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In the coming decades, exploration of the lunar surface is likely to increase as multiple nations execute ambitious lunar exploration programs. Among several environmental effects of such activities, increasing traffic near and on the lunar surface will result in the injection of anthropogenic neutral gases into the lunar exosphere. The subsequent ionization of such anthropogenic neutrals in the lunar environment may contribute to and ultimately exceed the generation of 'native' lunar pickup ions, thereby altering the fundamental space plasma interaction with the Moon. To better understand these possible effects, we conducted plasma simulations of the solar wind interaction with the Moon in the presence of increasing ion production rates from an anthropogenic lunar exosphere. At ionization levels between 0.1 and 10 times the native lunar exospheric ion production rate, little to no changes to the solar wind interaction to the Moon are present; however, ionization levels of 100 and 1000 times the native rate result in significant mass loading of the solar wind and disruption of the present-day structure of the Moon's plasma environment. Comparing to the planned Artemis landings, which are likely to contribute only an additional ∼ 10% of the native lunar exospheric ion production rate, we conclude that the Artemis program will have little effect on the Moon's plasma environment. However, more frequent landings and/or continual outgassing from human settlements on the Moon in the more distant future are likely to fundamentally alter the lunar plasma environment. [ABSTRACT FROM AUTHOR]

Three-dimensional global hybrid simulations are performed to explore how the interplanetary magnetic field (IMF), the Martian crustal fields, and planetary pickup ions affect the twisting of the Martian magnetotail. The results agree with previous studies that the crustal magnetic fields cause the Martian magnetotail to twist counterclockwise or clockwise depending on the sign of the IMF Y -component in the Mars solar orbital coordinates. However, the twist is more pronounced when the crustal fields are on the nightside, contradicting the early explanation that the crustal fields affect the twist through dayside magnetic reconnection between the crustal fields and the draped IMF. Additionally, planetary pickup ions also contribute to the twist because their mass loading slows down the plasma flow and leads to the bending of the magnetic field lines in the magnetotail. It is demonstrated that the twist inside Mars' shadow in the near magnetotail region (at X = −1.5 R M, where R M is Mars' radius) is mainly attributable to the crustal fields, while the influence of planetary pickup ions starts to dominate outside Mars' shadow and in regions further away from Mars. [ABSTRACT FROM AUTHOR]

We investigate the long-term persistence of the time series of energetic neutral atom (ENA) fluxes recorded by the Interstellar Boundary Explorer (IBEX). ENAs provide global information about the outer heliosphere and its interactions with the very local interstellar medium. To avoid the IBEX Ribbon, here, we focus our analysis solely on the polar regions N60°–N90° and S60°–S90°. Each year, IBEX takes two measurements of every pixel in the sky. We make use of the whole set of 14 yr of IBEX data and adhere to the correct time order for the construction of the flux time series. We examine in detail both the trend and the fluctuations of these time series. Using modern methods of time series analysis and persistence characterization, we show that the time series (i) trend is influenced by the solar cycle, (ii) persistence can be established independently of the presence of this trend, and (iii) statistical properties of the fluctuations differs between north and south, pointing to the existence of anisotropy and thus a north–south asymmetry. [ABSTRACT FROM AUTHOR]

The pickup process of newly ionized hydrogen (H) from the Martian extended exosphere represents an important H+ escape channel on Mars. It is generally believed that interplanetary (IP) shocks can accelerate the pickup ions and, in turn, affect the pickup process. However, the underlying processes inherent to the acceleration are not yet fully understood. Here, we concentrate on the dynamic processes involved in acceleration arising from IP shock compression. We examine two typical IP shock events characterized by a sudden increase in the average energy of upstream H+ pickup ions across the shock. The H+ pickup ions continuously enter the field of view of Supra-Thermal And Thermal Ion Composition or Solar Wind Ion Analyzer on board the Mars Atmosphere and Volatile EvolutioN spacecraft in a narrow angular range. Moreover, they correspond to a similar part of their respective ring distributions in velocity space. By comparing measured and theoretical H+ pickup ion energies, we attribute the more energetic H+ pickup ions to the enhanced convection electric field acceleration caused by the shock compression. An increase in the guiding center drift speed across the shock implies a higher escape rate of the H+ pickup ions. Furthermore, the pitch-angle scattering could facilitate the escape of the higher energy H+ pickup ions from Mars. The results may shed light on the understanding of the energization and escape of planetary pickup ions in the solar system. [ABSTRACT FROM AUTHOR]

The stability of a realistic multicomponent pickup ion (PUI) velocity distribution derived from a global model of neutral atoms in the heliosphere, which treats hydrogen and helium atoms self-consistently and includes equations for electrons and helium ions, is investigated using linear instability analysis and hybrid simulations. Linear instability analysis shows that the excited oblique mirror waves and the parallel/quasi-parallel Alfvén-cyclotron (AC) waves have lower growth rates than those obtained previously by A. Mousavi et al. for the PUI velocity distributions given by J. Heerikhuisen et al. The PUI scattering by each of the two modes alone is studied. In contrast to the previous investigations, our current simulations using the updated realistic distributions indicate that mirror waves alone do not effectively scatter PUIs in pitch angle. Instead, they primarily contribute to reducing the thermal spread anisotropy of the PUIs originating from the neutral solar wind. The unstable AC waves exhibit lower growth rates but higher saturation levels than the mirror waves. Two-dimensional (2D) simulation results show that when all unstable waves are present, the predominant contributor to the fluctuating magnetic field energy is the AC mode. The AC waves quickly scatter the PUIs with pitch angles away from 90 ^∘ toward isotropy, while the PUIs near 90 ^∘ pitch angle maintain a degree of anisotropy within our simulation timeframe. Moreover, several 1D and 2D hybrid simulations with different numbers of particles per cell are performed to examine the impact of numerical noise on PUI scattering. Finally, the implications of these results for the Interstellar Boundary Explorer energetic neutral atom ribbon are discussed.

