Interstellar Boundary Explorer Spacecraft Research Articles

Oblique Electromagnetic Instabilities Driven by Pickup Ion Ring-beam Distributions in the Outer Heliosheath. I. Linear Instability Analysis

The energetic neutral atom ribbon observed by the Interstellar Boundary Explorer spacecraft is believed to originate from the pickup ions in the outer heliosheath. The outer heliosheath pickup ions generally have a ring-beam velocity distribution at a certain pickup angle, α, the angle at which these ions are picked up by the interstellar magnetic field. The pickup ion ring-beam distributions can drive unstable waves of different propagation angles with respect to the background interstellar magnetic field, θ. Previous studies of the outer heliosheath pickup ion dynamics were mainly focused on ring-like pickup ion distributions with α ≈ 90° and/or the parallel- and antiparallel-propagating unstable waves (θ = 0° and 180°). The present study carries out linear kinetic instability analysis to investigate both the parallel and oblique unstable modes (0° ≤ θ ≤ 180°) driven by ring-beam pickup ion distributions of different pickup angles between 0° and 90°. Our linear instability analysis reveals that ring-beam pickup ion distributions can excite oblique mirror waves as well as parallel/quasi-parallel and oblique right- and left-helicity waves. The maximum growth rate among all the instabilities belongs to the parallel-propagating left-helicity waves at most pickup angles. Furthermore, the evolution of the unstable mirror waves by varying pickup angle indicates that as the pickup angle increases, the maximum growth rate of the mirror modes increases, while its propagation angle decreases.

Whence the Interstellar Magnetic Field Shaping the Heliosphere?

Abstract Measurements of starlight polarized by aligned interstellar dust grains are used to probe the relation between the orientation of the ambient interstellar magnetic field (ISMF) and the ISMF traced by the ribbons of energetic neutral atoms discovered by the Interstellar Boundary Explorer spacecraft. We utilize polarization data, many acquired specifically for this study, to trace the configuration of the ISMF within 40 pc. A statistical analysis yields a best-fit ISMF orientation, B magpol, aligned with Galactic coordinates ℓ = 42°, b = 49°. Further analysis shows the ISMF is more orderly for “downfield” stars located over 90° from B magpol. The data subset of downfield stars yields an orientation for the nearby ISMF at ecliptic coordinates λ, β ≈ 219° ± 15°, 43° ± 9° (Galactic coordinates l, b ≈ 40°, 56°, ±17°). This best-fit ISMF orientation from polarization data is close to the field direction obtained from ribbon models. This agreement suggests that the ISMF shaping the heliosphere belongs to an extended ordered magnetic field. Extended filamentary structures are found throughout the sky. A previously discovered filament traversing the heliosphere nose region, “Filament A,” extends over 300° of the sky, and crosses the upwind direction of interstellar dust flowing into the heliosphere. Filament A overlaps the locations of the Voyager kilohertz emissions, three quasar intraday variables, cosmic microwave background (CMB) components, and the inflow direction of interstellar grains sampled by Ulysses and Galileo. These features are likely located in the upstream outer heliosheath where ISMF drapes over the heliosphere, suggesting Filament A coincides with a dusty magnetized plasma. A filament 55° long is aligned with a possible shock interface between local interstellar clouds. A dark spot in the CMB is seen within 5° of the filament and within 10° of the downfield ISMF direction. Two large magnetic arcs are centered on the directions of the heliotail. The overlap between CMB components and the aligned dust grains forming Filament A indicates the configuration of dust entrained in the ISMF interacting with the heliosphere provides a measurable foreground to the CMB.

Termination Shock Measured by Voyagers and IBEX

Abstract Our heliosphere’s innermost boundary—the termination shock—slows and heats the supersonic solar wind and energizes anomalous cosmic rays (ACRs). We show that in addition to their termination shock crossings, the Voyager 1 and 2 spacecraft measurements identify additional points on the termination shock when they magnetically disconnect from the ACR source. These four points define a spherical approximation of the termination shock with radius 117 au, offset ∼32 au tailward, ∼27 au north, and ∼12 au to the port side of the Sun. Interstellar Boundary Explorer (IBEX) spacecraft observations independently confirm these general offsets, with the closest region of the termination shock ∼ 20° south of the interstellar inflow direction and a minimum distance ∼74 au. The maximum distance is ∼161 au, consistent with required ACR acceleration times. Thus, Voyager and IBEX spacecraft observations have directly revealed the global size and location of our heliosphere’s termination shock for the first time.

