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Context. The ESA Rosetta probe has not seen direct evidence of a fully formed bow shock at comet 67P/Churyumov–Gerasimenko (67P). Ion spectrometer measurements of cometary pickup ions measured in the vicinity of the nucleus of 67P are available and may contain signatures of the large-scale plasma environment. Aims. The aim is to investigate the possibility of using pickup ion signatures to infer the existence or nonexistence of a bow shock-like structure and possibly other large-scale plasma environment features. Methods. A numerical plasma model in the hybrid plasma description was used to model the plasma environment of a comet. Simulated pickup ion spectra were generated for different interplanetary magnetic field conditions. The results were interpreted through test particle tracing in the hybrid simulation solutions. Results. Features of the observed pickup ion energy spectrum were reproduced, and the model was used to interpret the observation to be consistent with a shock-like structure. We identify (1) a spectral break related to the bow shock, (2) a mechanism for generating the spectral break, and (3) a dependency of the energy of the spectral break on the interplanetary magnetic field magnitude and bow shock standoff distance.

Context. Anomalous cosmic rays are thought to be generated from pick-up ions that undergo Fermi-1 acceleration in the region close to the solar wind termination shock. This selective acceleration process depends on the efficiency at which upstream pick-up ions are injected into the first reflection mode, from where acceleration into higher energy anomalous cosmic rays starts. Aims. The injection efficiency is highly sensitive to the local inclination of the upstream magnetic field with respect to the shock normal. We determine the probability of pick-up ions being reflected at the shock depending on the magnetic inclination angle, which varies characteristically with latitude and longitude. This reflection is the relevant initial step in the acceleration of pickup ions to anomalous cosmic rays. Methods. We point out that the most dramatic change in the magnetic tilt angle occurs whenever two consecutive solar wind sector structures with different magnetic polarities, pass over the shock. We treat this effect by modeling the 3D- variation of the injection efficiency and start from calculations of the pick up ion reflection probability as function of the magnetic tilt angle. We first derive the magnetic tilt angle as function of latitude and longitude, representing the 3D-shape of the termination shock by a triaxial or prolate ellipsoid adaptable for conditions of different solar cycles, respectively. This allows us to semi-analytically express the 3D-variation of the injection efficiency and rate over a solar cycle. Results. We find a substantial variation in the injection efficiency by 5 orders of magnitude when passing from the upwind parts to the flanks of the termination shock or from low to high latitudes (except at the poles). Folding this with the normalized pickup ion density gives a normalized injection rate which increases from 0% to 30% from upwind to crosswind directions and analogously decreases from crosswind into downwind direction. The maximal value of the injection rate highly depends on the underlying sector structure of the heliospheric magnetic field. During solar minimum there is almost no injection in high latitudes, except at the pole itself, while the injection rate is everywhere fairly good during solar maximum. Conclusions. Our model indicates that the injection into the anomalous cosmic ray acceleration mainly takes place in crosswind directions e.g. in the flanks of the termination shock.

Context. Pickup ions are considered to be the seed population of the anomalous cosmic rays. The acceleration of the pickup ions at the solar wind termination shock depends on the efficiency with which they are injected into the first reflection mode, from where acceleration into high energy anomalous cosmic rays begins. In this letter, we study the longitudinal variation of this efficiency. Aims. We consider the specific MHD conditions at the longitudinal position of the termination shock, which are highly sensitive to the local inclination angle between the magnetic field and the shock normal. We determine the probability for pickup-ions to become reflected which depends on how the inclination angle varies with longitude. This is the first and relevant step in the acceleration of pickup ions to anomalous cosmic rays. Methods. The most dramatic change in the magnetic tilt angle occurs in the current sheet between two consecutive solar wind sector structures of different polarity, when they pass over the shock. We consider this effect by modeling the longitudinal variation in the injection efficiency. Starting from calculations of the pickup-ion reflectance probability as a function of the magnetic tilt angle with respect to the normal, we find an analytic expression for the injection efficiency: to do this we need the magnetic tilt angle as a function of longitude, which we obtain from an analytic representation of the termination shock shape. This enables us to determine the longitudinal dependence of the injection efficiency. Results. We find that a substantial variation in the injection efficiency of a factor of 3 occurs when passing from the upwind part to the flanks of the termination shock. Folding this with the longitudinal dependence of the pickup ion density gives an analytic expression the injection rate which varieis by a factor of 2 between up- and ecliptical crosswind direction and a factor of 5 between cross- and downwind direction. Conclusions. Our model indicates that injection into the anomalous cosmic ray acceleration predominantly occurs in the ecliptical crosswind direction e.g. in the flanks of the termination shock

