward. This surprising result was initially attributed to large well-collimated dayside jets directed ∼10° northward from the nucleus as observed by the DS1 MICAS camera. However, ground-based observations of Schleicher et al. (2003) indicate a symmetric gas cloud with respect to the comet-Sun line. In an attempt to resolve this conundrum, we have conducted a series of hybrid plasma simulations to investigate the effects of the large pickup ion gyroradius (i.e., comparable to the length scale of the cometary obstacle) at 1.36 AU. We find that that a maximum asymmetry, consistent with the observations, is generated when the pickup ion gyroradius is comparable to or less than the standoff distance of the bow shock (BIMF ∼ 4 nT). The degree of asymmetry is reduced for larger and smaller magnetic field strengths.

[1] Previously, hybrid simulation techniques using massless fluid electrons and kinetic ions have been successfully applied to study the electromagnetic plasma waves generated by ion pickup in the solar wind, where instability is driven by the large drift velocities of newborn ion populations. For ion pickup at Jupiter and Saturn's magnetospheres where instability is driven by heavy ions with a ring velocity distribution, we show that the one-dimensional hybrid simulation technique can successfully reproduce the behavior of this instability as predicted by linear dispersion theory as well as the important nonlinear wave-particle interactions. The simulated ion cyclotron waves have frequencies near the ion gyrofrequency and are generated as the anisotropic newborn ion ring distribution scatters to a more isotropic configuration. Simulated maximum wave amplitudes and instability growth rates increase with newborn ion density and pickup velocity. For appropriate heavy pickup ion densities and velocities the simulated wave amplitudes are within the range observed by spacecraft.

[1] We investigate the importance of nitrogen ions within Saturn’s magnetosphere and their contribution to the energetic charged particle population within Saturn’s inner magnetosphere. This study is based on the Voyager observations of Saturn’s magnetosphere and Cassini observations. The latter have shown that water group ions dominate both the plasma and energetic particle populations but that nitrogen ions over a broad range of energies were observed at � 5% abundance level. In the outer magnetosphere, methane ions were predicted to be an important pickup ion at Titan and were detected at significant levels in the outer magnetosphere and at Titan. O + ions were found to be the dominant heavy ion in the outer magnetosphere, � 60%, with methane ions being � 30% of the heavy ions and N + being a few percent. The two major sources of nitrogen ions within Saturn’s magnetosphere are Titan’s atmosphere and primordial nitrogen trapped in the icy crust of Saturn’s moons and its ring particles deep within the magnetosphere. It is important to understand the source, transport, and sinks of nitrogen in ordertodeterminewhethertheyhaveaprimordialoriginorarefromTitan’satmosphere.The energetic component is important, since it can come from Titan, be implanted into the surfaces of the icy moons, and reappear at plasma energies via sputtering obfuscating the ultimatesource.Aswewillshow,suchimplantationofnitrogenionscanproduceinteresting chemistry within the ice of Saturn’s moons. The emphasis will be on the nitrogen, but the oxygenandotherwatergroupionsarealsoconsidered.Wearguethatneutralcloudsofheavy atoms and molecules within Saturn’s outer magnetosphere may be the dominant source of energetic heavy ions observed within the inner magnetosphere. Pickup heavy ions in the outer magnetosphere have energies � 1–4 keV when born. If they diffuse radially inward, whileconservingthefirstandsecondadiabaticinvariants,theycanhaveenergiesgreaterthan several hundred keVinside of Dione’s L shell. We will show how observations relate to the various sources and acceleration processes such as ionization, collisions, wave-particle interactions, and radial diffusion.

[1] The composition and structure of neutral exospheres imbedded in the solar wind can be determined by measurements of their pickup ion progeny. Using Earth's Moon as the parent body, we consider transport of pickup ions from their birthplaces to measurement sites on possible spacecraft orbits. An ion observed to have a specific mass and velocity at a spacecraft position can be mapped back to its point of birth on the surface or in the atmosphere. Ion measurements of this type are shown to be a sensitive way to map the composition of surfaces and atmospheres of bodies with surface bound exospheres.

