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The Interstellar Boundary Explorer (IBEX) mission continues to provide energetic neutral atom (ENA) observations of the heliosphere and Earth’s magnetosphere from a global perspective and including spatial, temporal, and energy information. Due to its orbit, IBEX routinely observes the magnetosphere from a side-viewing vantage point, with a field-of-view that is nearly perpendicular to the day-night plane. This enables the construction of composite ENA images at different energies (0.5 – 6.0 keV) for convected solar wind conditions, which provides global insights into different magnetospheric plasma regions and processes.Earth’s plasma sheet plays a crucial role in the global circulation of plasma throughout the magnetosphere. The structure of the plasma sheet is driven by a combination of effects including interplanetary magnetic field (IMF) and solar wind conditions, internal magnetospheric processes, and Earth’s dipole tilt angle.This work examines the structure of the plasma sheet in the X-Z geocentric ecliptic plane (GSE) using ENA images from IBEX. The thickness and extent of the plasma sheet is compared for conditions of prolonged northward and southward IMF. We report on a North-South asymmetry in the plasma sheet that is related to the IMF orientation and is driven by a combination of dayside magnetic reconnection effects and high dipole tilt. Results provide evidence of dayside reconnection driving plasma sheet dynamics.

Direct sampling of interstellar neutral (ISN) atoms close to the Sun enables studies of the very local interstellar medium (VLISM) around the heliosphere. The primary population of ISN helium atoms has, until now, been assumed to reflect the pristine VLISM conditions at the heliopause. Consequently, the atoms observed at 1 au by the Interstellar Boundary Explorer (IBEX) were used to determine the VLISM temperature and velocity relative to the Sun, without accounting for elastic collisions with other species outside the heliopause. Here, we evaluate the effect of these collisions on the primary ISN helium population. We follow trajectories of helium atoms and track their collisions with slowed plasma and interstellar hydrogen atoms ahead of the heliopause. Atoms typically collide a few times in the outer heliosheath, and only ~1.5% of the atoms are not scattered at all. We use calculated differential cross sections to randomly choose scattering angles in these collisions. We estimate that the resulting primary ISN helium atoms at the heliopause are slowed down by ~0.45 km/s and heated by ~1100 K compared to the pristine VLISM. The resulting velocity distribution is asymmetric and shows an extended tail in the antisunward direction. Accounting for this change in the parameters derived from IBEX observations gives the Sun's relative speed of 25.85 km/s and temperature of 6400 K in the pristine VLISM. Finally, this paper serves as a source of the differential cross sections for elastic collisions with helium atoms., 15 pages, 5 figures, accepted for publication in ApJL

Energetic neutral atom (ENA) models typically require post-processing routines to convert the distributions of plasma and H atoms into ENA maps. Here we investigate how two different kinetic-MHD models of the heliosphere (the BU and Moscow models) manifest in modeled ENA maps using the same prescription and how they compare with Interstellar Boundary Explorer (IBEX) observations. Both MHD models treat the solar wind as a single-ion plasma for protons, which include thermal solar wind ions, pick-up ions (PUIs), and electrons. Our ENA prescription partitions the plasma into three distinct ion populations (thermal solar wind, PUIs transmitted and ones energized at the termination shock) and models the populations with Maxwellian distributions. Both kinetic-MHD heliospheric models produce a heliotail with heliosheath plasma organized by the solar magnetic field into two distinct north and south columns that become lobes of high mass flux flowing down the heliotail, though in the BU model the ISM flows between the two lobes at distances in the heliotail larger than 300 AU. While our prescription produces similar ENA maps for the two different plasma and H atom solutions at the IBEX-Hi energy range (0.5 - 6 keV), the modeled ENA maps require a scaling factor of ~2 to be in agreement with the data. This problem is present in other ENA models with the Maxwellian approximation of multiple ion species and indicates that a higher neutral density or some acceleration of PUIs in the heliosheath is required., Comment: 29 pages, 3 tables, 9 figures, accepted to ApJ

