Galactic Cosmic Rays in the Outer Heliosphere: Theory and Models

We review recent advances in the field of galactic cosmic ray transport in the distant heliosphere. The advent of global MHD models brought about a better understanding of the three-dimensional structure of the interface between the solar system and the surrounding interstellar space, and of the magnetic field topology in the outer heliosphere. These results stimulated a development of galactic cosmic ray transport models taking the advantage of the available detailed plasma backgrounds and of the new Voyager results from the heliosheath. It emerges that the heliosheath plays a prominent role in the process of modulation and filtration of low-energy galactic ions and electrons. The heliosheath stores particles for a duration of several years thus acting as a large reservoir of galactic cosmic rays. Cosmic-ray trajectories, transit times, and entry locations across the heliopause are discussed. When compared to observations model calculations of low energy electrons show almost no radial gradient up to the termination shock, irrespective of solar activity, but a large gradient in the inner heliosheath. Intensities are however sensitive to heliospheric conditions such as the location of the heliopause and shock. In contrast, high energy proton observations by both the Voyager spacecraft show a clear solar cycle dependence with intensities also increasing with increasing distance. By comparing these observations to model calculations we can establish whether our current understanding of long-term modulation result in computed intensities compatible to observations.     

Confronting Observations and Modeling: The Role of the Interstellar Magnetic Field in Voyager 1 and 2 Asymmetries

Magnetic effects are ubiquitous and known to be crucial in space physics and astrophysical media. We have now the opportunity to probe these effects in the outer heliosphere with the two spacecraft Voyager 1 and 2. Voyager 1 crossed, in December 2004, the termination shock and is now in the heliosheath. On August 30, 2007 Voyager 2 crossed the termination shock, providing us for the first time in-situ measurements of the subsonic solar wind in the heliosheath. With the recent in-situ data from Voyager 1 and 2 the numerical models are forced to confront their models with observational data. Our recent results indicate that magnetic effects, in particular the interstellar magnetic field, are very important in the interaction between the solar system and the interstellar medium. We summarize here our recent work that shows that the interstellar magnetic field affects the symmetry of the heliosphere that can be detected by different measurements. We combined radio emission and energetic particle streaming measurements from Voyager 1 and 2 with extensive state-of-the art 3D MHD modeling, to constrain the direction of the local interstellar magnetic field. The orientation derived is a plane ～60°–90° from the galactic plane. This indicates that the field orientation differs from that of a larger scale interstellar magnetic field, thought to parallel the galactic plane. Although it may take 7–12 years for Voyager 2 to leave the heliosheath and enter the pristine interstellar medium, the subsonic flows are immediately sensitive to the shape of the heliopause. The flows measured by Voyager 2 in the heliosheath indicate that the heliopause is being distorted by local interstellar magnetic field with the same orientation as derived previously. As a result of the interstellar magnetic field the solar system is asymmetric being pushed in the southern direction. The presence of hydrogen atoms tend to symmetrize the solutions. We show that with a strong interstellar magnetic field with our most current model that includes hydrogen atoms, the asymmetries are recovered. It remains a challenge for future works with a more complete model, to explain all the observed asymmetries by V1 and V2. We comment on these results and implications of other factors not included in our present model.     

3-D magnetic field and current system in the heliosphere

In this paper a global system of the magnetic field and current from the interaction of the solar wind plasma and the interstellar medium is modeled using a 3-D MHD simulation. The terminal shock, the heliopause and the outer shock are clearly determined in our simulation. In the heliosheath the toroidal magnetic field is found to increase with the distance from the sun. The magnetic field increases rapidly in the upstream region of the heliosheath and becomes maximum between the terminal shock and the heliopause. Hence a shell-type magnetic wall is found to be formed in the heliosheath. Because of this magnetic wall the radially expanding solar wind plasma changes its direction tailward in all latitudes except the equatorial region. Only the equatorial disk-like plasma flow is found to extend to the heliopause through the weak magnetic-field region around the equator. Two kinds of global current loops which sustain the toroidal magnetic field in the heliosphere are found in our simulation.The influence of the 11-year solar cycle variation of the magnetic polarity is also examined. It is found that the polarity of the toroidal magnetic field in the heliosheath switches at every solar cycle change. Hence the heliosheath is found to consist of such magnetized plasma bubbles. The neutral sheets are found to extend between such magnetized plasma bubbles in the 3-D heliosheath in a complicated form. The magnetic-pressure effect on the heliosheath plasma structure is also examined.     

