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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°.

Despite the long escort by the ESA Rosetta mission, direct observations of a fully developed bow shock around 67P/Churyumov–Gerasimenko have not been reported. Expanding on our previous work on indirect observations of a shock, we model the large-scale features in cometary pickup ions, and compare the results with the ESA Rosetta Plasma Consortium Ion Composition Analyser ion spectrometer measurements over the pre-perihelion portion of the escort phase. Using our hybrid plasma simulation, an empirical, asymmetric outgassing model for 67P, and varied interplanetary magnetic field (IMF) clock angles, we model the evolution of the large-scale plasma environment. We find that the subsolar bow shock standoff distance is enhanced by asymmetric outgassing with a factor of 2 to 3, reaching up to $18\,000\, \rm {km}$ approaching perihelion. We find that distinct spectral features in simulated pickup ion distributions are present for simulations with shock-like structures, with the details of the spectral features depending on shock standoff distance, heliocentric distance, and IMF configuration. Asymmetric outgassing along with IMF clock angle is found to have a strong effect on the location of the spectral features, while the IMF clock angle causes no significant effect on the bow shock standoff distance. These dependences further complicate the interpretation of the ion observations made by Rosetta. Our data-model comparison shows that the large-scale cometary plasma environment can be probed by remote sensing the pickup ions, at least when the comet’s activity is comparable to that of 67P, and the solar wind parameters are known.

Features of the response of pickup ions (PUIs) to the Kelvin--Helmholtz instability (KHI) on the heliopause (HP) are examined by means of two-dimensional hybrid simulations. We assume the supersonic neutral solar wind as the source of PUIs gyrating about the magnetic field in the outer heliosheath. These PUIs become energetic neutral atoms (ENAs) via charge exchange with interstellar hydrogen, and a portion of these ENAs are detected by spacecraft such as the Interstellar Boundary Explorer (IBEX}. To evaluate the possibility of identifying the KHI on HP from ENA observations, we assume that an imprint of the KHI may be displayed in spatial and temporal variations in the observed ENA profile. As an alternative to ENA, the column density of PUIs integrated across the HP is calculated. The KH-inducing vortex forces not only background protons but also PUIs to roll up deep in the inner heliosheath. The KH vortex also results in the emission of magnetosonic pulses that sweep PUIs in the outer heliosheath and lead to their local confinement. These effects elongate the PUIs spatial distribution in the direction normal to the HP. The appearance of the local confined structure in the PUIs column density is consequently confirmed, and this feature can be confirmed as the KHI evolution. Although the simulation cannot be quantitatively compared with the observations currently available because its resolution is too small, we expect that the derived properties will be useful for diagnosing the nature of HP fluctuation in future missions., Comment: 13 pages, 7 figures. Accepted for publication in ApJ

Mercury has a global dayside exosphere, with measured densities of 10−2 cm−3 at ~1500 km. Here we report on the inferred enhancement of neutral densities (, Mercury has a global dayside exosphere that is very tenuous and does not extend far from the planet. Here, the authors show enhancement of neutral densities at high altitudes inferred from pickup ions that is most likely caused by the impact of a meteroid.

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.

International audience; The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft arrived at Mars with the goal of determining the rates and mechanisms of atmospheric escape. Thermal hydrogen and hot oxygen escape are the two most important escape processes currently at work. Direct measurement of the escaping neutral hydrogen and oxygen atoms is impossible with current technology due to the low density and energy of escaping neutrals. However, when ionized and picked up by the solar wind, these escaping atoms can be detected by three particle detectors onboard MAVEN. By back-tracing the trajectories of measured pickup ions, constraints can be placed on the density of neutrals at altitudes not accessible by other measurement methods. In this work, pickup H+ and O+ data from the Solar Energetic Particle (SEP), Solar Wind Ion Analyzer (SWIA), and SupraThermal and Thermal Ion Composition (STATIC) instruments are used to assess the variability of neutral H and O exospheres at Mars. From an analysis of 2.5 Earth years of MAVEN data, we show that a strong H escape seasonal dependence is observed by SWIA and STATIC with inferred H escape rates as low as 3 × 1025 s-1 near aphelion and as high as 4 × 1026 s-1 near perihelion. Hot O escape rates derived from SEP, SWIA, and STATIC data imply a much less variable hot O exosphere with escape rates fluctuating by a factor of 2 around a mean value of 9 × 1025 s-1. Both escape rates are in general agreement with the most recent theoretical, modeled, and observationally inferred rates.

