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The interaction between the solar wind and the interstellar medium represents a collision between two plasma flows, resulting in a heliosphere with an extended tail. While the solar wind is mostly ionized material from the corona, the interstellar medium is only partially ionized. The ion and neutral populations are coupled through charge-exchange collisions that operate on length scales of tens to hundreds of astronomical units. About half the interstellar hydrogen flows into the heliosphere where it may charge-exchange with solar wind protons. This process gives rise to a nonthermal proton, known as a pickup ion, which joins the plasma. In this paper we investigate the effects of approximating the total ion distribution of the subsonic solar wind as a generalized Lorentzian, or κ distribution, using an MHD neutral code. We illustrate the effect different values of the κ parameter have on both the structure of the heliosphere and the energetic neutral atom flux at 1 AU. We find that using a κ distribution in our simulations yields levels of energetic neutral atom flux that are within a factor of about 2 or 3 over the IBEX-Hi range of energies from 0.5 to 6 keV. While the presence of a suprathermal tail in the proton distribution leads to the production of high-energy neutrals, the sharp decline in the charge-exchange cross section around 10 keV mitigates the enhanced transfer of energy from the ions to the neutrals that might otherwise be expected.

The solar wind in the outer heliosphere is fundamentally different from that in the inner heliosphere, with the influence from the local interstellar medium becoming significant. Observational evidence of pickup ions has accumulated as Voyager 2 continues its journey to the interstellar medium. Specifically, the nonadiabatic solar wind proton temperature profile, the slowdown of the solar wind, and an increased pressure associated with pickup ions are the manifestations of pickup ions in the outer heliosphere. In order to understand the solar wind in the outer heliosphere, we extend the three-fluid model first proposed by Isenberg [1986] to include the energy sharing between the solar wind protons and pickup ions. For simplicity, we introduce a parameter, the energy partition ratio e, to represent the division of the total energy provided by the pickup process between the solar wind protons and pickup ions. We find that only a small percentage of this total thermal energy is needed to heat the solar wind proton to produce the observed temperature profile. As expected, the higher the interstellar neutral hydrogen density, the smaller the percentage. The energy partition ratio has little effect on the slowdown of the solar wind and the pickup ion density distribution in the outer heliosphere, which is primarily controlled by the interstellar neutral hydrogen density. As far as the slowdown is concerned, a neutral hydrogen density of nH∞ = 0.08 cm−3 at the termination shock gives the best match to the observations [Wang et al., 2000]. In this case, the pickup ion density at 40 AU is also of the order of that estimated by Burlaga et al. [1996], and the energy partition ratio e = 0.05 gives the best fit to the observed proton temperature profile.

Charge exchange collisions between the solar wind alpha particles and heliospheric atomic hydrogen produce a unique emission in the 30.4-nm line. We demonstrate that an all-sky image in this emission would reveal the three-dimensional flow properties of the solar wind, including the flow properties in the regions over the Sun's poles and on the far side of the Sun. The boundary separating the fast (high-latitude) and slow (low-latitude) solar wind can be established remotely, from 1 AU. Most of the observed 30.4-nm emission would originate in the region extending up to 10 AU from the Sun. Solar wind flow velocities can be obtained both from radiance directional dependence (images) and directly from the Doppler shifts. The fast and slow solar wind flows would produce emissions spectrally separated by ∼0.03 nm (∼0.3 A). The feasibility of solar wind imaging depends on the spectral properties and relative strength of the background radiation. The main background sources include galactic diffuse and stellar radiation, glow of the interstellar plasma beyond the heliopause, and glow of the solar wind pickup ions. We show that the solar wind emissions are clearly spectrally separated from the background radiation. Metastable single-charged helium ions would also be produced in the solar wind charge exchange collisions. Two-photon decay of these ions would dominate in many directions the continuum extreme ultraviolet background at 1 AU in the 30–90 nm spectral range.

Voyager 2 continues to explore the outer heliosphere as Ulysses studies the latitudinal dependence of the solar wind. During the year 1991 these spacecraft were within 2° latitude and their radial separation was larger than 30 AU. This alignment presents a good opportunity to investigate the evolution of the solar wind and, in particular, the effect of pickup ions, which are an important component of the solar wind in the outer heliosphere. A numerical simulation study is used to model the evolution of solar wind structures from the location of Ulysses (1-5 AU) to Voyager 2 (33-36 AU) in 1991. The Ulysses observations were used as input into the numerical models and the theoretical predictions were compared with the Voyager 2 observations. The model produces profiles and magnitudes of the plasma parameters (e.g., flow speed and density) which are in reasonable agreement with the observations. The inclusion of pickup ions slows the solar wind, reduces the amplitudes of the speed variations and density spikes, and changes the speed of shock propagation. However, within 35 AU, pickup ions do not change the solar wind structures dramatically. Our calculations favor a low interstellar neutral hydrogen density, i.e., n H ∞ = 0.05 cm -3 , for this time period (1991).

