As the Sun moves through the local interstellar medium, its supersonic, ionized solar wind carves out a cavity called the heliosphere. Recent observations from the Interstellar Boundary Explorer (IBEX) spacecraft show that the relative motion of the Sun with respect to the interstellar medium is slower and in a somewhat different direction than previously thought. Here, we provide combined consensus values for this velocity vector and show that they have important implications for the global interstellar interaction. In particular, the velocity is almost certainly slower than the fast magnetosonic speed, with no bow shock forming ahead of the heliosphere, as was widely expected in the past.

Simulations of energetic neutral atom (ENA) maps predict flux magnitudes that are, in some cases, similar to those observed by the Interstellar Boundary Explorer (IBEX) spacecraft, but they miss the ribbon. Our model of the heliosphere indicates that the local interstellar medium (LISM) magnetic field (B(LISM)) is transverse to the line of sight (LOS) along the ribbon, suggesting that the ribbon may carry its imprint. The force-per-unit area on the heliopause from field line draping and the LISM ram pressure is comparable with the ribbon pressure if the LOS approximately 30 to 60 astronomical units and B(LISM) approximately 2.5 microgauss. Although various models have advantages in accounting for some of the observations, no model can explain all the dominant features, which probably requires a substantial change in our understanding of the processes that shape our heliosphere.

The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October 2008 with the sole, focused science objective to discover the global interaction between the solar wind and the interstellar medium. IBEX is designed to achieve this objective by answering four fundamental science questions: (1) What is the global strength and structure of the termination shock, (2) How are energetic protons accelerated at the termination shock, (3) What are the global properties of the solar wind flow beyond the termination shock and in the heliotail, and (4) How does the interstellar flow interact with the heliosphere beyond the heliopause? The answers to these questions rely on energyresolved images of energetic neutral atoms (ENAs), which originate beyond the termination shock, in the inner heliosheath. To make these exploratory ENA observations IBEX carries two ultra-high sensitivity ENA cameras on a simple spinning spacecraft. IBEX's very high apogee Earth orbit was achieved using a new and significantly enhanced method for launching small satellites; this orbit allows viewing of the outer heliosphere from beyond the Earth's relatively bright magnetospheric ENA emissions. The combination of full-sky imaging and energy spectral measurements of ENAs over the range from ∼10 eV to 6 keV provides the critical information to allow us to achieve our science objective and understand this global interaction for the first time. The IBEX mission was developed to provide the first global views of the Sun's interstellar boundaries, unveiling the physics of the heliosphere's interstellar interaction, providing a deeper understanding of the heliosphere and thereby astrospheres throughout the galaxy, and creating the opportunity to make even greater unanticipated discoveries.

Abstract.A coordinated effort to combine all three methods that are used to determine the physical parameters of interstellar gas in the heliosphere has been undertaken. In order to arrive at a consistent parameter set that agrees with the observations of neutral gas, pickup ions and UV backscattering we have combined data sets from coordinated observation campaigns over three years from 1998 through 2000. The key observations include pickup ions with ACE and Ulysses SWICS, neutral atoms with Ulysses GAS, as well as UV backscattering at the He focusing cone close to the Sun with SOHO UVCS and at 1 AU with EUVE. For the first time also the solar EUV irradiance that is responsible for photo ionization was monitored with SOHO CELIAS SEM, and the He I 58.4 nm line that illuminates He was observed simultaneously with SOHO SUMER. The solar wind conditions were monitored with SOHO, ACE, and WIND. Based on these data the modeling of the interstellar gas and its secondary products in the heliosphere has resulted in a consistent set of interstellar He parameters with much reduced uncertainties, which satisfy all observations, even extended to earlier data sets. It was also established that a substantial ionization in addition to photo ionization, most likely electron impact, is required, with increasing relative importance closer to the Sun. Furthermore, the total combined ionization rate varies significantly with solar latitude, requiring a fully three dimensional and time dependent treatment of the problem.