We revisit previous hybrid simulations of the heating and acceleration of interstellar pickup ions (PUIs) at the solar wind termination shock. In previous simulations, a relatively cold initial distribution of PUIs was assumed; and while the resulting shock-heated distribution was consistent with Voyager 2 LECP measurements at about 30 keV, the intensity of the distribution downstream of the shock in the ~1–10 keV energy range was lower than predictions based on analysis of energetic neutral atoms (ENAs) from the Interstellar Boundary Explorer-Hi and Cassini's Ion and Neutral Camera. Here we perform new simulations with more realistic initial PUI distributions. We assume the distribution is a partially filled spherical shell in velocity space with a radius that varies from 320 to 640 km s ^−1 . We then use the distributions downstream of the shock from these new simulations to estimate the ENA flux spectrum and compare with observations. We find that the predicted ENA spectrum from the new simulations much better matches the observations over a broad range of energies. We conclude that the hybrid simulations provide reasonable predictions for the distribution of charged particles in the energy range from ~0.5 to 50 keV.

The outer heliosphere is profoundly influenced by nonthermal energetic pickup ions (PUIs), which dominate the internal pressure of the solar wind beyond ~10 au, surpassing both solar wind and magnetic pressures. PUIs are formed mostly through charge exchange between interstellar neutral atoms and solar wind ions. This study examines the apparent heating of PUIs in the distant supersonic solar wind before reaching the heliospheric termination shock. New Horizons’ SWAP observations reveal an unexpected PUI temperature change between 2015 and 2020, with a notable bump in PUI temperature. Concurrent observations from the ACE and Wind spacecraft at 1 au indicate a ~50% increase in solar wind dynamic pressure at the end of 2014. Our simulation suggests that the bump observed in the PUI temperature by New Horizons is largely associated with the enhanced solar wind dynamic pressure observed at 1 au. Additional PUI temperature enhancements imply the involvement of other heating mechanisms. Analysis of New Horizons data reveals a correlation between shocks and PUI heating during the declining phase of the solar cycle. Using a PUI-mediated plasma model, we explore shock structures and PUI heating, finding that shocks preferentially heat PUIs over the thermal solar wind in the outer heliosphere. We also show that the broad shock thickness observed by New Horizons is due to the large diffusion coefficient associated with PUIs. Shocks and compression regions in the distant supersonic solar wind lead to elevated PUI temperatures and thus they can increase the production of energetic neutral atoms with large energy.

Anomalous cosmic-rays (ACRs) are thought to be originated from the acceleration of pickup ions (PUIs) at the termination shock or interplanetary shocks, and play important role for the plasma dynamics in the outer heliosphere. Due to limited observation, the effects of ACRs on the solar wind events is not well known. Under the approximation of spherical symmetry, we have developed a three-component magneto-hydrodynamic (MHD) numerical model that contains solar wind plasma, interstellar neutral atoms and ACRs, to investigate the evolution of the solar wind within a heliocentric distance from 1 to 150 astronomical units (AU). We use the solar wind observations from the OMNI database with the time from 2010.5 to 2016.0 (decimal years) at the inner boundary, and the effect of ACRs on the propagation of the solar wind events are compared with the observations from the spacecrafts of New Horizons, Voyager 1 and 2. The results show that ACRs may decrease the speed of the solar wind shocks to some extent, and the effect is positively correlated with the diffusion coefficient; a larger diffusion coefficient leads to a more pronounced effect. Moreover, the ACRs has a dissipation effect on the shock-like solar wind structures, and may play important roles on the dynamics of solar wind in the outer heliosphere. [ABSTRACT FROM AUTHOR]

The distribution of turbulence in the heliosphere remains a mystery, due to the complexity in not only modeling the turbulence transport equations but also identifying the drivers of turbulence that vary with time and spatial location. Beyond the ionization cavity (a few astronomical units (AU) from the Sun), the turbulence is driven predominantly by freshly created pickup ions (PUIs), in contrast to the driving by stream shear and compression. Understanding the source characteristics is necessary to refine turbulence transport models and interpret measurements of turbulence and solar wind temperature in the outer heliosphere. Using a recent latitude-dependent solar wind speed model and the ionization rate of neutral interstellar hydrogen (H), we investigate the temporal and spatial variation in the strength of low-frequency turbulence driven by PUIs from 1998 to 2020. We find that the driving rate is stronger during periods of high solar activity and at lower latitudes in the outer heliosphere. The driving rates for parallel and anti-parallel propagating (relative to the background magnetic field) slab turbulence have different spatial and latitude dependences. The calculated generation rate of turbulence by PUIs is an essential ingredient to investigate the latitude dependence of turbulence in the outer heliosphere, which is important to understand the heating of the distant solar wind and the modulation of cosmic rays. [ABSTRACT FROM AUTHOR]

Direct numerical simulations of three-dimensional compressible magnetohydrodynamic (MHD) turbulence have been performed in order to study the relation between wave modes and coherent structures and the consequent energization of test particles. Moreover, the question of which is the main mechanism of this particle energization is rigorously discussed. In particular, using the same initial conditions, we analyzed the nonlinear and linear evolution of a turbulent state along with the case of randomized phases. Then, the behaviors of the linear and nonlinear simulations were compared through the study of the time evolution of particle kinetic energy and preferential concentration. Also, spatiotemporal spectra were used to identify the presence of wave modes and quantify the fraction of energy around the MHD modes in linear and nonlinear simulations. Finally, the variation of the correlation time of the external forcing is studied in detail along with the effect on the particle energization (and clustering) and the presence of wave modes. More specifically, particle energization tends to decrease when the fraction of linear energy increases, supporting the idea that energization by structures is the dominant mechanism for particle energization instead of resonance with wave modes as suggested by Fermi energization theory. [ABSTRACT FROM AUTHOR]