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.

A key to understanding the evolution of the Martian climate over its history is the study of how the Martian atmosphere escapes to space. Studying the near-Mars space environment allows us to better understand atmospheric escape processes. One of these important processes is ion escape, in which atmospheric particles that are primarily ionized by the solar radiation above the exobase region can escape from the planet. Various model results, as well as MAVEN observations, have shown several important channels for ion escape in the Martian plasma environment. One of these channels forms when pickup ions are accelerated away from the planet by a motional electric field, creating a “plume” of escape organized by the upstream solar wind electric field. Although plasma models have predicted the existence of this plume before, only recently have we been able to regularly identify it in observations. Relatively little work has been done on how modeling choices influence the morphology of the plume. Here we presen...

Energetic neutral atoms (ENA) are an important tool for investigating the structure of the heliosphere. Recently, it was observed that fluxes of ENAs (with energy $\le$ 55 keV) coming from the upwind and downwind regions of the heliosphere are similar in strength. This led the authors of these observations to hypothesize that the heliosphere is bubble-like rather than comet-like, meaning that it has no extended tail. We investigate the directional distribution of the ENA flux for a wide energy range (3--88 keV) including the observations from IBEX (Interstellar Boundary Explorer), INCA (Ion and Neutral Camera, on board Cassini), and HSTOF (High energy Suprathermal Time Of Flight sensor, on board SOHO, Solar and Heliospheric Observatory). An essential element is the model of pickup ion acceleration at the termination shock (TS) proposed by Zank. We use state of the art models of the global heliosphere, interstellar neutral gas density, and pickup ion distributions. The results, based on the "comet-like" model of the heliosphere, are close in flux magnitude to ENA observations by IBEX, HSTOF and partly by INCA (except for the 5.2-13.5 keV energy channel). We find that the ENA flux from the tail dominates at high energy (in agreement with HSTOF, but not INCA). At low energy, our comet-like model produces the similar strengths of the ENA fluxes from the upwind and downwind directions, which, therefore, removes this as a compelling argument for a bubble-like heliosphere., Accepted for publication in The Astrophysical Journal

Voyager 2 observations throughout the heliosheath from the termination shock to the heliopause are used to normalize and constrain model pickup ion (PUI) fluxes. Integrating normalized PUI fluxes along the Voyager 2 trajectory through the heliosheath, and combining these integral fluxes with the energy-dependent charge-exchange cross section and the neutral hydrogen density, produces semi-empirical estimates of the energetic neutral atom (ENA) fluxes from the heliosheath. These estimated ENA fluxes are compared with observed ENA fluxes from the Interstellar Boundary Explorer (IBEX) to determine what percentage of the observed fluxes at each IBEX energy are from the heliosheath. These percentages are a maximum of ∼10% for most energies and depend strongly on termination shock properties, plasma density, bulk plasma flow characteristics, the shape of the heliopause, and turbulent energy diffusion in the heliosheath.