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) and December 13,2004 (flyby TB). Preliminary CAPS ion and electron measurements from these encounters (1,2) are compared with measurements made by the Voyager I Plasma Science Instrument (PLS). The comparisons are used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements (3,4). The plasma wake trajectories of flybys TA, TB 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. The inquiries stimulated by the previous interpretations and predictions made using PLS data have produced the following results from the CAPS ion measurements: A) The major ambient ion components of Saturn s rotating magnetosphere in the vicinity of Titan are H+, H2+, and O+. B) Finite gyroradius effects are apparent in ambient 0 as the result of its interaction with Titan s atmosphere. C) The principal pickup ions are composed of H+, H2+, CH4+ and N2+. D) There is clear evidence of slowing down of the ambient plasma due to pickup ion mass loading; and, as the ionopause~ is approached, heavier pickup ions such as N2+ become dominant. The similarities and differences between the magnitudes and structures of the electron densities and temperatures along the three flyby trajectories are described

[1] We investigate the details of Titan's interaction with Saturn's magnetosphere, which includes formation and location of an ionopause, mass loading via ion pickup, and the effects of finite gyroradii. We present new interpretations of the Voyager 1 plasma instrument measurements not addressed by Hartle et al. (1982). Pickup ions H+ and H2+ dominate in the outermost region with respect to Titan's “ionopause,” followed by CH4+ at intermediate distances and N2+ just outside the “ionopause.” Mass loading and slowing down of the ambient plasma is observed to increase as the pickup ion mass increases with decreasing radial distance from Titan's ionosphere. H2 and CH4 are molecules not originally included in the exosphere of Titan by Hartle and coworkers, and the pickup ions of H2+ and CH4+ are a new feature of our model calculations and should be present in Titan's exospheric region. Therefore Titan could be an important source of carbon to Saturn's magnetosphere. Finite gyroradius effects are identified in the plasma interaction with Titan's atmosphere, which results in an asymmetric removal of ambient plasma from Titan's exosphere region. The finite gyroradius effects also show that the observed hot keV ion component of the ambient plasma is a heavy ion such as N+/O+. A minimum “ionopause” altitude of 4800 km is estimated by a new approach using mass loading.

‘Torus-like’ velocity distribution of the helium pickup ions of interstellar origin has directly been observed by the GEOTAIL spacecraft. These distributions possess a free energy to excite an electromagnetic wave owing to the anisotropic distribution. Although there have been several reports on the quest of waves induced by hydrogen pickup ions based on the observation beyond ~4AU, there has been no report on the waves of helium pickup ions. In this brief report, power spectrum analysis for helium pickup ion events obtained by GEOTAIL is carried out to search for such a wave excitement. We report, however, that the expected wave could not be found owing to the low density of the helium pickup ions at 1AU. The upper limit of the waves is determined to be 24.3% above the background level.

[1] Unusual long wavelength (0.5–1 AU), low-frequency (∼3 × 10−5 s−1) velocity oscillations have been observed by Voyager 2 at 48 AU in the outer heliosphere (Paularena et al., 1996). The velocity oscillations appear to be superimposed on weak linear velocity gradients that are typically increasing. To date, no explanation has been advanced for the origin of either the flow gradients or the oscillations. Here we show that pickup ions can excite low-frequency fluctuations and drive the flow field linearly. The model is compared directly to velocity oscillations observed by the Voyager 2 PLS instrument between 40 and 55 AU over the 6-year period 1993–1998.