Abstract. The helium gravitational focusing cone has been observed using pickup He + , first during the solar minimum in [1984][1985] with the AMPTE/IRM spacecraft, and again in more detail from 1998 to 2002 with ACE and in 2000 with Nozomi. Five traversals of the cone allow us to obtain an accurate determination of the ecliptic longitude of the interstellar wind flow direction, λ = 74.43 • ± 0.33 • , while observations of pickup He ++ with Ulysses give us an estimate, relatively free of instrumental systematic uncertainties, of the neutral He density, n He = 0.0151 ± 0.0015 cm −3 , in the Local Interstellar Cloud. From best fits to the measured velocity distributions of pickup He + using time-stationary models we deduce the radial dependence and magnitude of electron-impact ionization rates that cannot presently be measured, and find this to be an important ionization process in the inner ( < ∼ 0.5 AU) heliosphere. We obtain excellent model fits to the 1998 cone profile using measured or deduced rates and known interstellar He parameters, and from this conclude that cross-field diffusion of pickup He + is small. Furthermore, we find no evidence for extra sources of He in or near the cone region. Best fits to the velocity distributions of He + are obtained assuming isotropic solar-wind-frame distributions, and we conclude from this that the scattering mean free path for pickup He + in the turbulent slow solar wind is small, probably less than 0.1 AU. We argue that application of 3D, time-dependent models for computation of the spatial distribution of interstellar neutral helium in the inner heliosphere may lead to excellent fits of short-term averaged pickup He + data without assuming loss rates that are significantly different from production rates.

Abstract. Energetic neutral atom (ENA) imaging is a powerful technique, which can remotely probe the properties of distant hot plasmas. Hot plasmas are abundant at the heliospheric boundary, the region where the expanding solar wind meets the surrounding local interstellar cloud. Here we present a new concept for imaging this boundary in ENA fluxes. Heliospheric ENAs are born from charge exchange between energetic protons and the background interstellar atomic hydrogen gas. The technique is ideal for studying the asymmetric threedimensional heliospheric interface region remotely, from 1 AU. We show that ENA imaging in the 0.2-6 keV energy range will establish the nature of the telTnination shock and properties of hot proton populations in the heliosheath. We also examine how the evolution of pickup proton populations at and beyond the shock can be explored. Global heliosphere ENA images will distinguish among the competing models of the interaction between the Sun and the local interstellar medium, and they will reveal the physics of important processes in the interface region. Heliospheric ENA fluxes are exceptionally weak, which makes imaging implementation difficult. Nonetheless, we show how single-pixel ENA sensors can image the heliosphere from a spicming spacecraft on a typical mission near 1 AU. The required instrumentation is briefly discussed. HeliosphereThe interaction of the expanding solar wind plasma and local interstellar medium (

Abstract. Circumterrestrial Lyman-α column brightness observations above 3 Earth radii (R e ) have been used to derive separate 3-D neutral hydrogen density models of the Earth's exosphere for solar minimum (2008, 2010) and near-solarmaximum (2012) conditions. The data used were measured by Lyman-α detectors (LAD1/2) onboard each of the TWINS satellites from very different orbital positions with respect to the exosphere. Exospheric H atoms resonantly scatter the near-line-center solar Lyman-α flux at 121.6 nm. Assuming optically thin conditions above 3 R e along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H-column density. We found significant differences in the density distribution of the terrestrial exosphere under different solar conditions. Under solar maximum conditions we found higher H densities and a larger spatial extension compared to solar minimum. After a continuous, 2-month decrease in (27 day averaged) solar activity, significantly lower densities were found. Differences in shape and orientation of the exosphere under different solar conditions exist. Above 3 R e , independent of solar activity, increased H densities appear on the Earth's nightside shifted towards dawn. With increasing distance (as measured at 8 R e ) this feature is shifted westward/duskward by between −4 and −5 • with respect to midnight. Thus, at larger geocentric distance the exosphere seems to be aligned with the aberrated Earth-solar-wind line, defined by the solar wind velocity and the orbital velocity of the Earth. The results presented in this paper are valid for geocentric distances between 3 and 8 R e .

The Interstellar Mapping and Acceleration Probe (IMAP) is a revolutionary mission that simultaneously investigates two of the most important overarching issues in Electronic supplementary material The online version of this article (