In this study, we extend the prior interstellar pickup ion (PUI) observations from the Solar Wind Around Pluto (SWAP) instrument on New Horizons out to nearly 47 au—essentially halfway to the termination shock in the upwind direction. We also provide significantly improved analyses of these and prior observations, including incorporating a cooling index, α, to characterize the nonadiabatic heating of PUI distributions. We find that the vast majority (93.6%) of all distributions show additional heating above adiabatic cooling. Speed jumps indicate compressional waves and shocks with associated enhancements in core solar wind and PUI densities and temperatures. Interestingly, additional heating of the PUIs as indicated by a peak in the cooling index follows the jumps by about a week. We characterize nearly continuous solar wind and H+ PUI data over ∼22–47 au, producing radial gradients, “fiducial” values at 45 au—halfway to the nominal upstream termination shock—for direct comparison to models, and extrapolated values at the shock. These termination shock values are n PUI = (4.1 ± 0.6) × 10−4 cm−3, T PUI = (5.0 ± 0.4) × 106 K, P PUI = 30 ± 4 fPa, α = 2.9 ± 0.2, n PUI/n Total = 0.24 ± 0.02, T PUI/T SW = 716 ± 124, P PUI/P SW = 173 ± 32, P PUI/P SW − Dyn = 0.14 ± 0.01. The PUI thermal pressure exceeds by more than an order of magnitude the thermal solar wind and magnetic pressures in the outer heliosphere. SWAP provides the first and only direct observations of interstellar PUIs in the outer heliosphere, which are critical for both inferring the plasma conditions at the termination shock and understanding PUI-mediated shocks in general. This study examines these observations and serves as the citable reference for these critical data.

Pickup ion detection at Titan is challenging because ion cyclotron waves are rarely detected in the vicinity of the moon. In this work, signatures left by freshly produced pick up heavy ions (m/q ∼ 16 to m/q ∼ 28) as detected in the plasma data by the CAPS/IMS instrument on-board Cassini are analyzed. In order to discern whether these correspond to ions of exospheric origin, one of the flybys during which the reported signatures were observed is investigated in detail. For this purpose, ion composition data from time of flight measurements and test particle simulations to constrain the ions' origin are used. After being validated, the detection method is applied to all the flybys for which the CAPS/IMS instrument gathered valid data, constraining the region around the moon where the signatures are observed. The results reveal an escape region located in the anti-Saturn direction as expected from the nominal corotation electric field direction. These findings provide new constraints for the area of freshly produced pickup ion escape, giving an approximate escape rate of 3.3−2+3×1023ions·s−1.

A Dione O 2 and CO 2 exosphere of similar composition and density to Rhea's is confirmed by Cassini spacecraft Ion Neutral Mass Spectrometer (INMS) flyby data. INMS results from three Dione and two Rhea flybys show exospheric spatial and temporal variability indicative of seasonal exospheres, modulated by winter polar gas adsorption and desorption at the equinoxes. Cassini Plasma Spectrometer (CAPS) pickup ion fluxes also show exospheric structure and evolution at Rhea consistent with INMS, after taking into consideration the anticipated charge exchange, electron impact, and photo-ionization rates. Data-model comparisons show the exospheric evolution to be consistent with polar frost diffusion into the surface regolith, which limits surface exposure and loss of the winter frost cap by sputtering. Implied O 2 source rates of ∼45(7) × 10 21 s −1 at Dione(Rhea) are ∼50(300) times less than expected from known O 2 radiolysis yields from ion-irradiated pure water ice measured in the laboratory, ruling out secondary sputtering as a major exospheric contributor, and implying a nanometer scale surface refractory lag layer consisting of concentrated carbonaceous impurities. We estimate ∼30:1(2:1) relative O 2 :CO 2 source rates at Dione(Rhea), consistent with a stoichiometric bulk composition below the lag layer of 0.01(0.13) C atoms per H 2 O molecule, deriving from endogenic constituents, implanted micrometeoritic organics, and (in particular at Dione) exogenous H 2 O delivery by E-ring grains. Impact deposition, gardening and vaporization may thereby control the global O 2 source rates by fresh H 2 O ice exposure to surface radiolysis and trapped oxidant ejection.