The Interstellar Boundary Explorer (IBEX) mission provides global energetic neutral atom (ENA) observations from the heliosphere and the Earth's magnetosphere, including spatial, temporal, and energy information. IBEX views the magnetosphere from the sides and almost always perpendicular to noon‐midnight plane. We report the first ENA images of the energization process in the Earth's ion foreshock and magnetosheath regions. We show ENA flux and spectral images of the dayside magnetosphere with significant energization of ENA plasma sources (above ~2.7 keV) in the region magnetically connected to the Earth's bow shock (BS) in its quasi‐parallel configuration of the interplanetary magnetic field (IMF). We also show that the ion energization increases gradually with decreasing IMF‐BS angle, suggesting more efficient suprathermal ion acceleration deeper in the quasi‐parallel foreshock., Key Points We present the first remote‐sensing global images of ion energization in the Earth's foreshockWe provide ion energization profile as a function of bow shock obliquityWe quantify the difference of energization in magnetosheath and its magnetically connected counterpart in the upstream foreshock

Interstellar neutral (ISN) atoms from the very local interstellar medium (VLISM) penetrate the heliosphere and are observed by detectors located near 1 au, e.g., on the Interstellar Boundary Explorer (IBEX), and in the future on the Interstellar Mapping and Acceleration Probe (IMAP). Interpretation of these observations provides insight into the physical conditions in the VLISM but requires modeling of the processes that change distributions of ISN atoms inside the heliosphere and beyond. Here, we focus on the consequences of collisional scattering in the outer heliosheath (OHS), beyond the heliopause. Charge exchange collisions create secondary atoms from the OHS ions, which have a different flow speed and temperature than pristine ISN atoms, especially close to the heliopause. It is widely assumed that these collisions do not change directions of interacting particle velocities. We show that this assumption is not justified for the typical collisions speed in the OHS, and therefore the distribution functions of secondary atoms are different than those calculated without this momentum exchange. This angular scattering in charge exchange collisions results in secondary atom production terms that show elongated distributions aligned with the relative bulk speed of the parent populations, as well as higher temperatures (up to ~3000 K) and shifted bulk speeds (up to ~2 km s-1). Distributions of ISN atoms are also affected by elastic collisions that similarly show significant scattering for collision energies typical in the OHS. Eventually, these scattering processes modify distributions of ISN atom observed in the heliosphere directly and as pickup ions. These effects may help explain systematical discrepancies between the IBEX data and current models.

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

The heliospheric energetic neutral atom spectrum observed by the Interstellar Boundary Explorer (IBEX) reveals that the heliosheath proton distribution is consistent with a power law. The origin of the spectrum is likely from interstellar pickup ions (PUIs) accelerated at the heliospheric termination shock (HTS). We present an explanation of the proton spectrum origin using a test particle simulation of PUIs accelerated at the HTS. PUIs experience preferential heating by the motional electric field in the shock foot, but do not develop a power-law tail without the presence of turbulence at wavenumbers (k) close to the PUI gyroradius scale (R g). Voyager 2 observations of the magnetic field downstream of the HTS indicate a moderate amount of turbulence at kR g ≅ 1, δ B / B 0 2 ≅ 0.01 , which we find to be sufficient for producing a downstream suprathermal PUI tail but not at intensities observed by IBEX. Within the shock ramp, however, Voyager observed the turbulence power at much smaller scales to be nearly 100 times stronger, suggesting the possibility of strong turbulence at the PUI gyroradius scale. We show that a proton distribution can develop a power law downstream of the HTS consistent with IBEX observations if δ B / B 0 2 ≳ 0.1 at kR g ≅ 1 in the shock foot. Shock drift acceleration of PUIs by the motional electric field is aided by interactions with turbulence upstream of the shock overshoot. Steepening of the IBEX proton spectrum in directions farther from the heliospheric nose suggests the HTS compression ratio and/or turbulence power weakens near the heliotail.