This paper presents a quantitative MHD model to study the effects of pickup protons on the solar wind. The model includes the kinetic theory solution of interstellar neutral hydrogen, the ionization process for the production of pickup protons, the spiral interplanetary magnetic field, and the full MHD equations. In order to study the effects of pickup protons, we first obtain the kinetic theory solutions of interstellar neutral hydrogen. This solution is then used to calculate the flow of interstellar hydrogen and the production of pickup protons. We study a one-dimensional, steady state model of the pickup proton solar wind in the upwind direction. The solar wind proton parameters predicted by this solution agree reasonably well with observational data from Voyager 2. Throughout the heliosphere the temperature of pickup protons is higher than the temperature of the solar wind protons by two orders of magnitude. Outside the ionization radius, the solar wind temperature and the fast speed begin to increase and the bulk speed and the Mach number of the wind begin to decrease. The component of the equation of motion along the streamline direction can be written in the form of a Bernoulli equation; it can clearly explain the deceleration of the solar wind. In the distant heliosphere the sum of the kinetic energy and the thermal energy decreases with the heliocentric distance due to the charge exchange process; at the same time, the thermal energy of the solar wind plasma increases due to heating by the pickup process. The combined action causes a significant radial decrease in the kinetic energy of the wind.

Results of a fully three-dimensional model of the interaction of the solar wind with the local interstellar medium are presented. The effects of charge-exchange with interstellar neutral hydrogen are taken into account self-consistently, while the effects of hot solar wind neutral hydrogen, as well as cosmic rays and magnetic fields, are ignored in this study. In accord with solar medium observations by Ulysses, the solar wind is assumed to depend on heliolatitude. Two large, long-lived polar coronal holes, one in the northern hemisphere and the other in the southern hemisphere, are assumed to produce a hot, low-density, high-speed wind which bounds a cooler, higher-density, low-speed ecliptic wind. The solar wind boundary conditions, which allow for a 1.5 increase in solar wind ram pressure over the poles of the Sun compared with the ecliptic plane, are drawn from published Ulysses data [Phillips et al., 1995, 1996]. The results of this simulation are compared with the no-charge-exchange asymmetric solar wind simulation, described by Pauls and Zank [1996]. It is found that the elongation of the heliosphere along the solar poles, resulting from the ram pressure increase with heliolatitude, induces a greater influx of interstellar hydrogen over the poles of the Sun than in the ecliptic plane. This, in turn, reduces the extent of elongation of the heliosphere along the poles of the Sun. The vorticity, found in the no-charge-exchange simulation, is absent in the presence of charge-exchange. Once again, as found by Baranov and Malama [1993] and Pauls et al. [1995], the interaction of the solar wind with the local interstellar medium is influenced strongly by charge-exchange processes.

Recent multidimensional numerical simulations [Pauls et al., 1995] of the solar wind interaction with the local interstellar medium have produced a self-consistent description of the interstellar neutral hydrogen at the heliopause. In this report, we develop a simple one-dimensional model of the solar wind interaction with interstellar hydrogen which is capable of describing the basic features of the interaction along the solar wind stagnation line that were obtained in the numerical simulations. It is shown that pickup ions in the subsonic solar wind can stagnate the plasma flow completely at a distance of 40 AU from the termination shock thus defining the position of the contact discontinuity relative to the termination shock. The plasma number density increases considerably but remains finite on the solar wind side of the contact discontinuity, for a finite velocity of the neutral hydrogen. For the interstellar wind, deceleration due to the interaction with solar hydrogen is found to be important, even though the solar hydrogen density flux is relatively small. The characteristic scale of this interaction is defined by the depth of penetration of the solar hydrogen atoms into the interstellar wind. We find that for observed solar wind and interstellar medium parameters, the deceleration is sufficient to stagnate the interstellar wind at the contact discontinuity.

