After July 2001, when Ulysses moved from 30 to 80 degrees in solar latitude, the Ulysses GAS instrument measured an apparent increase in the neutral He density. This is more naturally interpreted as a latitudinal dependence (decrease) of the loss rate due to solar photoionization rather than a true increase of the neutral He density. We have developed a three-dimensional model for solar EUV fluxes observed at any heliospheric position, using daily SOHO EIT observations, over successive Carrington rotations, projected to any heliospheric position. The combined effects of solar rotational and latitude-dependent flux variability are explicitly treated in this model. The flux model has been directly compared with other direct irradiance observations in the ecliptic plane with the SOHO/SEM irradiance time series for validation. We then use this flux to compute the photoionization rate of the in-flowing neutral He, and compare the modeled change with time along the spacecraft trajectory with the direct measurements from the out -of -ecliptic Ulysses GAS observations. The 3-D model developed will be directly applicable to STEREO EUV images from the SECCHI instrument suite. As the two spacecraft separate, the amount of the solar surface observed will increase through the mission, providing more accurate solar inputs.

Using SWAN photometric (Ly-alpha) data recently and cautiously cleaned from stellar and straylight contamination, a stationary "hot model" of the neutral hydrogen flow, and a classical least-mean-square method, we derive the total hydrogen ionization rate as a function of helio-latitude. As expected the latitudinal anisotropy decreases strongly with activity. We compare the results obtained for various phases of the solar cycle with the rates expected from solar wind and solar EUV measurements.

We present evolution of the latitudinal structure of the solar wind during the declining phase of the solar cycle (2002 -- 2004), derived from observations of the heliospheric Lyman-alpha glow by SWAN/SOHO. The present set complements an earlier analysis, performed from solar minimum until just after solar maximum. The observations were collected during the time when Ulysses due to ballistic conditions was unable to carry out relevant in situ observations. The groove in the helioglow seems to be back, suggesting recurrence of the bimodal structure of the solar wind, with a band of slow wind around the solar equator.

Since mid-2002, the Voyager 1 (V1) spacecraft has been returning low-energy particle data that we interpret as originating from the solar wind termination shock via direct connection along the interplanetary magnetic field line that passes through the position of V1. Strong field-aligned beaming has often been observed, indicating the source is along the field line in the direction back towards the Sun. Towards the end of 2004, V1 appears to have entered a region with new characteristics, while data from the Voyager 2 (V2) spacecraft is beginning to show unusual activity as well. We will report and characterize the latest observations from both spacecraft from this new, unexplored region of space. This work was supported by NASA under contract NAS7-03001.

In recent years it has become more and more evident that energetic neutral H-atoms (ENA`s) could be profitably used as messengers of fundamental plasma processes occuring in the up-to-now observationally unknown and unaccessed plasma sites of the heliospheric interface. In this respect in the recent past mainly shocked solar wind (SW) protons were discussed as candidates for the production of earth-targeted ENA`s between 5 and 50 KeV which could help to do helpful diagnostics of the heliospheric interface structures. Here we especially shall study an additional source of ENA`s also originating in the interface, however from pick-up ions (PUI`s), and competing in fluxes with ENA`s produced from shocked SW protons. As we show in the range between 5 to 50 KeV one obtains earth-targeted ENA fluxes from charge transfer reactions of H-atoms with H-PUI`s both in the region upstream and downstream of the termination shock. We solve a simplified PUI phase-space transport equation within the global plasma interface described by a time-dependent 5-fluid interaction code modelling the breathing of the heliosphere in reaction to the solar cycle activity. Our calculations show that depending on energy, viewing direction and period of the solar activity cycle, either PUI-generated or SW-generated spectral ENA fluxes dominate. This result should be taken as positive, since saying that this way both PUI-physics and SW-physics of the interface can be studied separately.

