SH51A-01
Towards High-accuracy MHD-Boltzmann Model of the Heliospheric Interface
With Voyager 1 crossing the solar wind (SW) termination shock and Voyager 2 approaching it, we need to develop highly accurate models of the heliospheric interface capable of reproducing a fine structure of the SW flow in the vicinity of the termination shock (TS) and in the heliosheath. A model of such type should, on the one hand, involve a kinetic treatment of the neutral component in the partially ionized SW and local interstellar medium plasmas and, on the other hand, take advantage of adaptive mesh refinement (AMR) techniques for resolving the heliospheric discontinuities (the TS, the heliopause, the bow shock, and additional transients) sharply enough so that they can be meaningfully compared with the spacecraft observations. Charge exchange between hydrogen atoms and ions substantially modifies the TS shape, its position with respect to the Sun, and, as a consequence, the properties of the SW in the heliosheath. We present new results obtained with our numerical code which models the ion flow magnetohydrodynamically, while transport of neutral hydrogen is described stochastically, using direct simulation Monte Carlo (DSMC) method. Solutions obtained with such calculations are compared with those obtained in the framework of our multi-fluid, AMR model, where different populations of neutral particles are treated hydrodynamically.
SH51A-02
Test-particle Orbit Simulations in Fields from a Realistic 3D MHD Simulation
Models designed to explore the global structure of the heliosphere have become increasing sophisticated. Incentives to increase and to further explore the predictive capabilities of such models include the entry of the Voyager spacecraft into the foreshock region of the termination shock (TS), Voyager 1 in mid-2002 and Voyager 2 in late 2004, and the crossing of the TS and passage into the heliosheath (HSH) of Voyager 1 in 2004 day 351. Using the electric and magnetic fields generated by a MHD model of a 3D, asymmetric heliosphere [Opher et al., Ap. J. L., 640, 2006], we have developed full-particle and adiabatic-orbit codes to simulate the motion of test particles in the solar wind, TS, and HSH environments. The full-particle orbits are necessary to investigate energetic ion (e.g., anomalous and galactic cosmic ray) motion at the TS and within the heliospheric current sheet that is included in the MHD model. Adiabatic orbits are used to study particle motion in the much larger volume of the HSH where the non-homogeneous model fields produce complex guiding center motions, including mirroring in local field compressions. We will present results from these orbit computations, which are intended to provide an initial, albeit simplified, look at the propagation of high-energy charged particles, in the scatter-free limit, in the best model of the TS/HSH field configurations currently available. We will also display drift paths of high-energy ions in the HSH fields using the guiding center drift equations that are applicable in the limit of diffusive propagation.
SH51A-03
Energetic Neutral Atom Distributions Observable at 1 AU
The three dimensional structure of the heliosphere, bounded by the heliopause which separates hot post termination shock solar wind from interstellar plasma, is poorly known. The crossing of the heliospheric termination shock by Voyager 1, in late 2005, has yielded valuable in-situ measurements, but does not help us much in determining the 3D structure of this interface. IBEX, due for launch in 2008, will provide all-sky images of energetic neutral atoms (ENA's), generated from charge-exchange encounters between interstellar neutral atoms and heliosheath plasma protons, observed from a highly eccentric Earth orbit. These skymaps will provide line of sight integrals of the heliosheath in all directions, and will require careful interpretation to deconvolve the 2D skymaps into 3D structure. We have previously investigated ENA fluxes at 1 AU using our self-consistently coupled 3D MHD-neutral models of the interaction between the solar wind and the interstellar medium. These synthetic skymaps were generated based on the assumption of a Maxwellian velocity distribution of plasma protons. Observations suggest, however, that the solar wind plasma may be better described using a Kappa distribution, which allows for high energy tails due to pick-up ions. In the preliminary results presented here, we have used several heliospheric configurations obtained from simulations which assume regular Maxwellian fluids, then we have constructed skymaps assuming a Kappa distribution in the heliosheath.
SH51A-04
The Composition of Energetic Particles in the Termination Foreshock and Heliosheath
Comparisons between the energetic particle measurements made with the Low Energy Charged Particle instrument on Voyager 1 in the termination foreshock (TFS) and the heliosheath (HS) have revealed some key features, using the observational tools of intensity, anisotropy, and spectral measurements. The shape and temporal evolution of ion spectra in both the TFS and the HS are organized by total kinetic energy. This total energy dependence is a robust feature that should be predicted by models and theories. In the TFS the anisotropies are organized by energy per nucleon but the spectra are organized by total kinetic energy. In the HS there is a period with anisotropy and spectra organized by total energy. Possibly this is evidence for two different transport regimes. There is at least one component of the spectra (dubbed "third source" particles) from ~1-100 MeV that exhibits different time profiles, spectral shape, and anisotropy from either the TFS particles or anomalous cosmic rays. In addition to the intensities, anisotropies, and spectral variations there remains an important tool to try to probe the energetic particle phenomena near the termination shock. This tool is elemental composition. We will report on the composition of energetic ions in the different regions of the TFS observed by Voyager 1 and 2, and in the HS observed by Voyager 1. In particular we will examine the comparative composition in these regions and relate this to previously measured pickup ions and anomalous cosmic ray composition.