The GAS-instrument onboard the space probe Ulysses is designed to measure the local angular distribution of the flow of interstellar neutral He-atoms within 3 AU distance from the sun. It allows to infer the kinetic parameters (velocity vector, temperature and density) of these particles outside the heliosphere (at infinity). During three observational periods, 1990/1991, shortly after launch and during the two fast latitude scans of Ulysses, from 9/1994 to 8/1996 and from 9/2000 to 8/2002, more than 300 measurements of the distributions were obtained and analyzed in detail. Known issues that relate to the pointing accuracy of the detector and to the efficiency calibration along with their impact on the results are addressed in this paper. The average values, derived from these observations and their refined analysis, are the bulk speed (v He ∞ = 26.3 ± 0.4 km s -1 ), the flow direction (ecliptic longitude λ∞ = 74.7° ± 0.5°, ecliptic latitude β∞ = -5.2° ± 0.2°) and temperature (T He ∞ = 6300 K ± 340 K). From 1990 to 2002, covering a complete solar cycle, no significant temporal variations of these parameters were observed, nor variations with solar latitude. In contrast, in the density n He ∞, derived from the series of local observations along the Ulysses-orbit, substantial apparent variations were seen. After a first attempt to explain these by an inadequate energy calibration of the efficiency they are now interpreted as variations in the loss processes (predominantly photo-ionization), which the neutral gas experiences along its trajectory to the observer. While the temporal variations of the ionization rate were taken into account using the measured solar EUV irradiance (from CELIAS/SEM on SOHO), the residual latitudinal variation has been attributed to a dependence of the solar irradiance on latitude, which thus far has not been accessible to direct observations. As a result of a simple model that includes these effects a density n He ∞ = 0.015 t 0.003 cm -3 has been deduced, which is in good agreement with an independent result from pickup ion observations.

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 .

Context. The interstellar flow longitude corresponds to the Sun’s direction of movement relative to the local interstellar medium. Thus, it constitutes a fundamental parameter for our understanding of the heliosphere and, in particular, its interaction with its surroundings, which is currently investigated by the Interstellar Boundary EXplorer (IBEX). One possibility to derive this parameter is based on pickup ions (PUIs) that are former neutral ions that have been ionized in the inner heliosphere. The neutrals enter the heliosphere as an interstellar wind from the direction of the Sun’s movement against the partially ionized interstellar medium. PUIs carry information about the spatial variation of their neutral parent population (density and flow vector field) in their velocity distribution function. From the symmetry of the longitudinal flow velocity distribution, the interstellar flow longitude can be derived.Aim. The aim of this paper is to identify and eliminate systematic errors that are connected to this approach of measuring the interstellar flow longitude; we want to minimize any systematic influences on the result of this analysis and give a reasonable estimate for the uncertainty.Methods. We use He+ data measured by the PLAsma and SupraThermal Ion Composition (PLASTIC) sensor on the Solar TErrestrial RElations Observatory Ahead (STEREO A) spacecraft. We analyze a recent approach, identify sources of systematic errors, and propose solutions to eliminate them. Furthermore, a method is introduced to estimate the error associated with this approach. Additionally, we investigate how the selection of interplanetary magnetic field angles, which is closely connected to the pickup ion velocity distribution function, affects the result for the interstellar flow longitude.Results. We find that the revised analysis used to address part of the expected systematic effects obtains significantly different results than presented in the previous study. In particular, the derived uncertainties are considerably larger. Furthermore, an unexpected systematic trend of the resulting interstellar flow longitude with the selection of interplanetary magnetic field orientation is uncovered.