We have developed a time-dependent, three-dimensional model of the dynamics of the heliosphere, which is based on a recent version of the ZEUS-3D code and employs parallel processing with MPI. It includes the solar and interstellar plasma components, neutral atoms and cosmic rays. First, results for the heliospheric dynamics caused by solar activity and a time-varying local interstellar medium will be presented. Second, the three-dimensional heliospheric distributions of galactic cosmic rays are derived for different fully anisotropic diffusion tensors. Finally, the influence of the pressure gradient of the galactic cosmic rays on heliospheric structure is studied for different choices of the diffusion tensor.

Interstellar material (ISM), including gas and dust, leave distinct signatures on heliosphere properties. Recent results on variations in the heliosphere-ISM interaction are presented. Small scale structure in the nearby warm ISM indicates the Sun will leave the low density ISM at the Local Interstellar Cloud (LIC) velocity during the next century, having entered it only during the past several thousands of years. The filling fraction of warm low density clouds, like the LIC that currently surrounds the Sun, is less than 35% in the nearest few tens of parsecs. This conclusions follows from a wide range of observations coupled with increasingly detailed theoretical models of ionization of the LIC (Slavin & Frisch 2002, Frisch & Slavin 2003). However, some of these cloudlets may be denser than the LIC. Results indicating the interstellar magnetic field is draped over the heliosphere and parallel to the galactic plane are summarized, although the underlying starlight polarization data are somewhat old.

The solar wind creates a bubble of plasma around the Sun which carves out a cavity in the super-sonic flow of interstellar gas. This cavity is the heliosphere and its interaction with the interstellar gas is not well understood. Various techniques exist to model this interaction, which is complicated by the non-Maxwellian natures of the interstellar hydrogen atoms passing through the heliosphere. One way to model this interaction is to use multiple fluids to model different parts of the neutral gas. Another method is a Monte-Carlo approach, where the hydrogen atom distribution function is approximated by a large number of test particles. We will present a comparison of these two methods and provide some examples where we investigate the effects of different collision mechanisms in the Monte-Carlo code.

We have used the first generation space weather forecasting HAFv.2 model of the solar wind to project simulated solar background and transient flare events, based on solar observations, out to 100 AU in three dimensions. We compared the modeled shock arrival times and solar wind velocities with in-situ data at Ulysses, Cassini, and Voyagers for the October/November 2003 events to suggest the necessity for consideration of solar generated disturbances when examining Voyager 1 and Voyager 2 observations in the outer heliosphere. To inspire further confidence in this effort, we then compared, out to 1AU, this kinematic, 3D solar wind model (HAFv.2) with a 3D MHD model that also incorporates a global, pre-event, inhomogeneous, background solar wind plasma and interplanetary magnetic field. Both solar wind models use source surface models to drive a quasi-steady background solar wind, and in both models transient events are superimposed on the background. Initial comparisons of the results from the two models with in-situ observations at ACE were favorable, and we now extend this work by comparing the results from both models beyond 1 AU. The three-dimensional models allow us to examine asymmetries in latitude and longitude that appear to be important in the evaluation of observations up to 10 AU and beyond. The work by DSI and JI was supported by Carmel Research Center. The work by MD, CDF, WS, and CSD was supported by the DoD project, University Partnering for Operational Support (UPOS), and by NASA's Living With a Star (LWS) Targeted Research and Development Program. The work of T.D. and Z.S. also was supported by the LWS grant, through NOAA Work Order No. W-10,118.