SH51A-05
Three-dimensional, Multi-fluid Model of the Heliospheric Interface
The problem of the solar wind (SW) interaction with the local interstellar medium (LISM), as many other physical phenomena, exhibits a multi-scale behavior. It is characterized by the disparity of scales in time and space, all of which must be properly resolved. The encounter of a supersonic source flow of the SW with a supersonic LISM flow creates a tangential discontinuity (the heliopause, HP). The SW consists of charged particles (mostly hydrogen ions), while the LISM is only partially ionized, being essentially a mixture of hydrogen ions and atoms. It is common to model the plasma interaction using ideal MHD equations. Neutral hydrogen influence on the heliospheric interface reveals itself via charge exchange between charged and neutral particles. The application of a multifluid approach for for modeling the transport of neutral particles proves to be particularly useful for unsteady interactions regimes. Since both plasma flows are supersonic, the interaction pattern involves a bow shock and a SW termination shock. Adaptive mesh refinement (AMR) techniques address the issue of disparate space scales. While preserving most of the advantages of regular gridding, an AMR covers areas that need higher resolution with increasingly fine grid patches. For time-dependent problems finer meshes are also advanced with smaller time steps. We present the first results of a 3D, AMR, multi-fluid modeling of the heliospheric interface for the LISM parameters agreeable with the observed and/or theoretically predicted asymmetries of the termination shock and the deviation between the neutral hydrogen and neutral helium flows at distances of about 10 AU from the Sun. A number of numerical tests are presented for 3D, multi-shock plasma flows.
SH51A-06
Comparing various multi-component global heliosphere models
Modeling of the global heliosphere seeks to investigate the interaction of the solar wind with the partially ionized local interstellar medium. Models that treat neutral hydrogen self-consistently and in great detail together with the plasma, but neglect magnetic fields, constitute a sub-category within global heliospheric models. There are several different modeling strategies used in the literature, and this contribution seeks to point out differences and commonalities in the modeling results from different strategies. Plasma-only models, and fully self- consistent models from four research groups, for which the neutral species is modeled with either one, three, or four fluids or kinetically, are run with the same boundary parameters and equations. They are compared to each other with respect to the locations of key heliospheric boundary locations, and with respect to the neutral hydrogen content throughout the heliosphere. In many respects, the models basically agree. In particular, the locations of the termination shock agree to within 7% in the nose direction and to within 14% in the downwind direction. The nose locations of the heliopause agree to within 5%. The filtration of neutral hydrogen from the interstellar medium into the inner heliosphere, however, is model dependent, and possible physical reasons for this are discussed.
SH51A-07
Transient Processes in the Distant Solar Wind and the Heliosheath: Voyager Data vs. Simulated Predictions
The fact that Voyager 1 crossed the termination shock (TS) at 94 AU at the end of the year 2004 possibly provides us with some information on pressure conditions of the LISM, as well as the time-varying distance of the TS before and after the crossing. Our three-dimensional MHD simulation aims to reproduce the real dynamical heliosphere of the recent period, Sept. 10, 2001-Dec. 3, 2006. The Voyager 2 (V2) solar-wind data were used as the inner-boundary condition of the simulation. Initially the total pressure of the LISM is roughly defined so that V1 crossed the TS at 94 AU. The relation between the solar-wind ram-pressure and the time-varying TS motion is investigated. The TS is found to move almost parallel to V1, and was always very near V1 during the period from about September 2002, when the TS-particles were first observed at V1, to the time of the TS-crossing. At the end of the high ram-pressure pulses of the Halloween events around August, 2004, the TS began to move inward and then V1 crossed the TS. This TS inward motion continued for 1.2 years and the final TS location was at about 85 AU from the sun. Then the TS started to move outward again, and it reached about 95 AU by the end of our simulation. The motion of the Heliopause (HP) is found to move synchronously with the TS motion with a time- delay of 0.82 year. This time is roughly equal to the propagation time of the sound wave between the TS and the HP along the sun-V1 line. The relation between the TS-particle events and the motion of the TS-position is also considered. The motion of the TS is found to be well synchronized to the observed count of the TS-particles. Structures of the TS along the sun-V1 line are also studied in detail to relate to the TS-particles observations more clearly. The observation that the TS-particle count did not increase but rather decreased at the TS-crossing can be explained if the high ram-pressure pulse carries accelerated-particles toward the downstream side when it collides the TS. We also discuss the solar-wind deceleration between the positions of V1 and V2 by estimating the time-correlation between the simulated-magnetic-field at the V1 position with the observed V1 magnetic field. Our analysis indicates that the time delay due to the deceleration is about 15 days in the course of about 80 days journey of the solar wind for about 20 AU between positions of V2 and V1. The motion of the TS along the sun-V2 line is also discussed.