First results from a two-dimensional (axisymmetric) time-dependent gas dynamic model for the interaction of the solar wind with the local interstellar medium are presented. The model includes the mutual influence of the interstellar and interplanetary plasma (protons and electrons) and the neutral interstellar hydrogen atoms. Neutrals created by charge exchange with the solar wind are ignored; this allows us to approximate the dynamical evolution of the system with the isotropic fluid equations. A supersonic interstellar wind is assumed. The global structure of the heliosphere and interaction region is described for both the plasma and neutral gas. The self-consistent model is compared to a noninteracting gas dynamic model, and dramatic differences in the size of the heliosphere are found. The distribution of neutral hydrogen in the heliosphere is also discussed.

obstructed by the spacecraft sunshade, a data analysis technique has been developed that recovers both bulk and thermal speeds to 10% accuracy and provides the first measurements of solar wind heavy ions (mass per charge >2 amu/e) at heliocentric distances within 0.5 AU. Solar wind alpha particles and heavy ions appear to have similar mean flow speeds at values greater than that of the protons by approximately 70% of the Alfven speed. From an examination of the thermal properties of alpha particles and heavier solar wind ions, we find a ratio of the temperature of alpha particles to that of protons nearly twice that of previously reported Helios observations, though still within the limits of excessive heating of heavy ions observed spectroscopically close to the Sun. Furthermore, examination of typical magnetic power spectra at the orbits of MESSENGER and at 1 AU reveals the lack of a strong signature of local resonant ion heating, implying that a majority of heavy ion heating could occur close to the Sun. These results demonstrate that the solar wind at � 0.3 AU is a blend of the effects of wave–particle interactions occurring in both the solar corona and the heliosphere.

[1] Although the solar wind deceleration in the terrestrial foreshock was noticed three decades ago, previous studies show some conflicting results. This paper presents direct evidence of solar wind deceleration in the foreshock of the Earth by using the data of two Cluster satellites. On 2 February 2003, the two satellites (C1 and C3) of Cluster missions are inside and outside the foreshock, respectively, approximately along the solar wind flow line, which can effectively exclude the uncertainty caused by the spatial and temporal changes of solar wind itself. Comparison of the plasma data recorded by two satellites shows that the solar wind velocity decreases in the foreshock and the largest deceleration reaches 22 km/s. The velocity distribution of ions in the phase space indicates that the solar wind ions undergo pitch angle scattering in the foreshock. The solar wind deceleration is associated with diffuse ions and ULF wave activities. The diffuse ion density reached 0.25 cm−3, about 7% of the solar wind density. The interaction of ULF waves with solar wind also deflects the solar wind away from the bow shock in both the ecliptic and the meridian planes. Meanwhile, the solar wind deceleration is accompanied by thermalization during which the solar wind temperature can reach 240 eV.

[1] The magnitude of the solar wind speed decrease from the inner to the outer heliosphere provides important information about the interaction of the heliosphere with the local interstellar medium (LISM), leading to better estimates of the size and shape of the heliosphere and of the LISM density. In this study we combine observations from multiple spacecraft (ACE, Ulysses, and Voyager 2) near solar maximum with a multifluid one-dimensional numerical model to examine the solar wind slowdown. We find a decrease in speed of 53-62 km s -1 at Voyager 2 (∼60 AU) with respect to ACE and Ulysses measurements or a 12-14% slowdown, the largest yet reported. The model results agree well with observations if we assume an interstellar neutral hydrogen density of 0.09 cm -3 at the termination shock.

It is commonly assumed that the supersonic solar wind terminates in a strong shock at positions where its pressure is balanced by the pressure of the interstellar medium. Presupposing that the solar wind is a fluid dynamic phenomenon, Parker has formulated equations for the stand-off distance as well as for the pressure on the subsonic side of such a shock. These relations were derived from the simplified Rankine-Hugoniot relations for highly supersonic (M ≫ 1) shocks, and from the Bernoulli equation as applied to the postshock flow. According to recent findings, however, the distant solar wind is more likely to have small Mach numbers (M ≳1) because it is heated and decelerated by new ions continuously created from interstellar neutral gas. Therefore, Parker's equations are extended to accommodate arbitrary Mach numbers using the unabridged Rankine-Hugoniot relations, plus the Bernoulli equation for the polytropic postshock flow, and taking into account the influence of ionizing and charge exchange reactions. The equations found are applicable to the solar wind as well as to other stellar winds of any M >1, irrespective of whether or not these winds are interacting with interstellar neutral gas.