We report fortuitous observations of low-energy lunar pickup ion fluxes near the Moon while in the solar wind by the Lunar Dust Experiment (LDEX) on board the Lunar Atmosphere and Dust Environment Explorer (LADEE). We describe the method of observation and the empirical calibration of the instrument for ion observations. LDEX observes several trends in the exospheric ion production rate, including a scale height of approximately 100 km, a positive, linear correlation with solar wind flux, and evidence of a slight enhancement near 7–8 h local time. We compare the LDEX observations to both LADEE Neutral Mass Spectrometer ion mode observations and theoretical models. The LDEX data are best fit by total exospheric ion production rates of ≈6 × 103 m−3 s−1 with dominant contributions from Al+, CO+, and Ar+, although the LDEX data suggest that the aluminum neutral density and corresponding ion production rate are lower than predicted by recent models.

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.

International audience; Dedicated in‐situ measurements of Mars' exospheric Ar density by NGIMS/MAVEN highlight profiles displaying two slopes altitude variation. Below 230 km, the Ar density is characterized by scale height of 13.5 ± 1.7 km and above 400 km by scale height of 156 ± 25 km. A comparison with the model HELIOSARES suggests that the most probable origin of the Ar non‐thermal component is the collisional interaction, below the exobase, between the atmospheric Ar and the non‐thermal O atoms produced by the dissociative recombination of O2+. These measurements therefore provide a new approach to constrain one of the main source of Mars' atmospheric escape. They also lead to set up an upper limit on the efficiency of pickup ion sputtering at Mars. The comparison with HELIOSARES suggests that NGIMS detection sensitivity might allow a positive detection of this mechanism if more measurements at high altitude deep in the nightside are performed.

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 present a new model of the Interstellar Boundary Explorer (IBEX) ribbon based on the secondary energetic neutral atom (ENA) mechanism, under the assumption that there is negligible pitch angle scattering of pickup ions (PUIs) outside the heliopause. Using the results of an MHD-plasma/kinetic-neutral simulation of the heliosphere, we generate PUIs in the outer heliosheath, solve their transport using guiding center theory, and compute ribbon ENA fluxes at 1 AU. We implement several aspects of the PUI dynamics, including (1) parallel motion along the local interstellar magnetic field (ISMF), (2) advective transport with the interstellar plasma, (3) the mirror force acting on PUIs propagating along the ISMF, and (4) betatron acceleration of PUIs as they are advected within an increasing magnetic field towards the heliopause. We find that ENA fluxes at 1 AU are reduced when PUIs are allowed to move along the ISMF, and ENA fluxes are reduced even more by the inclusion of the mirror force, which pushes particles away from IBEX lines-of-sight. Inclusion of advection and betatron acceleration do not result in any significant change in the ribbon. Interestingly, the mirror force reduces the ENA fluxes from the inner edge of the ribbon more than its outer edge, effectively reducing the ribbon's width by ∼6° and increasing its radius projected on the sky. This is caused by the asymmetric draping of the ISMF around the heliopause, such that ENAs from the ribbon's inner edge originate closer to the heliopause, where the mirror force is strongest.

We study the solar wind interaction with the Martian upper atmosphere during the 8 March 2015 interplanetary coronal mass ejection (ICME) by using a global multifluid MHD model. Comparison of the simulation results with observations from Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft shows good agreement. The total ion escape rate is increased by an order of magnitude, from 2.05 × 1024 s−1 (pre-ICME phase) to 2.25 × 1025 s−1 (ICME sheath phase), during this time period. Two major ion escape channels are illustrated: accelerated pickup ion loss through the dayside plume and ionospheric ion loss through the nightside plasma wake region. Interestingly, the tailward ion loss is significantly increased at the ejecta phase. Both bow shock and magnetic pileup boundary (BS and MPB) locations are decreased from (1.2RM, 1.57RM) at the pre-ICME phase to (1.16RM, 1.47RM), respectively, during the sheath phase along the dayside Mars-Sun line. Furthermore, both simulation and observational results indicate that there is no significant variation in the Martian ionosphere (at altitudes ≲ 200 km, i.e., the photochemical region) during this event.