Heliospheric ENA fluxes: How Sensitive Are They to the Ionization State of LIC?

We explore the sensitivity of the fluxes of heliospheric energetic neutral atoms (ENA) at 1 AU to the ionization state of the local interstellar cloud (LIC). The solar wind plasma is compressed and heated in the termination shock transition. The shocked solar plasma is convected toward the heliospheric tail in the heliosheath, the region between the termination shock and the heliopause. The ENAs are produced in charge exchange of the plasma protons and background neutral gas and can be readily detected at 1 AU. The expected ENA fluxes depend on the shocked plasma density, temperature, and velocity in the heliosheath. The size and structure of the heliospheric interface region depend on the parameters of the interstellar medium. ENA fluxes would thus reveal the LIC parameters. We demonstrate the sensitivity of the heliospheric ENA fluxes to the ionization state of the LIC. The axi-symmetric model of the solar wind/LIC interaction includes the self-consistent treatment of the plasma-gas coupling and Monte Carlo simulations of the neutral gas distribution. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of the Interstellar Magnetic Field and Neutrals on the Shape of the Outer Heliosphere

Formed as a result of the solar wind (SW) interaction with the circum-heliospheric interstellar medium (CHISM), the outer heliosphere is generically three-dimensional because of the SW asphericity and the action of the interstellar and interplanetary magnetic fields (ISMF and IMF). In this paper we show that charge exchange between neutral and charged components of the SW–CHISM plasmas plays a dominant role not only in determining the geometrical size of the heliosphere, but also in the modulation of magnetic-field-induced asymmetries. More specifically, charge exchange between SW and CHISM protons and primary neutrals of interstellar origin always acts to decrease the asymmetry of the termination shock and the heliopause, which can otherwise be very large. This is particularly pronounced because the ionization ratio of the CHISM plasma is rather low. To investigate the deflection of the CHISM neutral hydrogen flow in the inner heliosphere from its original orientation in the unperturbed CHISM, we create two-dimensional neutral H velocity distributions in the inner heliosphere within a 45-degree circular conical surface with the apex at the Sun and the axis parallel to the interstellar flow vector. It is shown that the distribution of deflections is very anisotropic, that is, the most probable orientation of the H-atom velocity differs from its average direction. We show that the average deflection of the H-atom flow, for reasonable ISMF strengths, occurs mostly in the plane formed by the ISMF and CHISM velocity vectors at infinity. The possibility that the ISMF orientation may influence the 2–3 kHz radio emission, which is believed to originate in the outer heliosheath, is discussed.     

The Heliospheric Termination Shock

The heliospheric termination shock is a vast, spheroidal shock wave marking the transition from the supersonic solar wind to the slower flow in the heliosheath, in response to the pressure of the interstellar medium. It is one of the most-important boundaries in the outer heliosphere. It affects energetic particles strongly and for this reason is a significant factor in the effects of the Sun on Galactic cosmic rays. This paper summarizes the general properties and overall large-scale structure and motions of the termination shock. Observations over the past several years, both in situ and remote, have dramatically revised our understanding of the shock. The consensus now is that the shock is quite blunt, is with the front, blunt side canted at an angle to the flow direction of the local interstellar plasma relative to the Sun, and is dynamical and turbulent. Much of this new understanding has come from remote observations of energetic charged particles interacting with the shock, radio waves and radiation backscattered from interstellar neutral atoms. The observations and the implications are discussed.     

IBEX—Interstellar Boundary Explorer

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 energy-resolved 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.     

The Dynamic Heliosphere: Outstanding Issues

Properties of the heliospheric interface, a complex product of an interaction between charged and neutral particles and magnetic fields in the heliosphere and surrounding Circumheliospheric Medium, are far from being fully understood. Recent Voyager spacecraft encounters with the termination shock and their observations in the heliosheath revealed multiple energetic particle populations and noticeable spatial asymmetries not accounted for by the classic theories. Some of the challenges still facing space physicists include the origin of anomalous cosmic rays, particle acceleration downstream of the termination shock, the role of interstellar magnetic fields in producing the global asymmetry of the interface, the influence of charge exchange and interstellar neutral atoms on heliospheric plasma flows, and the signatures of solar magnetic cycle in the heliosheath. These and other outstanding issues are reviewed in this joint report of working groups 4 and 6.     