Interstellar neutral (ISN) hydrogen is the most abundant species in the outer heliosheath and the very local interstellar medium (VLISM). Charge-exchange collisions in the outer heliosheath result in filtration, reducing the ISN hydrogen density inside the heliosphere. Additionally, these atoms are intensively ionized close to the Sun, resulting in a substantial reduction of their density within a few astronomical units from the Sun. The products of this ionization—pickup ions (PUIs)—are detected by charged particle detectors. The Solar Wind Around Pluto instrument on New Horizons provides, for the first time, PUI observations from the distant heliosphere. We analyze the observations collected between 22 and 52 au from the Sun to find the ISN hydrogen density profile and compare the results with predictions from global heliosphere models. We conclude that the density profile derived from the observations is inconsistent with steady-state model predictions. This discrepancy is not explained by time variations close to the Sun and thus may be related to the temporal evolution of the outer boundaries or VLISM conditions. Furthermore, we show that the cold and hot models of ISN hydrogen distribution are not a good approximation closer to the termination shock. Therefore, we recommend a new fiduciary point based on the available New Horizons observations at 40 au from the Sun, at ecliptic direction (285.°62, 1.°94), where the ISN hydrogen density is 0.11 cm ^−3 . The continued operation of New Horizons should give better insight into the source of the discussed discrepancy.

We investigate the role of the magnetohydrodynamic (MHD) turbulence measured by Voyager in the very local interstellar medium (VLISM) in modeling the Interstellar Boundary Explorer ribbon. We demonstrate that the mirroring by compressible modes of MHD turbulence dominates over that by the mean magnetic field. Based on the new mirror diffusion mechanism identified by Lazarian & Xu for particles with large pitch angles in MHD turbulence, we find that the mirror diffusion can both confine pickup ions and preserve their initial pitch angles, and thus it accounts for the enhanced intensity of energetic neutral atoms that return to the heliosphere. The ribbon width is determined by both the range of pitch angles for effective turbulent mirroring and the field line wandering induced by Alfvénic modes. It in turn provides a constraint on the amplitude of magnetic fluctuations of fast modes. The field line wandering also affects the coherence of the ribbon structure across the sky. By extrapolating the magnetic energy spectrum measured by Voyager, we find that the injection scale of the turbulence in the VLISM must be less than ∼500 au for the ribbon structure to be coherent. [ABSTRACT FROM AUTHOR]

A "Ribbon" of enhanced energetic neutral atom (ENA) emissions was discovered by the Interstellar Boundary Explorer in 2009, redefining our understanding of the heliosphere boundaries and the physical processes occurring at the interstellar interface. The Ribbon signal is intertwined with that of a globally distributed flux (GDF) that spans the entire sky. To a certain extent, Ribbon separation methods enabled examining its evolution independent of the underlying GDF. Observations over a full solar cycle revealed the Ribbon's evolving nature, with intensity variations closely tracking those of the solar wind (SW) structure after a few years delay, accounting for the SW–ENA recycling process. In this work, we examine the Ribbon structure, namely its ENA fluxes, angular extent, width, and circularity properties for two years, 2009 and 2019, representative of the declining phases of two adjacent solar cycles. We find that, (i) the Ribbon ENA fluxes have recovered in the nose direction and south of it down to ∼25° (for energies below 1.7 keV) and not at mid and high ecliptic latitudes; (ii) the Ribbon width exhibits significant variability as a function of azimuthal angle; (iii) circularity analysis suggests that the 2019 Ribbon exhibits a statistically consistent radius with that in 2009. The Ribbon's partial recovery is aligned with the consensus of a heliosphere with its closest point being southward of the nose region. The large variability of the Ribbon width as a function of azimuth in 2019 compared to 2009 is likely indicative of small-scale processes within the Ribbon. [ABSTRACT FROM AUTHOR]

In 2009, the Interstellar Boundary Explorer (IBEX) discovered a narrow "ribbon" of energetic neutral atom emissions across the sky with properties correlated with the solar wind latitudinal structure and the interstellar magnetic field draped around the heliosphere. It is widely believed that the ribbon is formed from the escape of heliospheric ENAs into the local interstellar medium and their eventual return as secondary ENAs. However, there is no consensus on the rate of pitch angle scattering of these PUIs before they become secondary ENAs. We test two opposing limits of scattering rates ("weak" versus "strong") by solving a time-dependent model of the ribbon that evolves with the solar cycle, and we compare them to IBEX observations over 2009–2019. First, we find that both models qualitatively reproduce the evolution of IBEX fluxes for most of the data set, with a few exceptions, although the strong (or "spatial retention") scattering model greatly underestimates the observed fluxes. Regardless, time dependence of fluxes cannot distinguish these models. Second, the ribbon's geometric properties, i.e., its center and radius, are significantly different between the models. The spatial retention model reproduces the observed ribbon centers as a function of energy and time slightly better than the weak scattering model, and the spatial retention model reproduces the observed ribbon radius over energy and time almost perfectly, whereas the weak scattering model compares poorly. Our analysis favors the spatial retention mechanism as the source of the IBEX ribbon, but it requires modification to increase the flux of ENAs observed at 1 au. [ABSTRACT FROM AUTHOR]