Collisional Heating of Interstellar Helium Flux at 1 AU

AbstractBecause of relative motion of the local interstellar medium (LISM) with respect to the Sun, the interstellar wind, interstellar neutral helium atoms enter the heliosphere. Instruments on the Ulysses and Interstellar Boundary Explorer spacecraft measured directional dependencies of intensities, or sky maps, of helium atoms and determined the interstellar wind velocity vector and temperature. The article focuses on heretofore unaccounted for heating of inflowing interstellar helium by elastic collisions with the solar wind ions and its effect on the LISM temperature obtained from the Interstellar Boundary Explorer‐type measurement. The calculations show that inferred LISM temperatures appear 175–270 K hotter than the actual interstellar gas due to collisions.

Secondary Interstellar Oxygen in the Heliosphere: Numerical Modeling and Comparison with IBEX-Lo Data

Abstract Quantitative analysis of the interstellar heavy (oxygen and neon) atom fluxes obtained by the Interstellar Boundary Explorer (IBEX) suggests the existence of the secondary interstellar oxygen component. This component is formed near the heliopause due to charge exchange of interstellar oxygen ions with hydrogen atoms, as was predicted theoretically. A detailed quantitative analysis of the fluxes of interstellar heavy atoms is only possible with a model that takes into account both the filtration of primary and the production of secondary interstellar oxygen in the boundary region of the heliosphere as well as a detailed simulation of the motion of interstellar atoms inside the heliosphere. This simulation must take into account photoionization, charge exchange with the protons of the solar wind and solar gravitational attraction. This paper presents the results of modeling interstellar oxygen and neon atoms through the heliospheric interface and inside the heliosphere based on a three-dimensional kinetic-MHD model of the solar wind interaction with the local interstellar medium and a comparison of these results with the data obtained on the IBEX spacecraft.

Kinetic modelling of primary and secondary interstellar oxygen atom fluxes in the heliosphere

The interstellar neutral (ISN) oxygen atoms penetrate into the heliosphere from the Local Interstellar Medium (LISM). Oxygen atoms that directly penetrate into the heliosphere are called primary, while secondary oxygen population is formed near the heliopause due to the process of charge exchange of interstellar oxygen ions with hydrogen atoms and its existence in the heliosphere was previously predicted theoretically (Izmodenov et al., 1997, 1999, 2004). In this work we study the distribution of both primary and secondary oxygen atoms in the heliosphere and its fluxes at 1 AU.The analysis of the distribution of ISN O atoms in the heliosphere is only possible in the frame of a model that takes into account both filtration of the primary and origin of the secondary interstellar oxygen in the region of interaction of the solar wind (SW) with the LISM, as well as a detailed simulation of the motion of interstellar atoms inside the heliosphere. This simulation should take into account the temporal and heliolatitudinal dependences of ionization, the process of charge exchange with SW protons and the effect of the solar gravitational attraction. This paper presents the results of modelling ISN O atoms inside the heliosphere based on a new three-dimensional kinetic-MHD model of the SW interaction with the LISM (Izmodenov and Alexashov, 2015). Oxygen atoms are of a particular interest because they are strongly coupled with the protons through the charge exchange process and due to the fact that the fluxes of interstellar oxygen are directly measured by the Interstellar Boundary Explorer (IBEX). The first quantitative measurements of the interstellar heavy neutral atoms (O and Ne) obtained on the IBEX spacecraft were presented in (Park et al., ApJS, 2015). Qualitative analysis of these data shows that the secondary component of the interstellar oxygen atoms was also measured along with the primary interstellar atoms.

NEUTRAL ATOM PROPERTIES IN THE DIRECTION OF THE IBEX RIBBON.

In this paper, we present results from our three-dimensional (3D) simulations of the interaction between the solar wind and local interstellar medium with an emphasis on the phase-space properties of energetic neutral atoms (ENAs) along a sight line that intersects the ribbon of enhanced ENA flux seen by NASA's Interstellar Boundary EXplorer spacecraft. The majority of these ENAs have velocities directed away from the heliosphere, but it is believed that interactions between heliospheric ENAs and ions outside the heliosphere may result in a population of secondary ENAs that return to the heliosphere and generate the ribbon. While we do not consider the ion dynamics that result in secondary ENAs, our analysis is of key importance to the process since the heliospheric ENAs we consider form the source population for those ions. We present the moments of the hydrogen distribution, along with moments parallel and perpendicular to the local magnetic field for the pick-up ions (PUIs) that these neutrals generate. Finally, we present gyro-averaged velocity distributions relative to the local magnetic field for the PUIs created from our simulated H-atoms, along with analytic fits to these distributions in the secondary ENA source region just beyond the heliopause.