[1] Two ionospheric modification experiments were carried out over the incoherent scatter radar (ISR) located at Millstone Hill, Massachusetts. These experiments are part of the Shuttle Ionospheric Modification with Pulsed Localized Exhaust (SIMPLEX) program at the Naval Research Laboratory. The experiments use 10-s burns of the dual orbital maneuver subsystem (OMS) engines to produce the injection of high-speed molecules in the ionosphere near 380 km altitude. Charge exchange between the high-speed exhaust molecules and the ambient oxygen ions yields molecular ion beams that disturb the natural state of the ionosphere. Radar scatter provides measurements of the ion velocity distributions and plasma turbulence that result from the ion beam interactions. Ground-based observations with the University of Massachusetts Digisonde record the ionospheric density depressions resulting from recombination of the molecular ions with electrons. Prompt signatures of nonequilibrium ion distributions in the OMS engine plume are seen in the data taken during the SIMPLEX III and IV experiments for the space shuttle flights STS-108 and STS-110, respectively. The SIMPLEX III observations are much weaker than those during SIMPLEX IV. These differences are primarily attributed to the changes in the viewing directions for the radar beam. During SIMPLEX IV, the radar is looking more downstream from the exhaust injection and the stimulation of plasma turbulence is seen with the ISR for over 30 s at distances up to 200 km from the burn altitude along the radar beam. Strong backscatter in the radar spectra is attributed to ion acoustic waves driven by the pickup ion beams. Both experiments provide large-scale cavities detected by the Digisonde for up to 20 min after the engine burn. These cavities are the result of ion-electron recombination of the pickup ions.

[1] Titan's interaction with Saturn's magnetosphere is studied using a combination of a three-dimensional (3-D) single-fluid magnetohydrodynamic (MHD) simulation and a test particle/Monte Carlo model. The MHD simulation includes an exosphere model based on the one used in the work of Cravens et al. (1998), simple ionospheric processes such as ion production, ion-neutral friction and dissociative recombination and provides a general picture of Titan's plasma environment. The fields from the MHD simulation are then used to calculate the trajectories of 1.4 × 106 ions. We calculate the velocity space distribution and differential energy flux for ambient H+ and N+ ions and for three generic species of pickup ions found upstream of, and within, Titan's plasma wake. The three generic pickup ion species we use are: light (e.g., H+ or H2+), medium (e.g., N+, CH4+, or CH5+), and heavy (e.g., N2+, HCNH+, or some other ionized heavy exospheric species) with representative masses 1, 14, and 28 amu. We also determine the ion flux into Titan's exobase for each species. The ambient ions are assumed to have a drifting Maxwellian distribution consistent with the Voyager observations, while the pickup ions are created with a radial distribution proportional to the neutral density profiles from the neutral exosphere model used in the 2-D MHD model of Cravens et al. (1998). The possibility that the ambient N+ ions may have a shell distribution rather than a Maxwellian is also considered. The modeled ion distributions are compared with data from the Voyager 1 Plasma Science Instrument (PLS).

[1] Voyager images of the icy satellites of Saturn, Dione and Enceladus, suggest that they may have been geologically active and are not only composed of ice. Recent observations by the Hubble Space Telescope have shown the presence of ozone at both Dione and Rhea, which also implies the presence of molecular oxygen at these bodies. Observations of Ariel, Europa, Ganymede, and Callisto indicate the presence of CO2, so its presence on the Saturnian satellites is also expected. The Cassini Plasma Spectrometer (CAPS) will provide the capability to determine the global composition of these bodies by measuring the pickup ions produced by the ionization of their sputter-produced atmospheres. We will present a model of these atmospheres and associated pickup ions and demonstrate CAPS ability to distinguish the freshly produced picked up ions from the ambient plasma. Such ions are expected to form a ring distribution that will have a uniquely different energy-angle dependence than the ambient plasma ions. In the case of Dione we expect the potential for a moderate strength interaction for which both Voyager 1 and Pioneer 11 spacecraft measured ion cyclotron waves centered on the Dione L shell and near the equatorial plane. SKR radio emissions also displayed emissions occurring at the orbital period of Dione which could indicate some intrinsic activity due to Dione. So again, something interesting may be going on at Dione. Since Enceladus, or material in orbit near Enceladus, may be the source of the E-ring, some surprises may be encountered during its close encounter with the Cassini spacecraft. In the case of Dione we will show that a wake pass at 500 km altitude is more than an order of magnitude better than an upstream pass at 500 km altitude. Pickup ion detection for minor ion species such as NH3+ is possible for 500 km altitude wake pass but not for ≈500 km altitude upstream pass at closest approach. For navigation reasons a 100 km pass is not allowed. Therefore it is essential to have a wake pass to maximize the science return for a targeted flyby with Dione. The CAPS observations when combined with magnetometer, plasma wave and energetic particle observations will allow us to estimate the source of ions into Saturn's magnetosphere due to these two bodies and to characterize the nature of the interaction with Saturn's magnetosphere.