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 estimate the heliospheric termination shock (HTS) compression ratio at multiple directions in the sky from a quantitative comparison of the observed and simulated inner heliosheath (IHS) energetic neutral atom (ENA) fluxes. We use a 3D steady-state simulation of the heliosphere to simulate the ENA fluxes by postprocessing the MHD plasma using a multi-Maxwellian distribution for protons in the IHS. The simulated ENA fluxes are compared with time exposure–averaged IBEX-Hi data for the first 3 yr of the mission. The quantitative comparison is performed by calculating the fractional difference in the spectral slope between the observed and simulated ENA fluxes for a range of compression ratios, where the simulated ENA spectrum is varied as a function of downstream pickup ion temperature as a function of compression ratio. The estimated compression ratio in a particular direction is determined by the minimum value of the fractional difference in spectral slope. Our study shows that the compression ratio estimated by this method is in close agreement with the large-scale compression ratio observed by Voyager 2 in its travel direction. Also, the compression ratio in other directions near the ecliptic plane is similar to the compression ratio at the Voyager 2 direction. The weakest shock compression is found to be on the port side of the heliosphere at direction (27°, 15°). This is the first study to estimate the HTS compression ratio at multiple directions in the sky from IBEX data.

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.

The effects of turbulence in the very local interstellar medium (VLISM) have been proposed by Giacalone & Jokipii (2015) to be important in determining the structure of the Interstellar Boundary Explorer (IBEX) ribbon via particle trapping by magnetic mirroring. We further explore this effect by simulating the motion of charged particles in a turbulent magnetic field superposed on a large-scale mean field, which we have considered to be either spatially-uniform or a mean field derived from a 3D MHD simulation. We find that the ribbon is not double-peaked, in contrast to Giacalone & Jokipii (2015). However, the magnetic mirror force still plays an important role in trapping particles. Furthermore, the ribbon$'$s thickness is considerably larger if the large-scale mean field is draped around the heliosphere. Voyager 1 observations in the VLISM show a turbulent field component that is stronger than previously thought, which we test in our simulation. We find that the inclusion of turbulent fluctuations at scales ${\gtrsim}$100 au and power consistent with Voyager 1 observations produces a ribbon whose large-scale structure is inconsistent with IBEX observations. However, restricting the fluctuations to ${\sim}$10 au or smaller produces a smoother ribbon structure similar to IBEX observations. Different turbulence realizations produce different small-scale features ${\lesssim}10��$ in the ribbon, but its large-scale structure is robust if the maximum fluctuation size is ${\sim}$10 au. This suggests that the magnetic field structure at scales ${\lesssim}$10 au is determined by the heliosphere$'$s interaction with the VLISM and cannot entirely be represented by homogeneous interstellar turbulence., Submitted to the Astrophysical Journal

In 2009, the Interstellar Boundary Explorer (IBEX) discovered the existence of a narrow "ribbon" of intense energetic neutral atom (ENA) emission projecting approximately a circle in the sky. It is believed that the ribbon originates from outside of the heliopause in radial directions ( r ) perpendicular to the local interstellar magnetic field (ISMF), B , i.e., B∙ r = 0. Swaczyna et al. (2016a) estimated the distance to the IBEX ribbon via the parallax method comparing the ribbon position observed from the opposite sides of the Sun. They found a parallax angle of 0.41° ± 0.15°, yielding a distance of 140 - 38 + 84 au to a portion of the ribbon at high ecliptic latitudes. In this study, we demonstrate how the apparent shift of the ribbon in the sky, and thus the apparent distance to the ribbon's source found via the parallax, depends on the transport effects of energetic ions outside the heliopause. We find that the apparent shift of the ribbon based on the "spatial retention" model with ion enhancement near B∙ r = 0, as proposed by Schwadron & McComas (2013), agrees with the parallax of the source region. Parallax is also accurate for a homogeneously-distributed emission source. However, if there is weak pitch angle scattering and ions propagate freely along the ISMF, the apparent shift is significantly smaller than the expected parallax because of the highly anisotropic source. In light of the results from Swaczyna et al. (2016a), our results indicate that the IBEX ribbon source is spatially confined.