WARMER LOCAL INTERSTELLAR MEDIUM: A POSSIBLE RESOLUTION OF THEULYSSES-IBEXENIGMA

Interstellar Boundary Explorer (IBEX) measurements from 2009-2010 identified a set of possible solutions with very tight coupling between the interstellar He inflow longitude, latitude, speed, and temperature. The center of this allowable parameter space suggested that the heliosphere could be moving more slowly and in a slightly different direction with respect to the interstellar medium than indicated by earlier Ulysses observations. In this study we examine data from 2012-2014 and compare results from an analytic analysis and a detailed computer model. For observations where the IBEX spacecraft pointing is near the ecliptic plane, the latest measurements indicate a different portion of IBEX's four-dimensional tube of possible parameters—one that is more consistent with the Ulysses flow direction and speed, but with a much higher temperature. Together, the current combined IBEX/Ulysses values we obtain are V ISM∞ ~ 26 km s–1, λISM∞ ~ 75°, βISM∞ ~ –5°, and T He∞ ~ 7000-9500 K. These indicate that the heliosphere is in a substantially warmer region of the interstellar medium than thought from the earlier Ulysses observations alone, and that this warmer region may be roughly isothermal. However, measurements taken when IBEX was pointing ~5° south of the ecliptic are inconsistent with this solution and suggest a slower speed, lower temperature, and flow direction similar to IBEX's prior central values. IBEX measures much deeper into the tails of the distributions of the inflowing interstellar material than Ulysses did and these observations indicate that the heliosphere's interstellar interaction is likely far more complex and interesting than previously appreciated.

Interstellar Gas Flow Vector and Temperature Determination over 5 Years of IBEX Observations

The Interstellar Boundary Explorer (IBEX) observes the interstellar neutral gas flow trajectories at their perihelion in Earth's orbit every year from December through early April, when the Earth's orbital motion is into the oncoming flow. These observations have defined a narrow region of possible, but very tightly coupled interstellar neutral flow parameters, with inflow speed, latitude, and temperature as well-defined functions of inflow longitude. The best- fit flow vector is different by ≈ 3° and lower by ≈ 3 km/s than obtained previously with Ulysses GAS, but the temperature is comparable. The possible coupled parameter space reaches to the previous flow vector, but only for a substantially higher temperature (by ≈ 2000 K). Along with recent pickup ion observations and including historical observations of the interstellar gas, these findings have led to a discussion, whether the interstellar gas flow into the solar system has been stable or variable over time. These intriguing possibilities call for more detailed analysis and a longer database. IBEX has accumulated observations over six interstellar flow seasons. We review key observations and refinements in the analysis, in particular, towards narrowing the uncertainties in the temperature determination. We also address ongoing attempts to optimize the flow vector determination through varying the IBEX spacecraft pointing and discuss related implications for the local interstellar cloud and its interaction with the heliosphere.

An MHD model of Ganymede's mini‐magnetosphere suggests that the heliosphere forms in a sub‐Alfvénic flow

AbstractDirectional fluxes of energetic neutral atoms (ENAs) measured by the Interstellar Boundary Explorer spacecraft reveal a Ribbon of strong emission whose source lies beyond the termination shock. Intense emissions of ENAs (energies 0.1 to 6 keV) appear along an extended arc significantly displaced from the nose of the heliosphere, the point on the boundary defined by the Sun's motion relative to the local interstellar medium (LISM). The locus of the Ribbon differs from expectations based on early models of the interaction of the solar wind with the LISM, assumed to flow at a super‐Alfvénic speed. Here we argue that the distribution of the ENA source can be understood if the flow is sub‐Alfvénic. We use a magnetohydrodynamic model of the mini‐magnetosphere of Ganymede, embedded in the sub‐Alfvénic flow of Jupiter's magnetospheric plasma, to establish where heated ions are distributed on the magnetopause. If the flow of the LISM is sub‐Alfvénic, reconnection would occur along an arc centered away from the nose for an appropriately chosen field orientation. Charge exchange with ions heated by reconnection would produce an ENA source distributed in a manner close to that observed. Heating of ions by reconnection can account also for the way ENA images vary with energy. Sub‐Alfvénic flow implies not only that reconnection on the heliopause can be centered well away from the nose, but also that no bow shock forms upstream of the heliopause. It also seems probable that the configuration of the heliosphere differs from the bullet shape frequently illustrated.