In the absence of an intrinsic dipole magnetic field, Mars' O+ planetary ions are accelerated by the solar wind. Because of their large gyroradius, a population of these planetary ions can precipitate back into Mars' upper atmosphere with enough energy to eject neutrals into space via collision. This process, referred to as sputtering, may have been a dominant atmospheric loss process during earlier stages of our Sun. Yet until now, a limited number of observations have been possible; Analyzer of Space Plasmas and Energetic Atoms-3/Mars Express observed such a precipitation only during extreme conditions, suggesting that sputtering might be not as intense as theoretically predicted. Here we describe one example of precipitation of heavy ions during quiet solar conditions. Between November 2014 and April 2015, the average precipitating flux is significant and in agreement with predictions. From these measured precipitating fluxes, we estimate that a maximum of 1.0 × 1024 O/s could have been lost due to sputtering.

We present a modeling study of interstellar pickup ion (PUI) distributions in co-rotating interaction regions (CIRs). We consider gradual compressions associated with CIRs formed when fast speed streams overtake slower streams in the inner heliosphere. For the analysis, we adopt a simplified magnetohydrodynamic model of a CIR [Giacalone et al., 2002]. The Energetic Particle Radiation Environment Module (EPREM) [Schwadron et al., 2010], a parallelized particle numerical kinetic code, is used to model PUI distributions using the focused transport equation, including adiabatic cooling/heating, adiabatic focusing, and parallel and perpendicular diffusion. The continuous injection of PUIs is handled as a source term with a ring distribution in velocity space that is produced from the local neutral density obtained from a hot model of the interstellar neutral gas. The simulated distributions exhibit a harder spectrum in the compression region and a softer spectrum in the rarefaction region than that in undisturbed solar wind. As an additional result, a v−5 power-law tail distribution above the PUI cut-off speed (a knee in the distribution) emerges for a particular velocity gradient in the CIR. The tail above the PUI cut-off is sensitive to the CIR velocity gradient, and in one observational case studied, this relationship adequately explains the observed spectrum from 2 to 4 times the solar wind speed. This suggests that the velocity gradient associated with the CIR formation can efficiently create a seed population of PUIs before a shock forms even without stochastic acceleration. Thus, local CIR compressions without shocks may play a significant role in the acceleration process as suggested previously [e.g., Chotoo et al., 2001; Giacalone et al., 2002; Ebert et al., 2012].

Without the shielding of a substantial intrinsic dipole magnetic field, the atmospheres of Mars and Venus are particularly susceptible to similar atmospheric ion energization and scavenging processes. However, each planet has different attributes and external conditions controlling its high altitude planetary ion spatial and energy distributions. This paper describes analogous test particle simulations in background MHD fields that allow us to compare the properties and fates, precipitation or escape, of the mainly O+ atmospheric pick-up ions at Mars and Venus. The goal is to illustrate how atmospheric and planetary scales affect the upper atmospheres and space environments of our terrestrial planet neighbors. The results show the expected convection electric field-related hemispheric asymmetries in both precipitation and escape, where the degree of asymmetry at each planet is determined by the planetary scale and local interplanetary field strength. At Venus, the kinetic treatment of O+ reveals a strong nightside source of precipitation while Mars' crustal fields complicate the simple asymmetry in ion precipitation and drive a dayside source of precipitation. The pickup O+ escape pattern at both Venus and Mars exhibits low energy tailward escape, but Mars exhibits a prominent, high energy ‘polar plume’ feature in the hemisphere of the upward convection electric field while the Venus ion wake shows only a modest poleward concentration. The overall escape is larger at Venus than Mars ( 2.1 × 10 25 and 4.3 × 10 24 at solar maximum, respectively), but the efficiency (likelihood) of O+ escaping is 2–3 times higher at Mars. The consequences of these comparisons for pickup ion related atmospheric energy deposition, loss rates, and detection on spacecraft including PVO, VEX, MEX and MAVEN are considered. In particular, both O+ precipitation and escape show electric field controlled asymmetries that grow with energy, while the O+ fluxes and energy spectra at selected spatial locations show characteristic signatures of the pickup related acceleration and precipitation.