The Interstellar Mapping and Acceleration Probe (IMAP) is a revolutionary mission that simultaneously investigates two of the most important overarching issues in Heliophysics today: the acceleration of energetic particles and interaction of the solar wind with the local interstellar medium. While seemingly disparate, these are intimately coupled because particles accelerated in the inner heliosphere play critical roles in the outer heliospheric interaction. Selected by NASA in 2018, IMAP is planned to launch in 2024. The IMAP spacecraft is a simple sun-pointed spinner in orbit about the Sun-Earth L1 point. IMAP’s ten instruments provide a complete and synergistic set of observations to simultaneously dissect the particle injection and acceleration processes at 1 AU while remotely probing the global heliospheric interaction and its response to particle populations generated by these processes. In situ at 1 AU, IMAP provides detailed observations of solar wind electrons and ions; suprathermal, pickup, and energetic ions; and the interplanetary magnetic field. For the outer heliosphere interaction, IMAP provides advanced global observations of the remote plasma and energetic ions over a broad energy range via energetic neutral atom imaging, and precise observations of interstellar neutral atoms penetrating the heliosphere. Complementary observations of interstellar dust and the ultraviolet glow of interstellar neutrals further deepen the physical understanding from IMAP. IMAP also continuously broadcasts vital real-time space weather observations. Finally, IMAP engages the broader Heliophysics community through a variety of innovative opportunities. This paper summarizes the IMAP mission at the start of Phase A development. Keywords: Heliosphere, Interstellar medium, IBEX, IMAP, ENA, Energetic particle

Properties of the inner heliosheath (IHS) plasma are inferred from energetic neutral atom (ENA) observations by ∼1 au spacecraft. However, the Compton–Getting effect due to the plasma velocity relative to the spacecraft is rarely taken into account, even though the plasma speed is a significant fraction of the ENA speed. In this study, we transform Interstellar Boundary Explorer (IBEX) ENA spectra to the IHS plasma frame using flow profiles from a 3D heliosphere simulation. We find that proton spectra in the plasma frame are steeper by ∼30% to 5% at ∼0.5 to 6 keV, respectively, compared to ENAs in the spacecraft frame. While radial plasma flows contribute most to the Compton–Getting effect, transverse flows at mid/high latitudes and the heliosphere flanks account for up to ∼30% of the frame transformation for IBEX-Hi at ∼0.7 keV and up to ∼60% for IBEX-Lo at ∼0.1 keV. We determine that the majority of IHS proton fluxes derived from IBEX-Hi measurements in 2009–2016 are statistically consistent with power-law distributions, with mean proton index ∼2.1 and standard deviation ∼0.4. We find significantly fewer spectral breaks in IBEX observations compared to early analyses, which we determine were a product of the “ion gun” background prevalent in ∼2009–2012 before corrections made by McComas et al. in subsequent data releases. We recommend that future analyses of the IHS plasma utilizing ENA measurements take into account the Compton–Getting effect including radial and transverse flows, particularly IBEX and Interstellar Mapping and Acceleration Probe measurements below ∼10 keV.

This study provides the first observations of Plutogenic ions and their unique interaction with the solar wind. We find ~20% solar wind slowing that maps to a point only ~4.5 RP upstream of Pluto and a bow shock most likely produced by comet-like mass loading. The Pluto obstacle is a region of dense heavy ions bounded by a “Plutopause” where the solar wind is largely excluded and which extends back >100 RP into a heavy ion tail. The upstream standoff distance is at only ~2.5 RP. The heavy ion tail contains considerable structure, may still be partially threaded by the interplanetary magnetic field (IMF), and is surrounded by a light ion sheath. The heavy ions (presumably CH4+) have average speed, density, and temperature of ~90 km s−1, ~0.009 cm−3, and ~7 × 105 K, with significant variability, slightly increasing speed/temperature with distance, and are N-S asymmetric. Density and temperature are roughly anticorrelated yielding a pressure ~2 × 10−2 pPa, roughly in balance with the interstellar pickup ions at ~33 AU. We set an upper bound of