The Interstellar Boundary Explorer (IBEX) has been observing the outer heliosphere and its interactions with the very local interstellar medium (VLISM) via measurements of energetic neutral atoms (ENAs) for over 14 yr. We discovered the IBEX Ribbon—a structure completely unanticipated by any prior theory or model—that almost certainly resides beyond the heliopause in the VLISM. We also characterized the other major source of heliospheric ENAs, the globally distributed flux (GDF), produced largely in the heliosheath between the termination shock and heliopause. In this study, we make three major new contributions. First, we validate, provide, and analyze the most recent 3 yr of IBEX-Hi (0.5–6 keV FWHM) data (2020–2022) for the first time. Second, we link these observations to the prior 11 yr of observations, exploring long-term variations. Finally, we provide the first IBEX team-validated Ribbon/GDF separation scheme and separated maps. Because of the uncertainty in separating different line-of-sight integrated sources, we provide not just best guess (median) maps, but also maps with upper and lower reasonable values of Ribbon and GDF fluxes, along with bounding fluxes that add the uncertainties to the upper and lower values. This allows theories and models to be compared with a range of possible values that the IBEX team believes are consistent with data. These observations, along with the reanalysis of the prior 11 yr of IBEX-Hi data, provide new insights and even further develop our detailed understanding of the heliosphere’s interaction with the local interstellar medium unlocked by IBEX.

The solar wind deceleration upstream of the Martian bow shock is examined using particle and magnetic field measurements obtained by the Mars Atmosphere and Volatile EvolutioN (MAVEN). Mars lacks a strong intrinsic magnetic field so its upper atmosphere extends beyond the Martian bow shock and interacts directly with the solar wind. Neutral atoms in the Martian upper atmosphere can be ionized through several physical processes and then start to move with the solar wind flow to form pickup ions. In return, the solar wind is expected to slow down due to the momentum transfer to the pickup ions. The present study surveys the MAVEN solar wind measurements between 2015 and 2019 to evaluate the solar wind deceleration upstream of the Martian bow shock. Different from previous studies of solar wind deceleration, our analysis carefully excludes the solar wind deceleration in the shock foot region. The average solar wind deceleration obtained is about 0.7% of the initial solar wind speed, much smaller than the values given by previous studies. Further calculation using several reasonable Martian upper atmosphere density profiles demonstrates that the deceleration observed is consistent with the pickup ion mass-loading scenario.

Ion pickup by the solar wind is ubiquitous in space plasma. Because pickup ions are originally produced by ionization of an exospheric neutral atmosphere, their measurements contain information on the exospheric neutral abundance. Here we established a method to retrieve exospheric number densities, by analyzing the ion velocity distribution functions of pickup ions measured by the Mars Atmosphere and Volatile EvolutioN spacecraft. We successfully retrieved exospheric oxygen density distributions at altitudes ranging from 1000 to 10,000 km around Mars except for the vicinity of the bow shock. This method can be applied to other space missions to study the upper atmosphere of planets, moons, and other small bodies in our solar system, where pickup ions exist.

The size, shape, and composition of the heliosphere are affected by its interaction with the local interstellar medium. A means of quantifying this interaction is by measuring the relative motion of the two media. We determine the flow direction, ${\lambda }_{\mathrm{ISN}}$ , of the interstellar wind through the heliosphere and its trend over an 11 yr solar cycle using Advanced Composition Explorer/Solar Wind Ion Composition Spectrometer He ^+ double-coincidence pickup ion (PUI) measurements from a data set spanning 13 full orbits, with focusing cone crossings occurring between the years 1998 and 2010. We measure the flow direction by fitting a kappa function to the focusing cone signature in the count rates of He ^+ at the PUI cutoff measured as a function of ecliptic longitude. We determine ${\lambda }_{\mathrm{ISN}}$ for each three-orbit boxcar centered on the years 1999 through 2009 and find the trend of the resulting linear fit. We account for solar transients by removing measurements associated with CMEs. However, there still may be effects from compression regions, such as stream–stream and corotating interaction regions. We additionally make considerations to ensure the PUI torus is in view, to account for varying interplanetary magnetic field angles, to ensure that we analyze newly injected interstellar PUIs largely unaffected by transport effects, and to account for general solar activity. We measure a slope in the flow direction to be 0.00° ± 0.51°​​​​ yr ^−1 , indicating that we do not measure a varying trend over this period. We repeat the focusing cone analysis for the combined data set and determine an overall flow direction of 75.37° ± 0.43°.

The distribution of ions in the heliosheath—the region between the heliospheric termination shock and the heliopause—is important for understanding remote observations of energetic neutral atoms (ENAs). The ion distributions were estimated previously based on hybrid simulations of the heating and evolution of solar wind and interstellar pickup ions across the solar wind termination shock, but these estimates only provide the distributions near the shock. In this work, we use self-consistent hybrid kinetic simulations to investigate the effects of turbulence on ion distributions in the heliosheath. The simulations are compared against Voyager observations, constraining the feasible amplitude and compressibility of turbulence. We find that the heating due to turbulent dissipation can lead to a significant increase in the temperature of thermal solar wind ions. Both turbulent velocity fluctuations and the heating of solar wind ions increase the charge-exchange source for ENAs at low energies (around 100 eV), where current ENA models underpredict observations by more than an order of magnitude. However, the effects of turbulence are likely not strong enough to fully explain these discrepancies.