Voyager 2 observations revealed that the hot solar wind ions (the so-called pickup ions) play a dominant role in the thermodynamics of the termination shock and the heliosheath. The number density and temperature of this hot population, however, have remained unknown, since the plasma instrument on board Voyager 2 can only detect the colder thermal ion component. Here we show that due to the multifluid nature of the plasma, the fast magnetosonic mode splits into a low-frequency fast mode and a high-frequency fast mode. The coupling between the two fast modes results in a quasi-stationary nonlinear wave mode, the “oscilliton,” which creates a large-amplitude trailing wave train downstream of the thermal ion shock. By fitting multifluid shock wave solutions to the shock structure observed by Voyager 2, we are able to constrain both the abundance and the temperature of the undetected pickup ions. In our three-fluid model, we take into account the nonnegligible partial pressure of suprathermal energetic electrons (0.022–1.5 MeV) observed by the Low-Energy Charged Particle Experiment instrument on board Voyager 2. The best fitting simulation suggests a pickup ion abundance of 20 ± 3%, an upstream pickup ion temperature of 13.4 ± 2 MK, and a hot electron population with an apparent temperature of ~0.83 MK. We conclude that the actual shock transition is a subcritical dispersive shock wave with low Mach number and high plasma β.

The Rosetta Ion and Electron Sensor (IES) has been measuring solar wind ions intermittently since exiting from hibernation in May 2014. On 19 August, when Rosetta was ~80 km from the comet 67P/Churyumov-Gerasimenko, which was ~3.5 AU from the Sun, IES began to see ions at its lowest energy range, ~4–10 eV. We identify these as ions created from neutral species emitted by the comet nucleus, photoionized by solar UV radiation in the neighborhood of the Rosetta spacecraft (S/C), and attracted by the small negative potential of the S/C resulting from the population of thermal electrons. Later, IES began to see higher-energy ions that we identify as having been picked up and accelerated by the solar wind. IES continues to measure changes in the solar wind and the development of the pickup ion structure.

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.

International audience; The Ion and Electron Sensor (IES) as well as other members of the Rosetta Plasma Consortium (RPC) on board the Rosetta spacecraft (S/C) measured the characteristics of the solar wind almost continuously since its arrival at 67P/Churyumov–Gerasimenko (CG) in 2014 August. An important process at a comet is the so-called pickup process in which a newly ionized atom or molecule begins gyrating about the interplanetary magnetic field, is accelerated in the process and is carried along with the solar wind. Within a month after comet arrival, while Rosetta was 10 keV for a few days and then intermittently through the remainder of the mission, but lower energy (a few keV) pickup ions were also observed. As of 2016 early February, the solar wind reappeared more consistently. We believe that the disappearance of the solar wind in the IES field of view is the result of interaction with the pickup ions and the eventual formation of a cavity that excluded the solar wind.

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.

Comet Siding Spring C/2013 A1 will pass Mars on 19 October 2014, entailing particle and dust precipitation in the Martian upper atmosphere and a potential dust hazard for orbiters. An estimate of the flux of energetic O+ ions picked up by the solar wind from the cometary coma is shown, with an increase of the O+ flux above 50 keV by 2 orders of magnitude. While the ionization of Mars’ upper atmosphere by precipitating O+ ions is expected to be negligible compared to solar EUV-XUV ionization, it is of the same order of magnitude at 110 km altitude during the cometary passage, leading to detectable increases in ionospheric densities. Cometary O+ pickup ion precipitation is expected to be the major nightside ionization source, creating a temporary ionosphere and a global airglow. These effects are dependent on the solar and cometary activities at the time of the encounter.