We have been investigating the turbulent heating of the equatorial solar wind, as modeled by a phenomenological description of the turbulent evolution under the action of spherical expansion, shear driving in the inner heliosphere, and interstellar pickup proton driving in the outer heliosphere. This model is able to reproduce the radial trend of solar wind temperature observed at the Voyager 2 spacecraft out to nearly 80 AU. However, a detailed comparison with the observed solar-rotation-averaged temperature, which shows considerable spatial/temporal variation, is less than satisfactory. In particular, the solar wind temperature beyond 55 AU, measured during the most recent solar maximum period, is found to be consistently lower than predicted by our model and the variations of the observed and model temperatures are not well matched in phase. These differences may indicate the need for modifications in the turbulent evolution model, which presently contains a number of simplifications. However, the present model does not include several important physical effects, which should be treated before considering the effect of more complicated turbulence models. In particular, the well-defined slowdown of the solar wind due to the momentum loading by the pickup protons has not been incorporated. Neither has the dependence of the charge-exchange cross-section on solar wind flux been taken into account. Both of these effects are expected to reduce the predicted model temperature in the distant solar wind. We will extend the turbulent evolution model to allow these effects in the theory, and determine whether they result in significant improvement in the comparison with the Voyager observations.

Recently Lallement et al. (2005) reported (from analysis of SOHO/SWAN measuments of backscattered solar Lyman-alpha radiation) that the direction of the interstellar H atom flow is 3-5° deviated from the direction of atoms of helium. The most probable physical phenomenon responsible for such a deviation is the interstellar magnetic field inclined to the direction of the interstellar wind. In this case the flow of interstellar charged component will be asymmetric in the heliospheric interface - the region of the solar wind interaction with the local interstellar medium. Interstellar H atoms penetrating through the heliospheric interface should have imprints of the asymmetry, because they are much stronger coupled to charged component as compared with interstellar atoms of helium. In this paper we present first results of our new self-consistent 3D kinetic-MHD model of the solar wind interaction with the magnetized interstellar plasma. It is shown that 3-5 % deviation of the direction of H atoms inside the heliosphere requires moderate interstellar magnetic field of ~2.5 µG inclined ~45° from the direction of interstellar flow. Different aspects of such a magnetic field on the structure of the heliospheric interface and on the shapes and locations of the termination shock and heliopause are discussed.

In this paper we present detailed comparison of two types of models of the heliospheric interface - the region of the solar wind interaction with the Local Interstellar Cloud (LIC). The first type of the models is based on the kinetic description of the interstellar H atom flow, which is required for this problem due to the fact that the mean free path of H atoms is comparable with the characteristic size of the heliospheric interface. The second type of the models is based on voluntary assumption that the flow of H atoms can be described hydrodynamically by set of Euler equations (one-fluid approach) or by 2, 3, and 4 sets of Euler equations for different populations of H atoms (respectively, 2-, 3-, or 4- fluid models). Physically, this assumption means that mean free path of H atoms with respect of H-H collisions between atoms of one population is effectively smaller as compared with both the characteristic size of the problem and the mean free path with respect of charge exchange. At the same time, H-H collisions between H atoms of different populations are ignored in the multi-fluid approach. Despite such an assumption has no physical background, multi-fluid models are still attractive for modelers because of relatively simple governing Euler equations, which are possible to solve by using standard numerical gasdynamic or MHD codes. The goal of this paper is to illuminate similarities and differences in the kinetic and multi-fluid models and explore physical reasons of the differences. Differences between models in observationally meaning values as locations of the termination shock and heliopause, filtration of H atoms through the heliospheric interface, and their bulk velocities and temperatures are clearly shown in the paper.

After a short description of the hydrogen absorption cell and its principle, we show the main characteristics of the cell data recorded by SWAN on board SOHO. The cell is absorbing a fraction of the Ly-alpha emission due to the interstellar neutral hydrogen flow within the solar system. In particular we illustrate and model the evolution of the so-called Zero Doppler Shift Cone along the Earth orbit. We then describe the different methods of analysis of these data. We focus on the method we have recently developed for the interstellar wind direction determination. This method is independent of the H cell characteristics and aging, and is extremely weakly sensitive to radiative transfer line profile broadening. We finally compare with other methods.