Interstellar neutral atoms provide a remote diagnostic of plasma in the outer heliosheath and the very local interstellar medium via charge exchange collisions that convert ions into atoms and vice versa. So far, most studies of interstellar atoms assumed that daughter hydrogen atoms directly inherit the kinetic properties of parent protons. This assumption neglects angular scattering of the interacting particles. However, for low relative velocities, as expected for charge exchanges in the outer heliosheath, this scattering is significant. In this study, we present how the parameters of daughter populations depend on the relative velocity and temperatures of parent populations. For this purpose, we numerically compute collision terms with and without this scattering. We find that the secondary population of interstellar hydrogen atoms, for the parent populations with the relative bulk velocity of 20 km s$^{-1}$ and equal temperatures of 7500 K, has ~2 km s$^{-1}$ higher bulk velocity if the scattering is taken into account. Additionally, temperatures are higher by ~2400 K and ~1200 K in parallel and perpendicular direction to the relative motion of parent populations, respectively. Moreover, a significant departure of secondary atoms from the Maxwell-Boltzmann distribution is expected for high relative velocities of parent populations. This process affects the distribution and density of interstellar atoms in the heliosphere and production of pickup ions. Thus, we show that angular scattering in charge exchange collisions is important to include in analyses of interstellar neutral atoms and pickup ions observed at 1 au and in the outer heliosphere., 22 pages, 13 figures, to appear in the Astrophysical Journal

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

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

Interstellar neutral gas atoms penetrate the heliopause and reach 1~au, where they are detected by IBEX. The flow of neutral interstellar helium through the perturbed interstellar plasma in the outer heliosheath (OHS) results in creation of the secondary population of interstellar He atoms, the so-called Warm Breeze, due to charge exchange with perturbed ions. The secondary population brings the imprint of the OHS conditions to the IBEX-Lo instrument. Based on a global simulation of the heliosphere with measurement-based parameters and detailed kinetic simulation of the filtration of He in the OHS, we find the number density of interstellar He$^+$ population at $(8.98\pm 0.12)\times 10^{-3}$~cm$^{-3}$. With this, we obtain the absolute density of interstellar H$^+$ $5.4\times 10^{-2}$~cm$^{-3}$ and electrons $6.3\times 10^{-2}$~cm$^{-3}$, and ionization degrees of H 0.26 and He 0.37. The results agree with estimates of the Very Local Interstellar Matter parameters obtained from fitting the observed spectra of diffuse interstellar EUV and soft X-Ray background., Accepted for ApJ

The ribbon of enhanced energetic neutral atom flux, discovered by the Interstellar Boundary Explorer (IBEX) in 2009, has redefined our understanding of the heliosphere’s interaction with the local interstellar medium (LISM). Yet, its origin continues to be a topic of scientific debate. The ribbon is circular and traces the region where the putative LISM magnetic field (B LISM) is perpendicular to the radial direction from the Sun. Using nine years of IBEX-Hi observations, we investigate the ribbon circularity and location as functions of time and energy. We provide updated locations of the ribbon center at five energy passbands (centered at 0.7, 1.1, 1.7, 2.7, and 4.3 keV) in ecliptic coordinates [longitude, latitude]: [217.°41 ± 0.°95, 44.°36 ± 0.°93], [219.°72 ± 0.°95, 41.°50 ± 0.°87], [220.°51 ± 1.°19, 39.°96 ± 1.°00], [218.°08 ± 1.°66, 38.°44 ± 1.°24], and [214.°68 ± 1.°48, 34.°13 ± 1.°19] respectively. The weighted mean center location over all energies and all years is [218.°33 ± 0.°68, 40.°38 ± 0.°88] and its radius is 74.°81 ± 0.°65. As viewed by IBEX at 1 au, we find that (1) the ribbon is stable over time, with distinct centers at each energy; (2) ribbon centers exhibit small temporal variations, likely caused by the solar wind (SW) speed and density variations; and (3) ribbon location in the sky appears to be driven by (i) the inherent alignment of the ribbon centers along the plane connecting the presumed B LISM and the heliospheric upwind direction, and (ii) the variable SW structure along the heliographic meridian, further emphasizing that the ribbon source is outside the heliosphere.

