Nikolai V. Pogorelov

Global Observations of the Interstellar Interaction from the Interstellar Boundary Explorer (IBEX)

Whats Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959, published online 15 October), Fuselier et al. (p. 962, published online 15 October), Funsten et al. (p. 964, published online 15 October), and Möbius et al. (p. 969, published online 15 October) present data taken by NASAs Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966, published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971, published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Suns travel through the galaxy as was previously thought. Observations by the Interstellar Boundary Explorer have revealed surprising features in the interaction between the heliosphere and the interstellar medium. 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.

Comparison of Interstellar Boundary Explorer Observations with 3D Global Heliospheric Models

Whats Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959, published online 15 October), Fuselier et al. (p. 962, published online 15 October), Funsten et al. (p. 964, published online 15 October), and Möbius et al. (p. 969, published online 15 October) present data taken by NASAs Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966, published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971, published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Suns travel through the galaxy as was previously thought. Observations by the Interstellar Boundary Explorer have revealed surprising features in the interaction between the heliosphere and the interstellar medium. 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 ~ 30 to 60 astronomical units and BLISM ~ 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 Effects of a κ-Distribution in the Heliosheath on the Global Heliosphere and ENA Flux at 1 AU

In this paper we investigate heliosheath energetic neutral atom (ENA) fluxes at keV energies, by assuming that the heliosheath proton distribution can be approximated using a κ-distribution. The choice of the κ-parameter derives from observational data of the solar wind (SW). Our work has direct applications to the upcoming IBEX mission, since we generate all-sky ENA maps within the IBEX energy range (10 eV-6 keV), as well as ENA energy spectra in several directions. We find that the use of κ, as opposed to a Maxwellian, gives rise to greatly increased ENA fluxes above 1 keV, while medium-energy fluxes are somewhat reduced. We show how IBEX data can be used to estimate the spectral slope of the proton distribution in the heliosheath, and that the use of κ reduces the differences between ENA maps at different energies. We also investigate the effect that introducing a κ-distribution has on the global interaction between the SW and the local interstellar medium (LISM), and find that there is generally an increase in energy transport from the heliosphere into the LISM, due to the modified profile of ENA energies. This results in a termination shock that moves out by 4 AU, a heliopause that moves in by 9 AU, and a bow shock 25 AU farther out, in the nose direction.

THREE-DIMENSIONAL FEATURES OF THE OUTER HELIOSPHERE DUE TO COUPLING BETWEEN THE INTERSTELLAR AND INTERPLANETARY MAGNETIC FIELDS. II. THE PRESENCE OF NEUTRAL HYDROGEN ATOMS

We investigate the effects of coupling the interstellar and interplanetary magnetic fields (ISMF and IMF) at the heliospheric interface while taking into account the momentum and energy transfer between the plasma and neutral components of the interacting solar wind (SW) and local interstellar medium (LISM). Investigation is performed on the basis of the multifluid model that treats different populations of neutral particles as fluids governed by separate sets of the Euler equations. As the properties of the local interstellar cloud (LIC) are rather poorly examined, with the exception of the interstellar plasma velocity and temperature, we assume several possible orientations of the ISMF. In particular, the case is considered where an ISMF vector is perpendicular to the LISM velocity vector and inclined at an angle of 60° toward the ecliptic plane of the Sun, as suggested in recent publications relating LIC properties to the radio emission observed by Voyager 1. Special attention is paid to the distribution of magnetic fields that can affect the cosmic-ray modulation throughout the heliosphere and the possibility of the heliospheric current sheet bending into one of the hemispheres. Differences are discussed between ideal MHD and multifluid calculations. Possibilities are analyzed for the IMF lines to cross the termination shock multiple times. Parameters are determined that affect the divergence between the neutral hydrogen and helium streamlines in the inner heliosphere.

Three-dimensional Features of the Outer Heliosphere Due to Coupling between the Interstellar and Interplanetary Magnetic Fields. I. Magnetohydrodynamic Model: Interstellar Perspective

The properties of the Local Interstellar Cloud (LIC) are rather poorly examined, with the exception of the interstellar plasma velocity and temperature. The poorest known quantities are the strength and direction of the interstellar magnetic field (ISMF). We review available observational data and perform three-dimensional, time-dependent parametric modeling of the solar wind (SW) interaction with the local interstellar medium (LISM) for a number of typical strengths and directions of the ISMF. The effect of coupling the ISMF and the interplanetary magnetic field on the SW-LISM interaction pattern is investigated. In addition, we consider the case of an ISMF vector perpendicular to the LISM velocity vector and inclined at an angle of 60° toward the ecliptic plane, as suggested in recent publications relating LIC properties to the radio emission observed by Voyager 1. Since the existence of nonevolutionary MHD shocks remains controversial, special attention is paid to the case where the ISMF strength ahead of the bow shock falls into the region of nonevolutionary parallel shocks. It is shown that a complicated, multishocked solution, which exists in this case in the axially symmetric statement, acquires a regular evolutionary structure in a genuinely three-dimensional formulation of the problem. Current views regarding the existence and character of nonevolutionary shocks in space are presented, and an attempt is made to distinguish important physical questions of the uniqueness of MHD shock-wave structures from numerical artifacts. This is the first in a series of papers investigating the three-dimensional structure of the outer heliosphere. Here we present the results of purely MHD calculations, adopting an interstellar medium perspective. Subsequent papers will address the combined effect of charge exchange between neutral and charged particles and magnetic fields.