Kurth and Gurnett [J. Geophys. Res.,98, 15,129, 1993] suggested that like other collisionless shocks in the solar wind, the termination shock would likely be a source of Langmuir waves. Furthermore, just as Langmuir waves often precede the crossing of a planetary bow shock, it was predicted that Langmuir waves might be observed several weeks prior to crossing the termination shock. We have closely examined recent Voyager 1 observations for bursty, narrowband emissions near the expected solar wind plasma frequency and have found a small number of intervals since early 2004 lasting from approximately one hour to a couple of days that appear to be Langmuir waves. The amplitudes are quite small, of the order of 1 µV/m, and their frequencies range from 178 to 562 Hz, within the range expected for the electron plasma frequency at the 91 - 95AU distance of Voyager 1 since early 2004. Given observations of energetic particles that have suggested that Voyager 1 is close to the termination shock, we examine the evidence for Langmuir waves as possible precursors of the crossing of that boundary.

Using the Solar Isotope Spectrometer (SIS) on NASA's Advanced Composition Explorer (ACE), we have measured the composition of energetic nuclei near 1 AU over energies of ~10 to 100 MeV/nucleon since August 1997. Oxygen fluxes at the low energy end of this interval are dominated by anomalous cosmic rays (ACRs) during solar quiet times at solar minimum. ACR intensities were suppressed by a factor of ~100 or more during solar maximum and are now once again recovering at 1 AU as the next solar minimum approaches. However, their present intensities are lower relative to higher rigidity galactic cosmic rays than they were during entry into solar maximum in 1998-2000.

We report ACE/SIS measurements of the ACR composition and spectra obtained during solar minimum and of the variation of the ACR oxygen intensity at 1 AU due to solar modulation during the solar cycle. Using the ACE measurements at 1 AU along with observations from Voyager 1 and 2 in the outer heliosphere, we determine the radial ACR oxygen intensity gradient during solar maximum and compare it with the galactic cosmic ray gradient during that time.

This work was supported by NASA at Caltech (under grants NAG5-6912 and NAS7-03001), JPL, and GSFC.

The neutral gas component of the local interstellar medium (LISM) flows through the inner heliosphere due to the relative motion of the Sun and the surrounding medium. Interstellar ions in contrast are diverted around the heliosphere by the magnetic field. The cross flow of the interstellar neutrals and ions at the heliopause leads to interactions that cause filtration of many species, in particular H and O. UV backscattering, pickup ion, and direct neutral gas observations for several species allow us to diagnose the flow pattern of the inflowing neutrals and to unravel the conditions in the LISM and the amount of filtering that occurs in the heliospheric boundary region. Using simultaneous observations with several solar, heliospheric, and astrophysics spacecraft, a recent coordinated analysis effort hosted by the International Space Science Institute (ISSI) has produced a consolidated set of the physical parameters for He. Starting with the reasonable assumption that all species in the undisturbed LISM have the same physical parameters (i.e. temperature, velocity) as He, which reaches the inner heliosphere without significant modification by the heliospheric interface, a similar effort is underway for interstellar H, whose characteristics are substantially altered. Making use of pickup ion, solar wind slowdown, and UV scattering observations at various distances from the Sun, and a full chain of modeling from the pristine LISM to the inner heliosphere, the interstellar H density, element abundances, and ionization fractions can be constrained. In addition, the effects of the heliospheric interface on flow vector and temperature can be inferred. Direct neutral gas observations provide the kinetic parameters and pickup ions return the density with the least uncertainties. The analysis results depend critically on the knowledge of the variable and three-dimensional solar radiation and solar wind flow. We report on the progress that is being made on the analysis efforts which combine comprehensive modeling with a broad range of observational data and promise to provide the best-constrained picture of the heliosphere and its interaction with the LISM to date.

