SH14A-1687
The Interstellar Heliopause Probe / Heliospheric Explorer: IHP/HEX
The Sun, driving a supersonic solar wind, cuts out of the local interstellar medium a giant plasma bubble, the heliosphere. Dedicated deep-space missions have greatly enhanced our understanding of our immediate neighborhood. Ulysses is the only spacecraft exploring the third, out-of-ecliptic dimension, while SOHO has allowed us to better understand the influence of the Sun and to image the glow of interstellar matter in the heliosphere. Voyager 1 has recently encountered the innermost boundary of this plasma bubble, the termination shock, and is returning exciting yet puzzling data of this remote region. The next logical step is to leave the heliosphere and to thereby map out in unprecedented detail the structure of the outer heliosphere and its boundaries, the termination shock, the heliosheath, the heliopause, and, after leaving the heliosphere, to discover the true nature of the hydrogen wall, the bow shock, and the local interstellar medium beyond. This will greatly advance our understanding of the heliosphere that is the best-known example for astrospheres as found around other stars. Thus, IHP/HEX will allow us to discover, explore, and understand fundamental astrophysical processes in the largest accessible plasma laboratory, the heliosphere. IHP addresses three core Science goals: H: How do solar wind and interstellar medium interact to form the heliosphere and how does this relate to the universal phenomenon of the formation of astrospheres? A: What are the properties of the very local interstellar medium and how do they relate to the typical ISM? F: How do plasma, neutral gas, dust, waves, particles, fields, and radiation interact in extremely rarefied and incompletely ionized plasmas?
SH14A-1688
The Interstellar Boundary Explorer Instrument Models and Predicted ENA Count Rates
The upcoming launch of the Interstellar Boundary Explorer (IBEX) promises a unique data set of heliospheric energetic neutral atoms (ENAs), rich with information about the global dynamics of the termination shock and heliosheath. We have developed a suite of tools to predict synthetic ENA flux from the internal energy, density, and bulk flow derived from large-scale magnetohydrodynamic simulations of the heliosheath, creating a visualization framework for ENA flux maps as well as making predictions for the IBEX mission. In the future these tools will be critical for interpreting the implications of IBEX observations. The impact of the ion distribution function is also explored in the context of ENA flux maps. Studies from ACE, WIND, and Ulysses have shown the solar wind ion population has a power law tail in suprathermal velocities between 2-10 times the solar wind speed. This solar wind characteristic is found to increase the predicted flux by more than an order of magnitude at the highest IBEX energies, with less but considerable impact at lower energies.
SH14A-1689
A Forecast of the Termination-Shock Position
The effects of heliospheric disturbances on the position of the termination shock (TS) are examined using a three-dimensional MHD model. Variations in the solar wind ram-pressure due to the interplanetary shock waves drive the TS from its steady-state equilibrium position, and emit shocks and waves downstream. Transmitted/emitted disturbances propagating from the TS to the heliopause (HP) are partially reflected at the HP, and the reflected waves return and collide with the TS. Thus, besides upstream solar wind disturbances, the TS position changes in response to incident downstream disturbances associated with waves reflected from the heliopause produced by earlier supersonic solar wind disturbances. To determine the time-varying TS position, we incorporate Voyager 2 (V2) plasma data as a boundary condition into our 3D MHD simulations, which allows us to forecast the termination shock movement for nearly a year after the present V2 data. Our simulations indicate that the TS was at about 92AU along the sun-V1 line on August 14, 2007, the last tentative available date of the V2 data. After this, our simulation forecasts that the TS position will continue to decrease to a minimum distance in late 2007 or early 2008. This decrease will be caused by the heliosheath returned-pulse driven by the March 2006 event. Whether V2 will cross the TS or not in this period depends on the future SW ram-pressure and also on the degree of the north-south asymmetry of the heliospheric structure. Some quantitative discussions are given.
SH14A-1690
Three-dimensional magnetohydrodynamic modeling of the solar corona and solar wind for a highly-tilted solar dipole
A fully three-dimensional MHD model is used to simulate the global steady-state structure of the solar corona and solar wind for a solar magnetic dipole tilted by 60 and 90° to the solar rotation axis. The model accounts for the energy and momentum addition from Alfvén waves in the region between the base of the corona and Earth's orbit and for the effects of interstellar pickup protons in the outer heliosphere (1-100~AU). We present simulation results on the evolution of the heliospheric current sheet and the corotating interaction regions with heliocentric distance and on the effects of pickup protons in this evolution.
SH14A-1691
Heliospheric current sheet in the distant solar wind plasma
Since Voyager 1 plasma instrument is not operational, understanding the data obtained by its magnetometer is of great importance for heliospheric community. One of the main difficulties one encounters when modeling the interplanetary magnetic field (IMF) in the solar wind (SW) is the necessity of a very fine resolution of the heliospheric current sheet (HCS). The angle between the Sun's rotation and magnetic-dipole axes is never zero, varying from about 8-9 degrees during solar minima to 90 degrees at solar maxima. As a result of Sun's rotation, the distance between two consecutive crossings of the ecliptic plane by the HCS becomes as small as about 3 AU in the supersonic SW and necessarily smaller in the inner heliosheath. As shown by Pogorelov (2006), charge exchange of the SW plasma with the interstellar medium neutrals can affect the HCS behavior qualitatively. This study is an attempt to investigate the HCS evolution in the SW from its origin at the inner boundary of the computational region out into the heliosheath. Comparison is made of the ideal MHD and MHD-neutral solutions.