A key process populating the oxygen exosphere at Mars is the dissociative recombination of ionospheric O2+, which produces fast oxygen atoms, some of which have speeds exceeding the escape speed and thus contribute to atmospheric loss. Theoretical studies of this escape process have been carried out and predictions made of the loss rate; however, directly measuring the escaping neutral oxygen is difficult but essential. This paper describes how energetic pickup ion measurements to be made near Mars by the SEP (Solar Energetic Particle) instrument on board the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft can be used to constrain models of photochemical oxygen escape. In certain solar wind conditions, neutral oxygen atoms in the distant Martian exosphere that are ionized and picked up by the solar wind can reach energies high enough to be detected near Mars by SEP.

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°).

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.

The upper atmosphere of Titan is highly variable as characterized by the variations of the thermospheric and exospheric temperatures from in situ measurements by Cassini at different Titan encounters. A related question has to do with the escape flux of CH4 that might also change with the complex plasma environment and ionospheric conditions. In this study, the atmospheric density profiles obtained by the INMS experiment on Cassini are examined in the context of a bi-Maxwellian approximation proposed by Jiang et al. The results are compared to the escape fluxes generated by magnetospheric and pickup ion sputtering and ionospheric processes. It is found that a CH4 flux at a level of the order of 1023–1025 CH4 s−1 could be maintained during the quiet condition. But episodic events with the corresponding CH4 escape rate reaching as large as 1027 s−1 might be possible. Such a time variability could be indicative of a global change of Titan's atmospheric temperature at relatively short timescales.

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.

[1] We use Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) measurements of lunar exospheric pickup ions in the terrestrial magnetotail lobes combined with a particle-tracing model to constrain the source species and distributions of the lunar neutral exosphere. These pickup ions, generated by photoionization of neutral species while the Moon is in the magnetotail lobes, undergo acceleration from both the magnetotail convection electric field and the lunar surface photoelectric field, giving rise to distinct pickup ion flux, pitch angle, and energy distributions. By simulating the behavior of lunar pickup ions in the magnetotail lobes and the response of the twin ARTEMIS probes under various ambient conditions, we can constrain several physical quantities associated with these observations, including the source ion production rate and the magnetotail convection velocity (and hence, electric field). Using the model-derived source ion production rate and established photoionization rates, we present upper limits on the density of several species potentially in the lunar exosphere. In certain cases, these limits are lower than those previously reported. We also present evidence that the lunar exosphere is displaced toward the lunar dawnside while in the terrestrial magnetotail based on fits to the observed pickup ion distributions.

During the MESSENGER spacecraft's interplanetary trajectory to Mercury, the Fast Imaging Plasma Spectrometer (FIPS) measured the first helium pickup ion distributions at a heliocentric distance (R) ranging between 0.3 and 0.7 AU. From several transits of MESSENGER through the interstellar helium gravitational focusing cone, we map the cone structure in the ecliptic at R=0.3 AU and compare it with observations of He+ at R=1 AU made with the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE). Average downwind enhancements of ≈8 and average cone widths of ≈30° measured by each sensor match known models of neutral helium during this most recent, unusually quiet, solar minimum. The average cone center direction is calculated to be at J2000 ecliptic longitude λ_∞=76.0°(±6.0°) and 77.0°(±1.5°) from FIPS and SWICS observations, respectively. These parameters are also in agreement with previous determinations of the downwind direction of interstellar flow and demonstrate the effectiveness of using pickup ion observations inside 1 AU to map the structure of the neutral helium distribution in the heliosphere, as well as to diagnose the conditions of the interstellar medium.

We apply a hybrid (kinetic ions, fluid electrons) simulation model to study Pluto's plasma environment during the New Horizons encounter on 14 July 2015. We show that Pluto's plasma interaction is dominated by significant north-south asymmetries, driven by large pickup ion gyroradii on the order of 200 Pluto radii. The transition region from the ambient solar wind to the population of plutogenic ions (called the “Plutopause”) also shows considerable asymmetries that cannot be explained by a fluid picture. Since the New Horizons spacecraft does not carry a magnetometer, we use our model to estimate the strength and direction of the interplanetary magnetic field (IMF) at the time of the flyby by comparing output from the hybrid simulation to the plasma signatures observed during the New Horizons encounter. We find that an IMF strength of at least 0.24 nT is required to generate the observed plasma signatures. An IMF orientation either parallel or anti-parallel to Pluto's orbital motion is able to explain the observed plasma densities and velocities along the New Horizons trajectory. Our simulations are able to quantitatively reproduce all key features of the plasma observations, specifically the gradual slowing of the solar wind, as well as the location and thickness of the Plutopause and bow shock.