New Horizons unveils the Pluto system In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanity's first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H 2 O, CH 4 , CO, N 2 , and NH 3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come. Science , this issue pp. 1284 , 10.1126/science.aad9189 , 10.1126/science.aad8866 , 10.1126/science.aae0030 , & 10.1126/science.aad9045

Since launch in Oct. 2008, IBEX, with its two energetic neutral atom (ENA) cameras, has provided humankind with the first-ever global images of the complex boundary separating the heliosphere from the local interstellar medium (LISM). IBEX’s energy-resolved all-sky maps, collected every six months, are yielding remarkable new insights into the heliospheres structure as it is shaped by the combined forces of the local interstellar flow, the local interstellar magnetic field (LISMF), and the evolving solar wind. IBEX has also acquired the first images of ENAs backscattered from the surface of the moon as well as global images of the magnetospheric response to solar wind disturbances. IBEX thus addresses all three Heliophysics science objectives set forth in the 2014 Science Plan for NASAs Science Mission Directorate (SMD) as well as the goals in the recent Solar and Space Physics Decadal Survey (NRC 2012). In addition, with the information it provides on the properties of the LISM and the LISMF, IBEX represents a unique bridge between heliophysics and astrophysics, and fills in critical knowledge for understanding the habitability of exoplanetary systems and the future habitability of Earth and the solar system. Because of the few-year time lag due to solar wind and ENA transport, IBEX observed the solar wind/ LISM interaction characteristic of declining phase/solar minimum conditions. In the continuing mission, IBEX captures the response of the interstellar boundaries to the changing structure of the solar wind in its transition toward the “mini” solar maximum and possibly the decline into the next solar minimum. The continuing IBEX mission affords never-to-be-repeated opportunities to coordinate global imaging of the heliospheric boundary with in-situ measurements by the Voyagers as they pass beyond the heliopause and start to directly sample the LISM.

The Pluto system was recently explored by NASA's New Horizons spacecraft, making closest approach on 14 July 2015. Pluto's surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto's atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto's diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto's large moon Charon displays tectonics and evidence for a heterogeneous crustal composition, its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected., 8 pages - Initial Science paper from NASA's New Horizons Pluto Encounter

We seek to understand the quantitative role of the dominant physical processes (charge-exchange, adiabatic heating, stochastic acceleration) governing the proton distribution in the heliotail using observations of hydrogen energetic neutral atoms (ENAs) from the Interstellar Boundary Explorer (IBEX ). We solve the Parker transport equation for solar wind protons and pickup ions (PUIs) as they propagate from the termination shock (TS) down the heliotail, including charge-exchange between protons and neutral hydrogen atoms as source terms derived from an MHD-fluid and kinetic-neutral simulation of the heliosphere. We compute ENA fluxes at 1 au from the results of the proton transport model and compare them with IBEX observations. We find that, under the assumptions of our model, a stochastic acceleration process is needed to counteract the energy-dependent losses of ∼0.1–5 keV PUIs from charge-exchange to reproduce IBEX data. The power-law velocity dependence of the diffusion coefficient (spectral index γ) is limited to the range 0.67

In-situ measurements taken by the Voyager 2 spacecraft suggest that the solar wind termination shock is significantly affected by the presence of pickup ions that are produced in the inner heliosphere due to charge exchange between interstellar neutrals and the solar wind ions. We use a fully kinetic particle-in-cell method to self-consistently simulate the shock with all physical properties available from Voyager 2. We have performed a set of simulations with varying velocity distribution functions for the pickup ions, since it was not determined by Voyager's measurements. We show that the measurements suggest that the pickup ions upstream of the shock are more energetic than generally believed. If their velocity distribution function assumes a filled-shell shape in the wind frame, the maximum cutoff speed for the pickup ions should be 650 km s−1 in order to reproduce the measurements, which is almost twice the local wind speed. We suggest that pickup ions upstream of the shock are energized by adiabatic compression of the solar wind plasma as well as due to an enhanced level of turbulence in a broad foreshock region.