Termination Shock Asymmetries as Seen by the Voyager Spacecraft: The Role of the Interstellar Magnetic Field and Neutral Hydrogen

We show that asymmetries of the termination shock due to the influence of the interstellar magnetic field (ISMF) are considerably smaller in the presence of neutral hydrogen atoms, which tend to symmetrize the heliopause, the termination shock, and the bow shock due to charge exchange with charged particles. This leads to a much stronger restriction on the ISMF direction and its strength. We demonstrate that in the presence of the interplanetary magnetic field the plane defined by the local interstellar medium (LISM) velocity and magnetic field vectors does not exactly coincide with the plane defined by the interstellar neutral helium and hydrogen velocity vectors in the supersonic solar wind region, which limits the accuracy of the inferred direction of the ISMF. We take into account the tilt of the LISM velocity vector with respect to the ecliptic plane and show that magnetic fields as strong as 3 μG or greater may be necessary to account for the observed asymmetry. Estimates are made of the longitudinal streaming anisotropy of energetic charged particles at the termination shock caused by the nonalignment of the interplanetary magnetic field with its surface. By investigating the behavior of interplanetary magnetic field lines that cross the Voyager 1 trajectory in the inner heliosheath, we estimate the length of the trajectory segment that is directly connected by these lines to the termination shock. A possible effect of the ISMF draping over the heliopause is discussed in connection with radio emission generated in the outer heliosheath.

Probing Heliospheric Asymmetries with an MHD-Kinetic model

New solar wind data from the Voyager 1 and Voyager 2 spacecraft, together with the SOHO SWAN measurements of the direction in which neutral hydrogen enters into the inner heliosheath and neutral helium measurements provided by multiple observations, are expected to provide more reliable constraints on the ionization ratio of the local interstellar medium (LISM) and the direction and magnitude of the interstellar magnetic field (ISMF). In this Letter we use the currently most sophisticated numerical model of the heliospheric interface, which is based on an MHD treatment of the ion flow and kinetic modeling of neutral particles, to analyze an ISMF-induced asymmetry of the heliosphere in the presence of the interplanetary magnetic field and neutral particles. It is shown that secondary hydrogen atoms modify the LISM properties leading to its shock-free deceleration at the heliopause. We determine the deflection of hydrogen atoms from their original trajectory in the unperturbed LISM and show that it occurs not only in the plane defined by the ISMF and LISM velocity vectors, but also, to a lesser extent, perpendicular to this plane. We also consider the possibility of using 2-3 kHz radio emission data to further constrain the ISMF direction.

On the Possibility of a Strong Magnetic Field in the Local Interstellar Medium

We analyze the consequences of the local interstellar magnetic field being almost 3 times larger than the Galactic average (ordered) field on the structure of the heliospheric interface in the axisymmetric case when the field is parallel to the relative direction of motion between the local interstellar medium (LISM) and the Sun. A field of such strength is expected to exist in the Local Interstellar Cloud, if the latter condensed from material inside a magnetic flux tube rebounding from the wall of the Local Bubble cavity. The analysis is performed using a newly developed multifluid neutral MHD model. We show that the bow shock ahead of the heliopause still exists for supersonic and sub-Alfvenic LISM parameters. Our results agree well with the observations of the Lyα absorption spectra and yield positions of the termination shock and the heliopause similar to those obtained from the standard super-Alfvenic model.

Consequences of the Heliopause Instability Caused by Charge Exchange

The heliopause (HP) is a tangential discontinuity that divides the interacting streams of the solar wind (SW) and local interstellar medium (LISM). It has been known for some time that the heliopause is subject to Rayleigh-Taylor instabilities driven and mediated byinterstellar neutralatoms. Here we identify anewform of instabilityontheflanks of HP driven by hot neutral hydrogen atoms created by charge exchange of interstellar neutrals with hot heliosheath plasma. To investigate the instabilities and the consequences of these on heliospheric structure we performed highresolution, shock-capturing, adaptive-mesh-refinementcalculations of the SW-LISMinteraction. Low numerical dissipation allows us to analyze the fine structure of the heliosheath resulting from the HP instability. We show that secondary neutrals play an essential role in destabilizing the flanks of the HP. We analyze the time-dependent location of the HP and the termination shock and the influence of their excursions on the plasma distribution in the inner heliosheath. It is found that perturbations generated by the HP instability can affect the distribution of plasma in the inner heliosheath on shorter timescales than the timescale of the instability as it develops near the stagnation axis. Finally, by way of application, we estimate the intensity of the soft X-ray emission generated in different regions of theHPandshowthatitsinstabilityleadstoasubstantialenhancementintheX-rayemissionwhichbearsanimprintof the HP shape. Subject headingg ISM: kinematics and dynamics — magnetic fields — MHD — shock waves — solar wind — X-rays: stars

The Effects of Global Heliospheric Asymmetries on Energetic Neutral Atom Sky Maps

The relationship between the all-sky distribution of energetic neutral atoms (ENAs) observable at 1 AU and the distribution and structure of plasma in the outer heliosphere is a promising tool for probing the structure of the heliospheric boundaries. The impetus for this work stems from the anticipated 2008 launch of the Interstellar Boundary Explorer, which will directly measure ENA fluxes from Earths orbit. We find, on the basis of global simulations, that the ENA flux depends sensitively on the shape of the heliopause as well as the velocity, density, and temperature of the postshock solar wind it envelopes. This Letter is intended as a preliminary, qualitative study of how ENA maps can help us identify heliospheric asymmetries.