Interplanetary Lyman α Observations: Intensities from Voyagers and Line Profiles from HST/STIS

We present an analysis of Voyager UVS data obtained between 1993 and mid-2007. These data are used to study the interplanetary background and the hydrogen number density in the outer heliosphere. Two types of observations are studied, first the heliospheric scans performed until 2003 and then the fixed line of sight observations, close to the upwind direction, which are still performed at the end of 2007. We make comparisons with models including multiple scattering and hydrogen distributions derived from self-consistent modeling of the interface region. It is found that there is a remaining discrepancy between models and data. The origin of this difference is unknown but it may be linked to a possible tilting of the heliospheric interface due to the presence of an interstellar magnetic field. We should also estimate alternate sources of emission which are not backscattering of solar photons like collisional excitation of hydrogen in the heliosheath and emission after charge transfer or recombination of proton and electron in HII regions. Line profiles from HST/STIS are also presented.     

Cosmic Rays in the Inner Heliosphere: Insights from Observations, Theory and Models

The global modulation of galactic cosmic rays in the inner heliosphere is determined by four major mechanisms: convection, diffusion, particle drifts (gradient, curvature and current sheet drifts), and adiabatic energy losses. When these processes combine to produce modulation, the complexity increases significantly especially when one wants to describe how they evolve spatially in all three dimensions throughout the heliosphere, and with time, as a function of solar activity over at least 22 years. In this context also the global structure and features of the solar wind, the heliospheric magnetic field, the wavy current sheet, and of the heliosphere and its interface with the interstellar medium, play important roles. Space missions have contributed significantly to our knowledge during the past decade. In the inner heliosphere, Ulysses and several other missions have contributed to establish the relative importance of these major mechanisms, leading to renewed interest in developing more sophisticated theories and numerical models to explain these observations, and to understand the underlying physics that determines galactic cosmic ray modulation at Earth. An overview is given of some of the observational and modeling highlights over the past decade.     

Filtration of Interstellar Atoms through the Heliospheric Interface

Interstellar atoms penetrate deep into the heliosphere after passing through the heliospheric interface—the region of the interaction of the solar wind with the interstellar medium. The heliospheric interface serves as a filter for the interstellar atoms of hydrogen and oxygen, and, to a lesser extent, nitrogen, due to their coupling with interstellar and heliospheric plasmas by charge exchange and electron impact ionization. The filtration has great importance for the determination of local interstellar abundances of these elements, which becomes now possible due to measurements of interstellar pickup by Ulysses and ACE, and anomalous cosmic rays by Voyagers, Ulysses, ACE, SAMPEX and Wind. The filtration of the different elements depends on the level of their coupling with the plasma in the interaction region. The recent studies of the filtration of the interstellar atoms in the heliospheric interface region is reviewed in this paper. The dependence of the filtration on the local interstellar proton and H atom number densities is discussed and the roles of the charge exchange and electron impact ionization on the filtration are evaluated. The influence of electron temperature in the inner heliosheath on the filtration process is discussed as well. Using the filtration coefficients obtained from the modeling and SWICS/Ulysses pickup ion measurements, the local interstellar abundances of the considered elements are determined.     

Composition of Anomalous Cosmic Rays

The “classic” anomalous cosmic ray (ACR) component originates as interstellar neutral atoms that drift into the heliosphere, become ionized and picked up by the solar wind, and carried to the outer heliosphere where the pickup ions are accelerated to hundreds of MeV, presumably at the solar wind termination shock. These interstellar ACRs are predominantly singly charged, although higher charge states are present and become dominant above ～350 MeV. Their isotopic composition is like that of the solar system and unlike that of the source of galactic cosmic rays. A comparison of their energy spectra with the estimated flux of pickup ions flowing into the termination shock reveals a mass-dependent acceleration efficiency that favors heavier ions. There is also a heliospheric ACR component as evidenced by “minor” ACR ions, such as Na, Mg, S, and Si that appear to be singly-ionized ions from a source likely in the outer heliosphere.     