Combined in situ observations of the interstellar wind through the solar system and of its pickup ions (PUIs), implanted after ionization in the solar wind, explain, in comparison with interstellar absorption lines of nearby stars, that the Sun is in an interaction region of the two nearest interstellar clouds. This new finding disrupts the long-held understanding that we are inside the local interstellar cloud (LIC). We discuss how space physics evolved toward such interdisciplinary studies between heliophysics and astrophysics. In 1984, the discovery of interstellar He+ PUIs exposed the very local interstellar medium to in situ diagnostics at 1AU. These PUIs provide the interstellar gas composition and form a stepping stone for the acceleration of ions, especially into anomalous cosmic rays. Using the Sun as a gravitational spectrograph, direct imaging of the neutral interstellar wind, first for He and then for H, O, and Ne, provides the interstellar gas velocity vector and temperature at the heliopause. Combining the interstellar gas flow vectors, those of secondary neutral He and O, and the interstellar magnetic field direction deduced from the interstellar H deflection and termination shock anisotropy seen by the Voyagers provides synergistically the heliosphere’s shape, its interaction with the interstellar medium, and constrains our radiation environment. This ISMF organizes the bright Ribbon seen in all-sky images of energetic neutral atoms with the potential to provide its precision determination. The elemental and isotopic composition from PUI and neutral gas observations constrains the galactic evolution and Big Bang cosmology, opening additional interdisciplinary opportunities.

The Voyager 2 crossing of the termination shock indicated that most of the upstream energy from the thermal solar wind ions was transferred to pickup ions (PUIs) and other energetic particles downstream of the shock. We use hybrid simulations at the termination shock for the Voyager 2, flank, and tail directions to evaluate the distributions of different ion species downstream of the shock over the energy range of 0.52–55 keV. Here, we extend the work of Gkioulidou et al., which showed an energy-dependent discrepancy between modeled and energetic neutral atom (ENA) observations, and fit distributions to a hybrid model to show that a population of PUIs accelerated via diffusive shock acceleration (DSA) to become low-energy anomalous cosmic rays (ACRs) can bridge the gap between modeled and observed ENA fluxes. Our results with the inclusion of DSA via hybrid fitting give entirely new and novel evidence that DSA at the termination shock is likely to be an important physical process. These ACRs carry a significant fraction of the energy density at the termination shock (22%, 13%, and 19% in the Voyager 2, flank, and tail directions, respectively). Using these ACRs in global ENA modeling of the heliosphere from 0.52 to 55 keV, we find that scaling factors as large as 1.8–2.5 are no longer required to match ENA observations at energies of ∼1–4 keV. Large discrepancies between modeled and observed ENAs only remain over energies of 4–20 keV, indicating that there may be a further acceleration mechanism in the heliosheath at these energies.

Pickup ions (PUIs) play a crucial role in the heliosphere, contributing to the mediation of large-scale structures such as the distant solar wind, the heliospheric termination shock, and the heliopause. While magnetic reconnection is thought to be a common process in the heliosphere due to the presence of heliospheric current sheets, it is poorly understood how PUIs might affect the evolution of magnetic reconnection. Although it is reasonable to suppose that PUIs decrease the reconnection rate since the plasma beta becomes much larger than 1 when PUIs are included, we show for the first time that such a supposition is invalid and that PUI-induced turbulence, heat conduction, and viscosity can preferentially boost magnetic reconnection in heliospheric current sheets in the distant solar wind. This suggests that it is critical to include the effect of the turbulence, heat conduction, and viscosity caused by PUIs to understand the dynamics of magnetic reconnection in the outer heliosphere.

Since its discovery in 2009, the IBEX energetic neutral atom (ENA) Ribbon has been a subject of numerous studies. It appears at energies ∼0.5–6 keV and is most pronounced at ∼1–3 keV. It is almost circular, ∼20°–40° wide, and its center lies near the pristine local interstellar magnetic field direction, whose field lines are draped around the heliosphere. The Ribbon intensity is enhanced above the more diffuse, globally distributed flux (GDF) and varies on timescales that are delayed compared to the underlying and slowly varying GDF. We present a novel method to infer the Ribbon boundaries and transverse profile of the Ribbon using sequential time variations of ENA fluxes, with minimal modeling assumptions involved. The method utilizes the difference in temporal evolution between the total Ribbon content and GDF fluxes. We then use the inferred Ribbon transverse profile to statistically quantify the GDF contribution to the observed peak Ribbon intensity to be ∼32.23% ± 3.15% in 2009–2011. This Ribbon separation method works best during times of gradual changes in solar wind output, and with high angular resolution and ENA counting statistics; results thus provide a proof of concept for the upcoming Interstellar Mapping and Acceleration Probe ENA measurements.

The role of pickup ions (PUIs) in the solar wind interaction with the local interstellar medium is investigated with 3D, multifluid simulations. The flow of the mixture of all charged particles is described by the ideal MHD equations, with the source terms responsible for charge exchange between ions and neutral atoms. The thermodynamically distinct populations of neutrals are governed by individual sets of gas dynamics Euler equations. PUIs are treated as a separate, comoving fluid. Because the anisotropic behavior of PUIs at the heliospheric termination shocks is not described by the standard conservation laws (a.k.a. the Rankine–Hugoniot relations), we derived boundary conditions for them, which are obtained from the dedicated kinetic simulations of collisionless shocks. It is demonstrated that this approach to treating PUIs makes the computation results more consistent with observational data. In particular, the PUI pressure in the inner heliosheath (IHS) becomes higher by ∼40%–50% in the new model, as compared with the solutions where no special boundary conditions are applied. Hotter PUIs eventually lead to charge-exchange-driven cooling of the IHS plasma, which reduces the IHS width by ∼15% (∼8–10 au) in the upwind direction, and even more in the other directions. The density of secondary neutral atoms born in the IHS decreases by ∼30%, while their temperature increases by ∼60%. Simulation results are validated with New Horizons data at distances between 11 and 47 au.