The description of the heliosphere as a system has emerged recently due to a deeper understanding of the heliosphere’s numerous interlinked components. The call for a system view is illustrated by several areas of discovery involving the sources and acceleration of energetic particles, the links between particle acceleration and magnetic field structure, and the dynamic solar sources of heliospheric magnetic field meso-scale and micro-scale structure. These research areas have seeded the discovery of the inner and outer pickup ion sources; the discovery of the outer source of anomalous cosmic rays; the discovery that nearly radial magnetic fields in co-rotating rarefaction regions and associated energetic particle dwells are caused by the motions of open magnetic field footpoints across coronal hole boundaries; the relevance of these footpoint motions for ubiquitous statistical acceleration in slow solar wind and for Favored Acceleration Locations at the Termination Shock (FALTS); and the Ulysses discoveries of multiple unplanned crossings of the distant cometary tails of Hyakutake and McNaught-Hartley. Here, we review these emerging areas of heliospheric research and discuss how they are leading to a deeper understanding of our intrinsically dynamic and interlinked heliosphere.

Pickup ion distributions vary substantially on a variety of time scales, although their sources may be relatively steady. This complicates their use as probes of the heliospheric and local interstellar particle populations. Interstellar He+ pickup ion observations from SOHO/CTOF and measurements of interplanetary conditions from SOHO and WIND enable a quantitative statistical analysis of these variations. Pickup ion distributions have been shown to correlate with IMF orientation, solar wind density, and IMF strength. Correlations of the pickup ions with IMF fluctuations are demonstrated, and it is shown that these are consistent with pitch angle scattering by waves. Further questions in pickup ion flux variations are discussed., 6 pages, 5 figures. To appear in the proceedings of the Riverside IGPP conference on the outer heliosphere, March. 2004

Kinetic aspects of a collisionless termination shock are discussed in the context of pickup ion injection into a diffusive acceleration mechanism. For the surfing (or multiply reflected ion) mechanism at an almost perpendicular termination shock to be effective the cross‐shock potential has to be of the order of several electron inertial lengths or smaller. Such small scales are only possible for shocks with an ion beta of about 0.1 or less. An alternative mechanism is discussed which is based on the result that pickup ions are easily reflected at quasi‐parallel shocks. Assuming that the termination shock, due to a turbulent upstream magnetic field, is locally quasi‐parallel, pickup ions are efficiently accelerated to several hundreds of keV within about 10 hours, from where a second diffusive process can accelerate them to ACR energies.

Conventionally, the heliospheric termination shock (HTS) is regarded as a sharp MHD discontinuity. More recent models suggest that the back‐reaction of energetic cosmic rays accelerated at the HTS will create a broad shock with an extended precursor. Here we suggest that a combination of upstream solar wind disturbances interacting with the HTS, the presence of cosmic ray gradient‐driven instabilities and their coupling to the pickup ion population yields a termination shock that is highly dynamical in both structure and location. We also suggest that the diffusive cosmic ray transport equation used to model particle acceleration at a shock may require modification to account for a highly turbulent velocity component.

Using time‐of flight mass spectrometers on Ulysses and ACE we measure the velocity distributions of particles in the poorly explored suprathermal range that extends from about twice to fifty times the solar wind speed. We find that H+, He++ and He+ are always present in this energy (or speed) range at variable intensity levels that depend on solar wind conditions. Contrary to most common expectations, even during most quiet times, in the absence of shocks and other interplanetary disturbances, suprathermal power‐law tails on solar wind distributions are observed that extend to the highest energies measured. Suprathermal tails are observed in distributions of solar wind and interstellar pickup ions, not just for protons but also in distributions of all heavy ions that can be measured. Tails are seen in the quiet and disturbed slow wind, as well as in the super‐quiet fast wind from polar coronal holes. Instead of diminishing rapidly with heliocentric distance, tails are found to persist to at least 5.4 AU and become harder. We suggest that pre‐accelerated particles contained in these tails may well be the population that is injected for further acceleration by shocks. It remains unknown what processes produce these tails and whether these tails persist to very large distances.