The Sun moves through the local interstellar medium, continuously emitting ionized, supersonic solar wind plasma and carving out a cavity in interstellar space called the heliosphere. The recently ...launched Interstellar Boundary Explorer (IBEX) spacecraft has completed its first all-sky maps of the interstellar interaction at the edge of the heliosphere by imaging energetic neutral atoms (ENAs) emanating from this region. We found a bright ribbon of ENA emission, unpredicted by prior models or theories, that may be ordered by the local interstellar magnetic field interacting with the heliosphere. This ribbon is superposed on globally distributed flux variations ordered by both the solar wind structure and the direction of motion through the interstellar medium. Our results indicate that the external galactic environment strongly imprints the heliosphere.
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 papersummarizes the IMAP mission at the start of Phase A development.
The Interstellar Boundary Explorer (IBEX) has obtained all-sky images of energetic neutral atoms emitted from the heliosheath, located between the solar wind termination shock and the local ...interstellar medium (LISM). These flux maps reveal distinct nonthermal (0.2 to 6 kilo-electron volts) heliosheath proton populations with spectral signatures ordered predominantly by ecliptic latitude. The maps show a globally distributed population of termination-shock-heated protons and a superimposed ribbonlike feature that forms a circular arc in the sky centered on ecliptic coordinate (longitude λ, latitude β) = (221°, 39°), probably near the direction of the LISM magnetic field. Over the IBEX energy range, the ribbon's nonthermal ion pressure multiplied by its radial thickness is in the range of 70 to 100 picodynes per square centimeter AU (AU, astronomical unit), which is significantly larger than the 30 to 60 picodynes per square centimeter AU of the globally distributed population.
First data from NASA's Interstellar Boundary EXplorer (IBEX) mission show a striking 'ribbon' feature of enhanced energetic neutral atom (ENA) emission. The enhancement in flux is between 2 and 3 ...times greater than adjacent regions of the sky. Yet the spectral index of ENAs appears to be the same both inside and outside the ribbon. While the ribbon itself was not predicted by any models of the heliospheric interface, its geometry appears to be related to the predicted interstellar magnetic field (ISMF) outside the heliopause (HP). In this Letter, we examine a process of ENA emission from the outer heliosheath, based on a source population of non-isotropic pick-up ions that themselves originate as ENAs from inside the HP. We find that our simplistic approach yields a ribbon of enhanced ENA fluxes as viewed from the inner heliosphere with a spatial location and ENA flux similar to the IBEX measurements, with the provision that the ions retain a partial shell distribution long enough for the ions to be neutralized. As a corollary, our idealized simulation of this mechanism suggests that ISMF is likely oriented close to the center of the observed ribbon.
Context. The boundary conditions of the heliosphere are set by the ionization, density, and composition of inflowing interstellar matter. Aims. Our aim is to constrain the properties of the Local ...Interstellar Cloud (LIC) at the heliosphere, which requires radiative transfer ionization models. Methods. We modeled the background interstellar radiation field using observed stellar FUV and EUV emission and the diffuse soft X-ray background. We also modeled the emission from the boundary between the LIC and the hot Local Bubble plasma, assuming that the cloud is evaporating because of thermal conduction. We created a grid of models covering a plausible range of LIC and Local Bubble properties, and used the modeled radiation field as input to radiative transfer/thermal equilibrium calculations using the Cloudy code. Data from in situ observations of He super(0), pickup ions and anomalous cosmic rays in the heliosphere, as well as from absorption line measurements towards epsilon CMa were used to constrain the input parameters. Results. A restricted range of assumed LIC H I column densities and Local Bubble plasma temperatures produced models that match all the observational constraints. The relative weakness of the constraints on N (H I) and T_{h} contrast with the narrow limits predicted for the H super(0) and electron density in the LIC at the Sun, n (H super(0)) = 0.19-0.20 cm super(-3) and n_{e} = 0.07 pm 0.01 cm super(-3). Derived abundances are mostly typical of low-density gas, with sub-solar Mg, Si, and Fe, possibly subsolar O and N, and S about solar; however, C is supersolar. Conclusions. The interstellar gas at the Sun is warm, low-density, and partially ionized, with n (H) approx 0.23-0.27 cm super(-3), T approx 6300 K, X (H super(+)) similar to 0.2, and X (He super(+)) similar to 0.4. These results appear to be robust since acceptable models are found for substantially different input radiation fields. Our results favor low values for the reference solar abundances for the LIC composition.