Fast solar wind (SW) flows outward from polar coronal holes (PCHs). The latitudinal extent of the fast SW varies during different phases of the solar cycle. The fast SW in the inner heliosheath produces a flatter proton spectrum than the slow SW that can be observed through energetic neutral atoms (ENAs) by the Interstellar Boundary Explorer (IBEX). In this study, we investigate the evolution of PCHs as reflected in the high-time resolution ENA flux measurements from IBEX-Hi, where the PCHs are identified by ENA spectral indices

Interstellar neutral atoms enter the heliosphere at a relatively slow speed corresponding to the motion of the Sun through the local interstellar medium, which is approximately 25 km s ^−1 . Neutral hydrogen atoms enter from the approximate location of the Voyager spacecraft and are eventually ionized primarily by collision with thermal solar wind ions. An earlier analysis by Hollick et al. examined low-frequency magnetic waves observed by the Voyager spacecraft from launch through 1990 that are thought to arise from the scattering of newborn interstellar pickup H ^+ and He ^+ . We report an analysis of Voyager 1 observations in 1991, which is the last year of high-resolution magnetic field data that are publicly available, and find 70 examples of low-frequency waves with the characteristics that suggest excitation by pickup H ^+ and 10 examples of waves consistent with excitation by pickup He ^+ . We find a particularly dense cluster of observations at the tail end of what is thought to be a Merged Interaction Region (MIR) that was previously studied by Burlaga & Ness using Voyager 2 observations. This is not unexpected if the MIR is followed by a large rarefaction region, as they tend to be regions of reduced turbulence levels that permit the growth of the waves over the long time periods that are generally required of this instability.

Nearly incompressible magnetohydrodynamic (NI MHD) theory for β ∼ 1 (or β ≪ 1) plasma has been developed and applied to the study of solar wind turbulence. The leading-order term in β ∼ 1 or β ≪ 1 plasma describes the majority of 2D turbulence, while the higher-order term describes the minority of slab turbulence. Here, we develop new NI MHD turbulence transport model equations in the high plasma beta regime. The leading-order term in a β ≫ 1 plasma is fully incompressible and admits both structures (flux ropes or magnetic islands) and slab (Alfvén waves) fluctuations. This paper couples the NI MHD turbulence transport equations with three fluid (proton, electron, and pickup ion) equations, and solves the 1D steady-state equations from 1–75 au. The model is tested against 27 yr of Voyager 2 data, and Ulysses and NH SWAP data. The results agree remarkably well, with some scatter, about the theoretical predictions.

In this paper, we develop the transport equation of kappa, the fundamental thermodynamic parameter that labels kappa distributions of particle velocities. Using the recently developed concept of entropy defect, we are able to formulate the transport equation of kappa as a function of a general, positive or negative, rate of entropy change. Then, we derive the particular case of exchanging plasma ions with low-dimensionality, newly born pickup protons, which interact and decrease the entropy of the flow of otherwise kappa-distributed plasma protons. Finally, we apply the transport equation of kappa to the solar wind plasma protons, which leads to the radial profile of kappa values, as well as the evolution of the kappa distributions through the heliosphere. The results show that the solar wind kappa decreases with increasing heliocentric distance, corresponding to plasmas residing in stationary states far from classical thermal equilibrium. Moreover, in the outer heliosphere and the heliosheath, kappa reaches its lowest values and is spread across the far-equilibrium region of 1.5 < κ < 2.5, which coincides with independent observations provided by NASA’s Interstellar Boundary Explorer mission.

We revisit the question of how the unstable scattering of interstellar pickup ions (PUIs) may drive turbulence in the outer solar wind and why the energy released into fluctuations by this scattering appears to be significantly less than the standard bispherical prediction. We suggest that energization of the newly picked-up ions by the ambient turbulence during the scattering process can result in a more spherical distribution of PUIs and reduce the generated fluctuation energy to a level consistent with the observations of turbulent intensities and core solar wind heating. This scenario implies the operation of a self-regulation mechanism that maintains the observed conditions of turbulence and heating in the PUI-dominated solar wind.

The present study investigates the unstable waves driven by the pickup ions of ring-beam distributions with various pickup angles and pickup ion temperatures in the outer heliosheath, using both linear instability analysis and hybrid simulations. While previous papers have generally assumed specific pickup ion temperatures, this study takes a more comprehensive approach by examining a wide range of pickup ion temperatures that may occur in the outer heliosheath. Our one-dimensional simulations demonstrate that regardless of the initial pickup ion temperature, the pitch-angle scattering of the pickup ions at small pickup angles saturates before the pickup ions can reach the hemisphere of negative parallel velocities with respect to the background magnetic field in velocity space, while at a 90° pickup angle, the pickup ions of ring distributions attain a significant level of isotropy. In contrast, the two-dimensional simulation results show that at all pickup angles, increasing the pickup ion temperature only slightly reduces the pickup ion scattering rate, but does not prevent the pickup ions from reaching the hemisphere of negative parallel velocities. Overall, the results do not align with the requirements of the earlier proposed scenarios for the secondary energetic neutral atom (ENA) mechanism of the Interstellar Boundary Explorer ENA ribbon, which demand either weak pickup ion scattering in the outer heliosheath or at least incomplete pickup ion scattering in the off-ribbon directions.

This study provides a detailed analysis of five distant interplanetary shocks observed by the Solar Wind Around Pluto instrument on board New Horizons, which exhibit the signature of a suprathermal H ^+ pickup ion (PUI) tail in the downstream distribution. These shocks were observed with a PUI data cadence of approximately 24 hr, covering a heliocentric distance range of 23.71–36.75 au. The shock compression ratio varies between approximately 1.4 and 3.2. The H ^+ PUI density and temperature show a gradual increase across the shock, while the H ^+ solar wind density shows erratic behavior without a distinct downstream compression. The H ^+ PUI cooling index variation across the shock displays different characteristics in each shock. This study demonstrates, for the first time, the variation of the number density of downstream H ^+ PUI tails with the shock compression ratio, revealing an increase in tail density with stronger shocks. Additionally, theoretical estimates of reflected PUI number densities derived from the electrostatic cross-shock potential agree very well with the observed H ^+ PUI tail densities for stronger shocks.