Diagnosing the Neutral Interstellar Gas Flow at 1 AU with IBEX-Lo

Every year in fall and spring the Interstellar Boundary Explorer (IBEX) will observe directly the interstellar gas flow at 1 AU over periods of several months. The IBEX-Lo sensor employs a powerful triple time-of-flight mass spectrometer. It can distinguish and image the O and He flow distributions in the northern fall and spring, making use of sensor viewing perpendicular to the Sun-pointing spin axis. To effectively image the narrow flow distributions IBEX-Lo has a high angular resolution quadrant in its collimator. This quadrant is employed selectively for the interstellar gas flow viewing in the spring by electrostatically shutting off the remainder of the aperture. The operational scenarios, the expected data, and the necessary modeling to extract the interstellar parameters and the conditions in the heliospheric boundary are described. The combination of two key interstellar species will facilitate a direct comparison of the pristine interstellar flow, represented by He, which has not been altered in the heliospheric boundary region, with a flow that is processed in the outer heliosheath, represented by O. The O flow distribution consists of a depleted pristine component and decelerated and heated neutrals. Extracting the latter so-called secondary component of interstellar neutrals will provide quantitative constraints for several important parameters of the heliosheath interaction in current global heliospheric models. Finding the fraction and width of the secondary component yields an independent value for the global filtration factor of species, such as O and H. Thus far filtration can only be inferred, barring observations in the local interstellar cloud proper. The direction of the secondary component will provide independent information on the interstellar magnetic field strength and orientation, which has been inferred from SOHO SWAN Ly-α backscattering observations and the two Voyager crossings of the termination shock.     

Kinetic-Gasdynamic Modeling of the Heliospheric Interface

Heliospheric energetic neutral atoms (ENAs) that will be measured by the Interstellar Boundary Explorer (IBEX) originate from the heliosheath. The heliosheath is formed as a result of the interaction of the solar wind (SW) with the circum-heliospheric interstellar medium (CHISM). The expected fluxes of ENAs are strongly dependent on the nature of this interaction. In turn, the interaction of the solar wind with the local interstellar cloud has a complex and multi-component nature. Detailed theoretical modeling of the interaction between the SW and the local interstellar medium is required to understand the physics of the heliosheath and to predict and explain the heliospheric ENAs. This paper summarizes current state-of-art kinetic-gasdynamic models of the SW/CHISM interaction. We shall restrict our discussion to the kinetic-gasdynamic and kinetic-magnetohydrodynamic (MHD) models developed by the Moscow group. This paper summarizes briefly the main results of the first self-consistent, two-component, kinetic-gasdynamic model by Baranov and Malama (J. Geophys. Res. 98:15157–15163, 1993), presents new results obtained from the 3D kinetic-MHD model by Izmodenov et al. (Astron. Astrophys. 437:L35–L38, 2005a), describes the basic formulation and results of the model by Malama et al. (Astron. Astrophys. 445:693–701, 2006) as well as reports current developments in the model. This self-consistent model considers pickup protons as a separate non-equilibrium component. Then we discuss a stochastic acceleration model for pickup protons in the supersonic solar wind and in the heliosheath. We also present the expected heliospheric ENA fluxes obtained in the framework of the models.     

Magnetospheric Science Objectives of the <Emphasis Type="Italic">Juno</Emphasis> Mission

In July 2016, NASA’s Juno mission becomes the first spacecraft to enter polar orbit of Jupiter and venture deep into unexplored polar territories of the magnetosphere. Focusing on these polar regions, we review current understanding of the structure and dynamics of the magnetosphere and summarize the outstanding issues. The Juno mission profile involves (a) a several-week approach from the dawn side of Jupiter’s magnetosphere, with an orbit-insertion maneuver on July 6, 2016; (b) a 107-day capture orbit, also on the dawn flank; and (c) a series of thirty 11-day science orbits with the spacecraft flying over Jupiter’s poles and ducking under the radiation belts. We show how Juno’s view of the magnetosphere evolves over the year of science orbits. The Juno spacecraft carries a range of instruments that take particles and fields measurements, remote sensing observations of auroral emissions at UV, visible, IR and radio wavelengths, and detect microwave emission from Jupiter’s radiation belts. We summarize how these Juno measurements address issues of auroral processes, microphysical plasma physics, ionosphere-magnetosphere and satellite-magnetosphere coupling, sources and sinks of plasma, the radiation belts, and the dynamics of the outer magnetosphere. To reach Jupiter, the Juno spacecraft passed close to the Earth on October 9, 2013, gaining the necessary energy to get to Jupiter. The Earth flyby provided an opportunity to test Juno’s instrumentation as well as take scientific data in the terrestrial magnetosphere, in conjunction with ground-based and Earth-orbiting assets.     