Interstellar neutral gas flows through the inner heliosphere because of the Sun’s relative motion with respect to the surrounding local interstellar medium. Even for the contemporary conditions of a warm and dilute interstellar medium with a neutral H density of about 0.18 cm^-3 and a He density of 0.015 cm^-3 interstellar He constitutes the largest neutral gas contributor in interplanetary space at Earth’s orbit. This neutral gas is the source of a small, but noticeable population of pickup ions in the solar wind, which in turn is injected into ion acceleration processes much more efficiently than solar wind ions. In fact, He⁺ has been found to be the third most abundant energetic ion species at 1 AU after H⁺ and He²⁺. During its lifetime the Sun has encountered a variety of different interstellar medium conditions. In fact, it has apparently entered the current local cloud only a few ten thousand years ago coming from the even more dilute and hot environment of the so-called local bubble. It is expected that the solar system has encountered much denser clouds in the past and will again in the future. We investigated denser interstellar clouds with a variation in temperature and bulk flow velocity by using multi-fluid models of the global heliosphere and kinetic models of the He flow to derive the spatial distribution of interstellar neutral H and He in the inner heliosphere. We also estimated the corresponding populations of pickup ions and energetic particles. Under all circumstances interstellar He maintains its dominant role as neutral gas contributor in the inner heliosphere. For total interstellar densities up to 100 times the current value, a parameter range for which the heliosphere remains large compared to the Earth’s orbit, the interstellar neutral gas generally does not yet affect the solar wind dynamically at 1 AU, except for the region of the He focusing cone on the downwind side of the interstellar flow. Because dense clouds usually are also cold a rather narrow cone with a drastically increased density develops, which acts like a huge and stationary comet in the sense that the solar wind would be decelerated to approximately the interstellar flow speed at 1 AU. In cases when the flow vector is almost aligned with the ecliptic plane the Earth would routinely encounter a region dominated by the interstellar gas. Here neutral solar wind and neutrals generated from the He pickup ions by charge exchange would bombard the Earth’s atmosphere, not being blocked the magnetic field. Under such conditions energetic particles generated from interstellar pickup ions would also dominate the energetic particle population during solar minimum.

On time scales as short as 10kyrs, the heliosphere is exposed to different interstellar environments, as suggested by the observation of interstellar cloud types within several hundred pc of the Sun which vary in their density, temperature, degree of ionization, and galactic velocity. In particular, the Sun was once located in the hot, ionized, low-density plasma interior of the Local Bubble, and may occasionally be battered by high-velocity interstellar shock fronts. By means of numerical modeling, the interaction of the solar wind with a variety of partially ionized interstellar media is investigated. A range of ISM densities, relative velocities, and temperatures is considered. The resulting changed heliospheric structures are discussed, as are the altered distributions of neutral atoms and galactic cosmic ray spectra.

The numerical modeling of the self-consistent interaction of the partially ionized local interstellar medium with the solar wind yields detailed estimates of the distribution of neutral particles throughout the heliosphere. The solar wind and the solar radiation undergo long-term variability on the time scale of the periodic 11 year solar sunspot cycle. We study the consequences of this variability on the neutrals in the heliosphere through a time-dependent global heliospheric model, assuming an idealized 11 year solar variation. Charge exchange between the neutrals of interstellar origin (and secondary neutrals) and solar wind heavy ions produces characteristic x-ray emission. We evaluate the time-dependent x-ray (volume) emission and present line-of-sight integrated x-ray intensity predictions. The integration will mix features from different phases in the solar cycle.