Observations of the plasma environment outside of the Earth's magnetosphere by the Magnetospheric Multiscale Mission Hot Plasma Composition Analyzers (MMS‐HPCAs) allow density measurements of geocoronal hydrogen pickup ions (PUIs). A study of this environment from December 4, 2019 to December 6, 2019, part of a single orbital period of the MMS spacecraft, revealed an inverse square dependence on PUI density. This dependence corresponds well with an inverse‐cube dependence on the neutral hydrogen density within the geocorona. Plain Language Summary: Measuring pickup ions produced from the Earth's geocorona provide an in‐situ analysis of their geocentric density dependence. These measurements also provide a model‐dependent evaluation of the neutral geocorona's expanse and geocentric dependence, which is part of the Earth's extended atmosphere, also known as the exosphere. Key Points: Pickup ions (PUIs) produced from the hydrogen exosphere are observable with near‐Earth plasma instrumentsPickup ions are distinguishable from solar wind ions through their respective velocity distributionsGeocoronal PUI density decreases with geocentric radius and corresponds to an inverse radius cube neutral dependence [ABSTRACT FROM AUTHOR]

We investigate a possible negative ion feature observed by the Cassini Plasma Spectrometer (CAPS) during a flyby of Saturn's moon Dione that occurred on 7 April 2010. By examining possible particle trajectories, we find that the observed particles are consistent with negative pickup ions originating near the moon's surface. We find that the mass of the negative pickup ions is in the range of 15–25 u and tentatively identify this species as O−, likely resulting from ionization and subsequent pickup from Dione's O2‐CO2 exosphere. Our estimates show that the negative ion density is ~3 × 10−3 cm−3. This is comparable to, but slightly smaller than, that previously reported for the density of O2+ pickup ions for the same flyby, indicating that negative pickup ions may represent a major loss channel for Dione's exosphere. Key Points: Analysis of data from the Cassini Plasma Spectrometer confirms negative pickup ions originating from Saturn's moon DioneThe negative pickup ions are consistent with O− originating from Dione's exosphereDensity estimates indicate that negative pickup ions may represent a significant loss channel for Dione's exosphere [ABSTRACT FROM AUTHOR]

On 05 December 2015 the Magnetospheric MultiScale constellation observed interstellar pickup ion distributions during an inbound crossing of Earth's perpendicular bow shock near the subsolar point, which provides new insights into shock acceleration of pickup ions throughout the heliosphere. In this study we analyze the upstream and downstream velocity distributions of H+ and He+ using data from the Hot Plasma Composition Analyzer on Magnetospheric MultiScale. We derive average two‐dimensional pitch angle distributions in the upstream and downstream field‐aligned bulk plasma frame, as well as integrated one‐dimensional velocity distributions. By comparing the upstream and downstream distributions, we find evidence of accelerated H+ and He+ downstream of the shock. By comparing measured and theoretical reflection ratios for He+, we attribute a significant part of this acceleration to a single reflection at the shock. Plain Language Summary: By studying distributions of ions before and after a shock in space, we gain valuable insights into shock acceleration mechanisms that produce high‐energy ion populations, observed throughout the heliosphere. On 05 December 2015 the Magnetospheric MultiScale constellation crossed Earth's perpendicular bow shock and measured velocity distributions of He+ ions before and after the shock. This study analyzes these distributions in search for signs of accelerated He+. We find that He+ is accelerated mainly perpendicular to the local magnetic field, and that a significant fraction of the accelerated He+ is consistent with the theory of shock reflection at the bow shock, in which an ion is reflected at the shock and then energized by the solar wind electric field (pointing parallel to the shock surface) as the ion gyrates back to the shock. Key Points: MMS observed interstellar pickup He+ upstream and downstream of the Earth's perpendicular bow shockComparison of upstream and downstream velocity distributions revealed accelerated pickup He+The observations are consistent with single reflection shock acceleration [ABSTRACT FROM AUTHOR]

Abstract: Saturn's largest icy moon, Rhea, hosts a tenuous surface‐sputtered exosphere composed primarily of molecular oxygen and carbon dioxide. In this Letter, we examine Cassini Plasma Spectrometer velocity space distributions near Rhea and confirm that Cassini detected nongyrotropic fluxes of outflowing CO 2 + during both the R1 and R1.5 encounters. Accounting for this nongyrotropy, we show that these possess comparable along‐track densities of ∼2 × 10−3 cm−3. Negatively charged pickup ions, also detected during R1, are surprisingly shown as consistent with mass 26 ± 3 u which we suggest are carbon‐based compounds, such as CN−, C 2 H −, C 2 −, or HCO−, sputtered from carbonaceous material on the moon's surface. The negative ions are calculated to possess along‐track densities of ∼5 × 10−4 cm−3 and are suggested to derive from exogenic compounds, a finding consistent with the existence of Rhea's dynamic CO2 exosphere and surprisingly low O2 sputtering yields. These pickup ions provide important context for understanding the exospheric and surface ice composition of Rhea and of other icy moons which exhibit similar characteristics. [ABSTRACT FROM AUTHOR]

Observations of plasma and turbulence in the outer heliosphere (the distant supersonic solar wind and the subsonic solar wind beyond the heliospheric termination shock) made by the Voyager Interstellar Mission and the energetic neutral atom observations made by the IBEX spacecraft have revealed that the underlying plasma in the outer heliosphere and very local interstellar medium (VLISM) comprises distinct thermal proton and electron and suprathermal pickup ion (PUI) populations. Estimates of the appropriate collisional frequencies show that the multi-component plasma is not collisionally equilibrated in either the outer heliosphere or VLISM. Furthermore, suprathermal PUIs in these regions form a thermodynamically dominant component. We review briefly a subset of the observations that led to the realization that the solar wind-VLISM interaction region is described by a non-equilibrated multi-component plasma and summarizes the derivation of suitable plasma models that describe a PUI-mediated plasma. [ABSTRACT FROM AUTHOR]