Physical Processes in the Outer Heliosphere

This chapter covers the theory of physical processes in the outer heliosphere that are particularly important for the IBEX Mission, excluding global magnetohydrodynamic/Boltzmann modeling of the entire heliosphere. Topics addressed include the structure and parameters of the solar wind termination shock, the transmission of ions through the termination shock including possible reflections at the shock electrostatic potential, the acceleration and transport of suprathermal ions and anomalous cosmic rays at the termination shock and in the heliosheath, charge-exchange interactions in the outer heliosphere including mass and momentum loading of the solar wind, the transport of interstellar pickup ions, and the production and anticipated intensities of energetic neutral atoms (ENAs) in the heliosphere.     

Composition of Interstellar Neutrals and the Origin of Anomalous Cosmic Rays

Knowledge of the elemental composition of the interstellar gas is of fundamental importance for understanding galactic chemical evolution. In addition to spectroscopic determinations of certain element abundance ratios, measurements of the composition of interstellar pickup ions and Anomalous Cosmic Rays (ACRs) have provided the principal means to obtain this critical information. Recent advances in our understanding of particle acceleration processes in the heliosphere and measurements by the Voyagers of the energy spectra and composition of energetic particles in the heliosheath provide us with another means of determining the abundance of the neutral components of the local interstellar gas. Here we compare the composition at the termination shock of six elements obtained from measurements of (a) pickup ions at ～5 AU, (b) ACRs in the heliosphere at ～70 AU, and (c) energetic particles as well as (d) ACRs in the heliosheath at ～100 AU. We find consistency among these four sets of derived neutral abundances. The average interstellar neutral densities at the termination shock for H, N, O, Ne and Ar are found to be 0.055±0.021 cm^?3, (1.44±0.45)×10^?5 cm^?3, (6.46±1.89)×10^?5 cm^?3, (8.5±3.3)×10^?6 cm^?3, and (1.08±0.49)×10^?7 cm^?3, respectively, assuming the He density is 0.0148±0.002 cm^?3.     

The Heliospheric Magnetic Field and Its Extension to the Inner Heliosheath

The general structure of the heliospheric magnetic field is well known and has been extensively studied, mostly in the inner heliosphere, out to the orbit of Saturn. Beyond 10 AU, the Pioneer and now the Voyager spacecraft have provided a view of the outer heliosphere. Its structure is strongly affected by large-scale phenomena originating in the Sun’s activity, such as the pattern of fast and slow solar wind streams around solar minimum that lead to Corotating Interaction Regions, and the increased frequency and strength of Coronal Mass Ejections around solar maximum. The large current sheet that separates the dominant magnetic polarities in the heliospheric medium, the Heliospheric Current Sheet, provides a variable structure that evolves from a relatively simple geometry close to the solar equatorial plane to what is likely to be a highly complex and dynamic surface reaching to high heliolatitudes at high levels of solar activity. The magnetic field observed in a fluctuating, dynamical heliosheath differs considerably from that in a static heliosheath. In particular, the time between current-sheet crossings (sectors) is quite sensitive to the radial speed of the solar-wind termination shock. If an inwardly moving termination shock moves past an observer on a slowly moving spacecraft, the time between current-sheet crossings in the heliosheath becomes larger, and can become very large, for reasonably expected inward shock speeds. This effect may help to explain recent observations of the magnetic field from the Voyager 1 spacecraft, where, in the heliosheath, the magnetic field remained directed outward from the Sun for several months without a current-sheet crossing. The crossings finally resumed and now occur somewhat regularly. In addition, the magnetic fluctuations in the heliosheath are observed to be quite different from those in the supersonic upstream solar wind.     