In late 2014, the solar wind dynamic pressure increased by ~50% over a relatively short time (~6 months). In early 2017, the Interstellar Boundary Explorer (IBEX) observed an increase in heliospheric energetic neutral atom (ENA) fluxes from directions near the front of the heliosphere. These enhanced ENA emissions resulted from the increase in SW pressure propagating through the inner heliosheath (IHS), affecting the IHS plasma pressure and emission of ~keV ENA fluxes. We expand on the analysis by McComas et al. on the effects of this pressure change on ENA fluxes observed at 1 au using a three-dimensional, time-dependent simulation of the heliosphere. The pressure front has likely already crossed the termination shock (TS) in all directions, but ENA fluxes observed at 1 au will change over the coming years, as the TS, heliopause, and IHS plasma pressure continue to change in response to the SW pressure increase. Taken in isolation, the pressure front creates a "ring" of increasing ENA fluxes projected in the sky that expands in angular radius over time, as a function of the distances to the heliosphere boundaries and the ENA propagation speed. By tracking the position of this ring over time in our simulation, we demonstrate a method for estimating the distances to the TS, heliopause, and ENA source region that can be applied to IBEX data. This will require IBEX observations at 4.3 keV up through ~2020, and longer times at lower ENA energies, in order to observe significant changes from the heliotail.

Our heliosphere—the bubble in the local interstellar medium produced by the Sun's outflowing solar wind—has finally responded to a large increase in solar wind output and pressure in the second half of 2014. NASA's Interstellar Boundary Explorer (IBEX) mission remotely monitors the outer heliosphere by observing energetic neutral atoms (ENAs) returning from the heliosheath, the region between the termination shock and heliopause. IBEX observed a significant enhancement in higher energy ENAs starting in late 2016. While IBEX observations over the previous decade reflected a general reduction of ENA intensities, indicative of a deflating heliosphere, new observations show that the large (~50%), persistent increase in the solar wind dynamic pressure has modified the heliosheath, producing enhanced ENA emissions. The combination of these new observations with simulation results indicate that this pressure is re-expanding our heliosphere, with the termination shock and heliopause already driven outward in the locations closest to the Sun. The timing between the IBEX observations, a large transient pressure enhancement seen by Voyager 2, and the simulations indicates that the pressure increase propagated through the heliosheath, reflected off the heliopause, and the enhanced density of the solar wind filled the heliosheath behind it before generating significantly enhanced ENA emissions. The coming years should see significant changes in anomalous cosmic rays, galactic cosmic radiation, and the filtration of interstellar neutral atoms into the inner heliosphere.

The Interstellar Boundary Explorer (IBEX) has now operated in space for 7 years and returned nearly continuous observations that have led to scientific discoveries and reshaped our entire understanding of the outer heliosphere and its interaction with the local interstellar medium. Here we extend prior work, adding the 2014-2015 data for the first time, and examine, validate, initially analyze, and provide a complete 7-year set of Energetic Neutral Atom (ENA) observations from ~0.1 to 6 keV. The data, maps, and documentation provided here represent the 10th major release of IBEX data and include improvements to various prior corrections to provide the citable reference for the current version of IBEX data. We are now able to study time variations in the outer heliosphere and interstellar interaction over more than half a solar cycle. We find that the Ribbon has evolved differently than the globally distributed flux (GDF), with a leveling off and partial recovery of ENAs from the GDF, owing to solar wind output flattening and recovery. The Ribbon has now also lost its latitudinal ordering, which reflects the breakdown of solar minimum solar wind conditions and exhibits a greater time delay than for the surrounding GDF. Together, the IBEX observations strongly support a secondary ENA source for the Ribbon, and we suggest that this be adopted as the nominal explanation of the Ribbon going forward., Comment: Pre-print, 59 pages, 33 figures, published in ApJS