Abstract: 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 , and 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. [Copyright &y& Elsevier]

Abstract: 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 and 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. [Copyright &y& Elsevier]

Abstract: Titan has a very extensive atmosphere and exosphere which interact strongly with the corotating magnetospheric plasma. Some of the new pickup ions created in the vicinity of the exosphere will re-impact the upper atmosphere causing additional energy input. Previous investigations of the atmospheric collisional interaction of the pickup ions have generally assumed the atmosphere to be spherically symmetric. From the Voyager and Cassini observations, we know that the magnetic field configuration and plasma flow field is highly asymmetric. To study the possible spatial variation in the pickup ion influx, we have employed the plasma data of the three dimensional MHD simulation of Kopp and Ip [Kopp, A., Ip, W.-H. Asymmetric mass loading effect at Titan’s ionosphere. J. Geophys. Res. 106, 8323–8332, 2001] to compute the trajectories of the pickup ions. In this work, we calculate the ion influx and energy deposit into Titan’s exobase for the , and pickup ions separately. The model results of four different Titan’s orbital locations are also presented. [Copyright &y& Elsevier]

Abstract: In December 1995 and November 2002, the Galileo spacecraft traversed the inner Io plasma torus on the J0 and A34 passes, respectively. Observations of electron density showed a steep drop off at the inner boundary located between 4.5R J and 5R J on the two passes, and a much more gradual decrease in the outer torus. The [Wang, Y.-L., Russell, C.T., Raeder, J. The Io mass loading disk: model calculations. Journal of Geophysical Research 106, 26243–26260, 2001] model of torus formation considers multiple ionization and neutralization stages for the pickup ions, and predicts an inner torus boundary location which depends on the local plasma velocity at Io. For an inner torus boundary at ∼4.7R J, ions are picked up into a plasma flowing at ∼37 km/s (74 km/s is the corotation velocity at 5.9R J). The steepness of the boundary and the double peaked density structure seen on the two Galileo passes is not reproduced by the [Wang, Y.-L., Russell, C.T., Raeder, J. The Io mass loading disk: model calculations. Journal of Geophysical Research 106, 26243–26260, 2001] model, which considers the source of mass loading at Io to be uniformly distributed around the moon and does not include outward radial convection. Adding this convection to the model, as would be required to maintain a steady state magnetodisk, steepens the inner torus boundary, but does so over very long timescales. The different inner torus density structure observed on the J0 and A34 passes is not reproduced by either the [Wang, Y.-L., Russell, C.T., Raeder, J. The Io mass loading disk: model calculations. Journal of Geophysical Research 106, 26243–26260, 2001] or outward radial convection models and indicates the possible occurrence of a large mass loading event in the 7 years between the passes or longitudinal asymmetry in the torus structure. [Copyright &y& Elsevier]

Pickup ions provide us with a powerful tool to probe remote regions in and beyond the heliosphere and determine the composition of the neutral gas in these regions. Interstellar pickup ions, observed inside the heliosphere at 3–5 AU from the Sun, are created by charge exchange and photoionization of the local interstellar gas that penetrates deep into the heliosphere. Comprehensive measurements of H, He, C, N, O, and Ne, especially with the Solar Wind Ion Composition Spectrometer (SWICS) on both the Ulysses and ACE spacecraft, are giving us a wealth of data that we use to infer the chemical and physical properties of the Local Interstellar Cloud (LIC). Using updated and improved measurements of pickup ions and state-of-the-art modeling, we find that in the LIC the proton and neutral hydrogen densities are respectively (0.061 ± 0.018) and (0.176 ± 0.019) cm-3. The neutral helium density is (0.0154 ± 0.0015) cm-3 and the ionized helium density (0.0083 ± 0.0021) cm-3, giving a LIC total gas density of hydrogen plus helium of (0.261 ± 0.026) cm-3. Using these updated densities, the location of the heliospheric termination shock is calculated to be at about 96 AU in 2002, and the crossing of Voyager 1 through this shock is predicted to occur between 2003 and 2009. The metallicity of the gas in the LIC is found to be comparable to that of the Protosolar Cloud. This unexpected result may be explained by a recent model of quasi-continuous excess infall of extra-galactic material that is only moderately processed by stellar nucleosynthesis [Astrophys. J. 578 (2002) 862]. [Copyright &y& Elsevier]

A search for ULF waves associated with pickup ions was undertaken for Venus and for Mars. The Pioneer Venus Orbital Plasma Analyzer (OPA) measurements of ion energy per unit charge spectra provided signatures of the probable presence of pick-up ions. At the locations of these energetic ion signatures we derived power spectra and propagation characteristics of structures in the local magnetic field from measurements by the Pioneer Venus Orbiter Magnetometer. Some of these features are characteristic of effects previously identified in the ionosheath of Venus, analogous to wave activity properties in the Earth’s magnetosheath. However, for the first time examples have been found at Venus of ULF waves near the ion gyrofrequencies in the ionosheath and in the far magnetic tail. Both linear and elliptically polarized waves are found in the vicinity of ion pickup detections. The signatures of these features are presented using one orbit of Pioneer Venus Orbiter data. The same wave analysis techniques are applied to the magnetic field data measured on one characteristic orbit from the Mars Global Surveyor Magnetometer. This is used to demonstrate how a comparative analysis of the two planets can be useful. [Copyright &y& Elsevier]