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.

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.

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.

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.

Context. Pickup ions in the inner heliosphere mainly originate in two sources, one interstellar and one in the inner solar system. In contrast to the interstellar source that is comparatively well understood, the nature of the inner source has not been clearly identified. Former results obtained with the Solar Wind Ion Composition Spectrometer on-board the Ulysses spacecraft revealed that the composition of inner-source pickup ions is similar, but not equal, to the elemental solar-wind composition. These observations su ered from very low counting statistics of roughly one C + count per day. Aims. Because the composition of inner-source pickup ions could lead to identifying their origin, we used data from the Charge-TimeOf-Flight sensor on-board the Solar and Heliospheric Observatory. It o ers a large geometry factor that results in about 100 C + counts per day combined with an excellent mass-per-charge resolution. These features enable a precise determination of the inner-source heavy pickup ion composition at 1 AU. To address the production mechanisms of inner-source pickup ions, we set up a toy model based on the production scenario involving the passage of solar-wind ions through thin dust grains to explain the observed deviations of the inner-source PUI and the elemental solar-wind composition. Methods. An in-flight calibration of the sensor allows identification of heavy pickup ions from pulse height analysis data by their mass-per-charge. A statistical analysis was performed to derive the inner-source heavy pickup ion relative abundances of N + , O + , Ne + , Mg + , Mg 2+ , and Si + compared to C + . Results. Our results for the inner-source pickup ion composition are in good agreement with previous studies and confirm the deviations from the solar-wind composition. The large geometry factor of the Charge-Time-of-Flight sensor even allowed the abundance ratios of the two most prominent pickup ions, C + and O + , to be investigated at varying solar-wind speeds. We found that the O + /C + ratio increases systematically with higher solar-wind speeds. This observation is an unprecedented feature characterising the production of inner-source pickup ions. Comparing our observations to the toy model results, we find that both the deviation from the solar-wind composition and the solar-wind-speed dependent O + /C + ratio can be explained.

Context. He+ pickup ions are either born from the ionization of interstellar neutral helium inside our heliosphere, the so-called interstellar pickup ions, or through the interaction of solar wind ions with small dust particles, the so-called inner source of pickup ions. Until now, most observations of pickup ions were limited to reduced 1D velocity spectra, which are insufficient to study certain characteristics of the He+ velocity distribution function (VDF).Aims. It is generally assumed that rapid pitch-angle scattering of freshly created pickup ions quickly leads to a fully isotropic He+ VDF. In light of recent observations, this assumption has found to be oversimplified and needs to be reinvestigated.Methods. Using He+ pickup ion data from the PLASTIC instrument on board the STEREO A spacecraft, we reconstruct a reduced form of the He+ VDF in two dimensions. This allows us to study relative changes of the 2D He+ VDF as a function of the configuration of the heliospheric magnetic field.Results. Our observations show that the He+ VDF is highly anisotropic and even indicates that, at least for certain configurations of B , it is not fully gyrotropic. Our results further suggest, that the observed velocity and pitch angle of He+ depends strongly on the local solar magnetic field vector, B , the ecliptic longitude, λ , the solar wind speed, v sw , and the global distribution of B . Conclusions. We found two distinct signatures that systematically change as a function of the alignment of B : (1) a ring beam distribution that is most pronounced at w sw > 0.5 and likely attributed to interstellar He+ ; (2) a beam signature aligned parallel to B that is most pronounced at w sw . The strong anisotropy and the aforementioned dependencies of the He+ VDF also imply that observations of 1D velocity spectra of He+ pickup ions are potentially deceiving.