Relations between ISM inside and outside the heliosphere

Thanks to remarkable new tools, such as the Goddard High Resolution Spectrograph (GHRS) on board the HST and the EUVE spectrometer on the interstellar side, and Ulysses particle detectors on the heliospheric side, it is possible now to begin to compare abundances and physical properties of the interstellar matter outside the heliosphere (from absorption features in the stellar spectra), and inside the heliosphere (from in situ or remote detection of the interstellar neutrals or their derivatives, the pick-up ions or the Anomalous Cosmic Rays detected by the two Voyager spacecraft).Ground-based and UV spectra of nearby stars show that the Sun is located between two volumes of gas of different heliocentric velocities V and temperatures T (see also Linsky et al, this issue). One of these clouds has the same velocity (V= 25.6 km s^–1 from = 255 and =8) and temperature (6700 K) as the heliospheric helium of interstellar origin probed by Ulysses, and is certainly surrounding our star (and then the Local Interstellar Cloud or LIC). This Identification allows comparisons between interstellar constituents on both sides of the heliospheric interface.Ly-alpha background data (absorption cell and recent HST-GHRS spectra) suggest that the heliospheric neutral H velocity is smaller by 5–6 km s^–1 than the local cloud velocity, and therefore that H is decelerated at its entrance into the heliosphere, in agreement with interaction models between the heliosphere and the ISM which include the coupling with the plasma. This is in favor of a non negligible electron density (at least 0.05 cm³). There are other indications of a rather large ionization of the ambient ISM, such as the ionization equilibrium of interstellar magnesium and of sodium. However the resulting range for the plasma density is still broad.The heliospheric neutral hydrogen number density (0.08–0.16 cm^–3) is now less precisely determined than the helium density (0.013–0.017 cm^–3, see Gloeckler, Witte et al, Mobius, this issue). The comparison between the neutral hydrogen to neutral helium ratios in the ISM (recent EUVE findings) and in the heliosphere, suggests that 15 to 70% of H does not enter the heliosphere. The comparison between the interstellar oxygen relative abundance (with respect to H and He) in the ISM and the heliospheric abundance deduced from pick-up ions is also in favor of some filtration, and thus of a non-negligible ionization.For a significant ISM plasma density, one expects a Hydrogen wall to be present as an intermediate state of the interstellar H around the interface between inside and outside. Since 1993, the two UVS instruments on board Voyager 1 and 2 indeed reveal clearly the existence of an additional Ly-alpha emission, probably due to a combination of light from the compressed H wall, and from a galactic source. On the other hand, the decelerated and heated neutral hydrogen of this H wall has recently been detected in absorption in the spectra of nearby stars (see Linsky, this issue).     

Plasma transport across the heliopause

Existing heliopause models are critically rediscussed under the new aspect of possible plasma mixing between the solar wind and the ambient ionized component of the local interstellar medium (LISM). Based on current kinetic plasma theories, effective diffusion rates across the heliopause are evaluated for several models with turbulence caused by electrostatic or electromagnetic interactions that could be envisaged in this context. Some specific cases that may lead to high diffusion rates are investigated, especially in regard to their LISM magnetic field dependence.For weak fields (less than 10^–7 G), macroscopic hydrodynamic instabilities, such as of Rayleigh-Taylor or Kelvin-Helmholtz-types, can be excited. The resulting plasma mixing rates at the heliopause may amount to 20–30% of the impinging mass flow.Recently, an unconventional new approach to the problem for the case of tangential magnetic fields at the heliopause was published in which a continuous change of the plasma properties within an extended boundary layer is described by a complete set of two-fluid plasma equations including a hybrid MHD-formulation of wave-particle interaction effects. If a neutral sheet is assumed to exist within the boundary layer, the magnetic field direction is proven to be constant for a plane-parallel geometry. Considering the electric fields and currents in the layer, an interesting relationship between the field-reconnection probability and the electric conductivity can be derived, permitting a quantitative determination of either of these quantities.An actual value for the electrical conductivity is derived here on the basis of electron distribution functions given by a superposition of Maxwellians with different temperatures. Using two-stream instability theory and retaining only the most unstable modes, an exact solution for the density, velocity, and magnetic and electric fields can be obtained. The electrical conductivity is then shown to be six orders of magnitude lower than calculated by conventional formulas. Interestingly, this leads to an acceptable value of 0.1 for the reconnection coefficient.By analogy with the case of planetary magnetopauses, it is shown here for LISM magnetic fields of the order of 10^–6 G or larger that field reconnection processes may also play an important role for the plasma mixing at the heliopause. The resulting plasma mixing rate is estimated to amount to an average value of 10% of the incident mass flow. It is suggested here that the dependence of the cosmic-ray penetration into the heliosphere on the distribution of reconnecting areas at the heliopause may provide a means of deriving the strength and orientation of the LISM field.A series of observational implications for the expected plasma mixing at the heliopause is discussed in the last part of the paper. In particular, consequences are discussed for the generation of radio noise at the heliopause, for the penetration of LISM neutrals into the heliosphere, for the propagation of cosmic rays towards the inner part of the solar system and for convective electric field mergings into the heliosphere during the course of the solar cycle, depending on the solar cycle variations. With concern to a recent detection of electrostatic plasma waves by plasma receivers on Voyagers 1 and 2, we come to an interesting alternate explanation: the heliopause, rather than the heliospheric shock front, could be responsible for the generation of these waves.