International audience; Soon after the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft started orbiting Mars, the SEP (Solar Energetic Particle), SWIA (Solar Wind Ion Analyzer), and STATIC (Supra-Thermal and Thermal Ion Composition) instruments on board the spacecraft detected planetary pickup ions. SEP can measure energetic (>60 keV) oxygen pickup ions, the source of which is the extended hot oxygen exosphere of Mars. Model results show that these pickup ions originate from tens of Martian radii upstream of Mars and are energized by the solar wind motional electric field as they gyrate back toward Mars. SWIA and STATIC can detect both pickup oxygen and pickup hydrogen with energies below 30 keV and created closer to Mars. In this study, data from the SEP, SWIA, and STATIC instruments containing pickup ion signatures are provided and model-data comparisons are shown. During the times when MAVEN is outside the Martian bow shock and in the upstream undisturbed solar wind, the solar wind velocity measured by SWIA and the solar wind (or interplanetary) magnetic field measured by the MAG (magnetometer) instrument can be used to model pickup oxygen and hydrogen fluxes. By comparing measured pickup ion fluxes with model results, the Martian thermal hydrogen and hot oxygen neutral densities can be probed outside the bow shock, providing a helpful tool in constraining estimates of neutral oxygen and hydrogen escape rates. Our analysis reveals an order of magnitude density change with Mars season in the hydrogen exosphere, whereas the hot oxygen exosphere was found to remain steadier.

[1] We present a method for deriving constraints on the structure and composition of the lunar atmosphere by using pickup ion measurements from ARTEMIS, mapping observed fluxes from the spacecraft location to derive production rates at the source region, and fitting to a parameterized neutral atmosphere model. We apply this technique to ~12 min of high-resolution burst data collected by ARTEMIS P2 above the sunlit lunar surface, in the dawnside terrestrial magnetosheath. During this time period, ARTEMIS observed multiple velocity components, requiring the presence of multiple species and/or source regions. We use species at or near masses 12, 16, 24, 28, and 40 to derive a best-fit model that proves consistent with most known abundances and limits on neutral densities as well as predictions thereof. However, we find indications of large neutral abundances at mass ~16, exceeding optical limits on oxygen by a factor of ~20, possibly indicating either “seeding” of the Moon by terrestrial oxygen during its magnetotail passage or significant contributions by OH or CH4. We also derive new upper limits on the abundance of OH and Al in the atmosphere.

Interstellar Pickup ions (PUIs) play a significant part in mediating the solar wind (SW) interaction with the interstellar medium. In this paper, we examine the details of spatial variation of the PUI velocity distribution function (VDF) in the SW by solving the PUI transport equation. We assume the PUI distribution is isotropic resulting from strong pitch-angle scattering by wave-particle interaction. A three-dimensional model combining the MHD treatment of the background SW and neutrals with a kinetic treatment of PUIs throughout the heliosphere and the surrounding local interstellar medium (LISM) has been developed. The model generates PUI power law tails via second-order Fermi process. We analyze how PUIs transform across the heliospheric termination shock (TS) and obtain the PUI phase space distribution in the inner heliosheath including continuing velocity diffusion. Our simulated PUI spectra are compared with observations made by New Horizons, Ulysses, Voyager 1, 2 and Cassini, and a satisfactory agreement is demonstrated. Some specific features in the observations, for example, a cutoff of PUI VDF at $v = V_{SW}$ and a $f \propto v^{-5}$ tail in the reference frame of the SW, are well represented by the model., 17 pages, 5 figures, Accepted for publication in the Astrophysical Journal on 09/16/2016

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).