Observations from the Interstellar Boundary Explorer (IBEX) of energetic neutral atoms (ENAs) reveal two populations, those emitted from a narrow (∼20°-40°) ribbon that is centered on the local ...interstellar magnetic field, and a globally distributed flux (GDF) that is controlled by processes in the heliosheath. This is a third study utilizing a previously developed technique to separate ENA emissions in the ribbon from the GDF. In the first ribbon separation study, we analyzed the first year of IBEX data at the energies of 0.7 keV and above; the second study analyzed data down to 0.2 keV using the first five years of IBEX data. Here, we utilize the separation analysis from 0.7 keV and above to study time evolution in 3 year intervals over the first nine years of IBEX data. This study is the first to reveal the global time evolution of the GDF distinct from that of the IBEX ribbon. We show that the time evolution of the GDF within 40° of the upwind pressure maximum is driven by changes in the solar wind ram pressure through compression and rarefaction in the heliosheath. In contrast, the GDF is relatively stable in the region centered on the heliotail downwind with respect to the interstellar flow. The evolution of the IBEX ribbon is observed to have a time lag with respect to the upwind GDF evolution, likely due to the secondary (neutral) solar wind source. The time lag observed in the ribbon evolution is consistent with the generation of ions retained for several years beyond the heliopause. These observations lend further support to secondary solar wind models of the IBEX ribbon, but also require that there is a significant several year time lag for reneutralization of ions that form the IBEX ribbon. We use this study of the 9 year separation of the IBEX ribbon from the globally distributed flux to prepare for a formal IBEX data release of ribbon and globally distributed flux maps to the heliophysics community.
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 (BLISM) 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 BLISM 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.
ABSTRACT The Interstellar Boundary Explorer (IBEX) has been directly observing neutral atoms from the local interstellar medium for the last six years (2009-2014). This paper ties together the 14 ...studies in this Astrophysical Journal Supplement Series Special Issue, which collectively describe the IBEX interstellar neutral results from this epoch and provide a number of other relevant theoretical and observational results. Interstellar neutrals interact with each other and with the ionized portion of the interstellar population in the "pristine" interstellar medium ahead of the heliosphere. Then, in the heliosphere's close vicinity, the interstellar medium begins to interact with escaping heliospheric neutrals. In this study, we compare the results from two major analysis approaches led by IBEX groups in New Hampshire and Warsaw. We also directly address the question of the distance upstream to the pristine interstellar medium and adjust both sets of results to a common distance of ∼1000 AU. The two analysis approaches are quite different, but yield fully consistent measurements of the interstellar He flow properties, further validating our findings. While detailed error bars are given for both approaches, we recommend that for most purposes, the community use "working values" of ∼25.4 km s−1, ∼75 7 ecliptic inflow longitude, ∼ −5 1 ecliptic inflow latitude, and ∼7500 K temperature at ∼1000 AU upstream. Finally, we briefly address future opportunities for even better interstellar neutral observations to be provided by the Interstellar Mapping and Acceleration Probe mission, which was recommended as the next major Heliophysics mission by the NRC's 2013 Decadal Survey.
Light from nearby stars becomes linearly polarized while traversing a dichroic interstellar medium. Polarization data which reveal heliosphere features are summarized. Polarizing dust grains trace a ...magnetic field direction consistent with the field affecting the warm breeze of secondary interstellar helium found by IBEX. Polarization data also trace the field direction that dominates the configuration of the IBEX ribbon of energetic neutral atoms (ENA). The polarizations that echo heliospheric features appear to arise from magnetically-aligned submicron interstellar dust grains that are excluded from the inner heliosphere by large charge-to-mass ratios, but, due to long collision times, retain their alignment with respect to the interstellar magnetic field draping over the heliosphere.
Interstellar neutral gas atoms penetrate the heliopause and reach 1 au, where they are detected by Interstellar Boundary Explorer (IBEX). The flow of neutral interstellar helium through the perturbed ...interstellar plasma in the outer heliosheath (OHS) results in the creation of a 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 the interstellar He+ population to be (8.98 0.12) × 10−3 cm−3. With this, we obtain the absolute density of interstellar H+ as 5.4 × 10−2 cm−3 and that of electrons as 6.3 × 10−2 cm−3, with ionization degrees of 0.26 for H and 0.37 for He. The results agree with estimates of the parameters of the Very Local Interstellar Matter obtained from fitting the observed spectra of diffuse interstellar EUV and the soft X-ray background.
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