Interstellar dust close to the Sun

The low density interstellar medium (ISM) close to the Sun and inside of the heliosphere provides a unique laboratory for studying interstellar dust grains. Grain characteristics in the nearby ISM are obtained from observations of interstellar gas and dust inside of the heliosphere and the interstellar gas towards nearby stars. Comparison between the gas composition and solar abundances suggests that grains are dominated by olivines and possibly some form of iron oxide. Measurements of the interstellar Ne/O ratio by the Interstellar Boundary Explorer spacecraft indicate that a high fraction of interstellar oxygen in the ISM must be depleted onto dust grains. Local interstellar abundances are consistent with grain destruction in ~150 km s−1 interstellar shocks, provided that the carbonaceous component is hydrogenated amorphous carbon and carbon abundances are correct. Variations in relative abundances of refractories in gas suggest variations in the history of grain destruction in nearby ISM. The large observed grains, > 1 μm, may indicate a nearby reservoir of denser ISM. Theoretical three-dimensional models of the interaction between interstellar dust grains and the solar wind predict that plumes of ~0.18 μm dust grains form around the heliosphere.

THE INTERSTELLAR MAGNETIC FIELD CLOSE TO THE SUN. II.

A key indicator of the galactic environment of the Sun is provided by the magnetic field in the interstellar medium (ISM), which influences the shape of the heliosphere. The direction of the nearby interstellar magnetic field (ISMF) is determined from starlight polarized in the ISM. The local ISMF direction is found from the ISMF direction that provides the best fit to the polarization position angles of nearby stars, using weighted fits to the data. New polarization observations are included in the analysis. The best-fitting ISMF is close to the magnetic field direction traced by the center of the Ribbon of energetic neutral atoms, discovered by the Interstellar Boundary Explorer spacecraft. Both the magnetic field and kinematics of the local ISM are consistent with a scenario where the local ISM is a fragment of the Loop I superbubble. An ordered component of the local ISMF is found in a region where PlanetPol data show that polarization increases with distance. It extends to within 8 parsecs of the Sun and implies a weak curvature in the nearby ISMF. Variations from the ordered component indicate turbulence of +/-23 deg. The local ISMF is generally uniform in direction over spatial scales of 8-200 parsecs so that it appears similar to interarm magnetic fields. The best-fitting ISMF direction also agrees with the position of tail-in spatial asymmetries in GeV-TeV galactic cosmic rays. The peculiar geometrical relation between the CMB dipole moment, the heliosphere nose, and local ISMF is supported by these new results. Radiative torques are not likely to play a role in grain alignment for these polarizations.

COMPARISONS OF THE INTERSTELLAR MAGNETIC FIELD DIRECTIONS OBTAINED FROM THEIBEXRIBBON AND INTERSTELLAR POLARIZATIONS

Variations in the spatial configuration of the interstellar magnetic field (ISMF) near the Sun can be constrained by comparing the ISMF direction at the heliosphere found from the Interstellar Boundary Explorer spacecraft (IBEX) observations of a 'Ribbon' of energetic neutral atoms (ENAs), with the ISMF direction derived from optical polarization data for stars within ~40 pc. Using interstellar polarization observations towards ~30 nearby stars within 90 deg of the heliosphere nose, we find that the best fits to the polarization position angles are obtained for a magnetic pole directed towards ecliptic coordinates of lambda, beta 263 deg, 37 deg (or galactic coordinates of L,B 38 deg, 23deg), with uncertainties of +/- 35 deg, based on the broad minimum of the best fits and the range of data quality. This magnetic pole is 33 deg from the magnetic pole that is defined by the center of the arc of the ENA Ribbon. The IBEX ENA ribbon is seen in sightlines that are perpendicular to the ISMF as it drapes over the heliosphere. The similarity of the polarization and Ribbon directions for the local ISMF suggest that the local field is coherent over scale sizes of tens of parsecs. The ISMF vector direction is nearly perpendicular to the flow of local ISM through the local standard of rest, supporting a possible local ISM origin related to an evolved expanding magnetized shell. The local ISMF direction is found to have a curious geometry with respect to the cosmic microwave background dipole moment.