The SWAN instrument on SOHO has been recording measurements of the interplanetary ultraviolet background for more than a decade now. These measurements are made by two identical sensor units placed on opposite sides of the SOHO spacecraft. A periscopic system allows to point the line of sight of each unit in any direction of the sky which is not blocked by the spacecraft. So each unit can see half of the sky in a period of roughly one day. The SWAN instrument makes 4 full-sky maps per week The intensity data obtained with these full-sky maps have been used to derive information on the ionization fluxes emitted by the sun and on their variation during the solar cyle. The SWAN instrument also contains hydrogen cells which can be used to study the line profile of the interplanetary UV emission. We will show how the H cell data can be used to reconstruct the interplanetary line profile. This requires to combine the H cell measurements obtained during weekly full-sky observations over a period of one year. The line profile reconstruction technique developed for the SWAN data has been used over the ten years of available data thus allowing to see the variations of the interplanetary line profile over a solar cycle. Line profiles are used to derive apparent velocities and temperatures of the interplanetary hydrogen atoms (averages on the line of sight). This constrains the models of the heliospheric interface. We will show that the line profiles can be used to constrain the radiation pressure from the Sun, the solar ionization flux but also give information about the heliospheric interface which influences the interplanetary UV background.

Voyager 2 is now beyond 75 AU and observing the descending phase of the solar cycle. Voyager currently observes low speeds, near 400 km/s, suggesting coronal hole flow is not observed. Since the latitudinal speed gradient which occurs in the inner heliosphere at solar minimum has not yet developed, we can use the speed decrease from Earth to Voyager 2 to determine the solar wind slowdown due to interaction with interstellar H and thus the interstellar H density at the termination shock. We present a profile of the speed slowdown versus distance and compare it to model predictions. Voyager 2 is also the spacecraft in best position to predict the plasma conditions at Voyager 1 (whose plasma instrument failed). We show that some of the Voyager 1 energetic particle events are associated with plasma changes observed at Voyager 2. We also discuss how the solar wind plasma has evolved with distance since Voyager was last in the descending phase of the solar cycle 11 years ago.

The Interstellar Boundary Explorer (IBEX) will measure energetic neutral atoms (ENAs), which are mainly produced by charge exchange processes between protons and interstellar hydrogen in the inner heliosheath. On the basis of a 3D model of the heliosphere ("Bochum model"), we extend the 2D computations with the Bonn model by Fahr and Scherer (2004) to derive full-sky ENA flux maps. According to these authors, there are three seed populations that contribute to the total ENA flux in the energy range to be observed by IBEX, namely the shocked solar wind protons and the pick-up ions (PUIs) upstream and downstream of the termination shock. While the solar wind contribution can be directly obtained from the Bochum code, those from the PUIs can been estimated combining 2D and 3D model results. The resulting full-sky ENA flux maps enable us to study their sensitivity to different structures of the heliospheric interface and to compare them with forthcoming IBEX maps.

The solar wind is too weak to be able to directly detect around another solar-like star with current remote sensing capabilities. However, the existence of such winds around other cool main sequence stars can be inferred indirectly from observations of Lyman-α absorption from the interaction regions between the winds and the interstellar medium. Since the amount of absorption depends on the strength of the stellar wind, this absorption diagnostic has led to the first estimates of mass loss rates for truly solar-like stars. By analyzing how mass loss correlates with stellar age and activity we can infer what the solar wind was like in the past. This is not only of interest to solar/stellar astronomers but is also of vital importance to those who study planetary atmospheres due to the eroding effects that the solar wind is believed to have had on the atmospheres of planets such as Mars.

Effects of anomalous component of cosmic rays (ACRs) influence on variation of the location of the termination shock with the solar cycle are investigated. The dynamic pressure of the solar wind changes by factor of two during 11-year solar cycle. The heliospheric termination shock varies under the influence of these dynamic pressure variations. In this paper we consider spherically symmetric time-dependent model of the solar wind interaction with the local interstellar medium, which takes into account influence of the solar wind protons, electrons, pickup ions, interstellar H atoms and ACRs components. Mutual influences of the components are treated self-consistently. We present results of a parametric study varing diffusion coefficient, efficiency of injection of ACRs at the termination shock and number density of interstellar H-atoms. Results are compared with previously published global models of the time-dependent heliosphere (e.g. Izmodenov and Malama, 2004; Izmodenov et al. 2005; Scherer and Fahr, 2003; Mueller and Zank, 2003).