SH14A-1692
The Importance of Using Continuous Solar Inputs in 3D Models to Simulate the Distant Solar Wind and Heliosheath
Solar transients and the background solar wind give rise to asymmetries in the distant solar wind and heliosheath. Entering continuous solar data into time-dependent, 3D models originating at the Sun is crucial for accurately characterizing the distant solar wind and the heliosheath. We employ our time-dependent, 3D MHD (magnetohydrodynamic) model the HHMS - the Hybrid Heliospheric Modeling System - to obtain insights into the propagation of the solar wind to ACE, Ulysses, and beyond. In addition, we use our quick look tool, the HAFv.2, a 3D, time-dependent kinematic model, to complement the HHMS results. Comparisons of simulations from both models with spacecraft data show very good agreement. We present case study results that contrast the significant differences between including and excluding specific features of models. These case studies illustrate that, in order to obtain a meaningful depiction of the heliosphere in 3D, it is necessary to use models that take primary solar source data as continuous inputs and extrapolate phenomena in three dimensions from the solar surface throughout the heliosphere and into the heliosheath. Models that are limited in their dimensionality (i.e., 1D or 2D), or limited temporally (i.e., non- continuous input data), or limited in terms of the starting or boundary conditions (e.g., starting at Earth), all lead to erroneous depictions of the time varying 3D heliosphere. For example, whereas the results from the 3D models with continuous inputs starting at the Sun agree with observations at Ulysses, even the results from the best 1D models differ considerably from the observations at Ulysses (e.g. the Halloween 2003 events). 3D modeling a solar event in isolation and not in the context of the continuous solar inputs leads to a very different longitudinal distribution of the event and its asymmetries (e.g., the January 2005 events). Modeling the heliospheric latitude variations in solar wind density starting with observations made at L1 rather than at the Sun leads to erroneous distributions (e.g. the Halloween 2003 events). Overall our results emphasize the significant asymmetries in three dimensions within the solar wind throughout the heliosphere. Moreover, these model and data comparisons provide the context for our studying solar transients and their effects on energetic particles, galactic cosmic rays, and turbulence in the distant solar wind and heliosheath.
SH14A-1693
Hybrid Simulations of the Heliospheric Termination Shock
We use the Los Alamos hybrid plasma code to simulate the quasi-perpendicular heliospheric termination shock in one spatial dimension. The purpose of this study is to shed light on the shock-heated thermal and suprathermal populations in the heliosheath that have not yet been measured directly. Our study will make predictions that will be compared to energetic neutral atom (ENA) observations to be made by the Interstellar Boundary Explorer (IBEX) Mission after its launch in 2008. The ENAs to be measured by IBEX are generated from charge exchange of interstellar neutrals with protons heated at the termination shock. We use the simulations to characterize the downstream proton velocity distribution as a function of Alfven Mach number and plasma beta for a range of parameters expected at the termination shock. These distributions will then be used in a model to predict the fluxes of energetic neutrals to be observed by IBEX.
SH14A-1694
Diffusive Acceleration at the Blunt Termination Shock
Voyager 1's recent and long anticipated passage into the heliosheath contradicted the prediction that we would observe the source of anomalous cosmic rays accelerated by the termination shock. The observed energetic protons reveal a power-law spectrum below several MeV, but above several MeV the spectrum falls more sharply, and we observe the familiar bump caused by modulation of anomalous cosmic rays. Here, we develop the theoretical framework to include the motions and drift of particles during diffusive shock acceleration at a 3-D termination shock, including cross-field diffusion, and curvature and gradient drifts. Our model supports the concept of McComas and Schwadron that because of the termination shock's blunt structure, there should be a strong deficit of locally accelerated anomalous cosmic rays near the nose. With reasonable parameters for the scattering mean free path parallel to the magnetic field of ~3 AU, and a ratio of perpendicular to parallel diffusion of ~0.03, the model produces an energy spectrum that agrees well with Voyager 1 observations near the termination shock. These parameters also lead to an acceleration time to 10 MeV of about 1 year, which is comparable to previous estimates derived from the observed charge states of ACRs. Thus, we provide a theory for diffusive acceleration at the blunt termination shock. The predictions of this theory are consistent with Voyager 1's observations showing a lack of ACRs accelerated near the nose of the termination shock and the ACR acceleration timescale derived from ACR charge-states.
SH14A-1695
The Effects of Using a Differential Cross-section for Charge-exchange on the Geometry of the Heliosphere
The flow of the solar wind plasma carves out the heliosphere, and creates a large scale region of interaction between itself and the interstellar medium. Although the solar wind and interstellar plasmas do not mix -- they are instead separated by a contact discontinuity known as the heliopause -- neutral hydrogen atoms of interstellar origin flow freely through the entire region. Charge-exchange weakly couples the plasma and neutral gasses, and this process leads to a significant cooling and shrinking of the heliosheath compared with a plasma-only model). Secondary neutral atoms produced in the solar wind by this process may travel back into the interstellar medium and deposit their energy upon further charge-exchange. Models of the heliosphere have traditionally included neutral hydrogen atoms either kinetically or as a fluid, and coupled these to the plasma through charge-exchange. Charge-exchange collisions, however, represent a limiting case of a more general H-p collision, where particle momentum interchange exactly. In this poster we compare this commonly used approximation with an approach which utilizes the differential cross-section. We will present a detailed analysis of the ion and neutral distributions, along with comparisons of the overall heliospheric structure in axially symmetric geometry.
SH14A-1696
Interstellar Neutral Atoms in the Heliosphere: Predictions for IBEX-LO
The flow of local interstellar gas into the heliosphere has been observed indirectly with several methods. These neutral atoms are unperturbed by the interplanetary magnetic field and provide a probe of the conditions in the local interstellar cloud. The low energy instrument on board the Interstellar Boudary Explorer (IBEX-LO) will convert these particles to negative ions (via scattering from a CVD diamond surface) for measurement by standard mass and energy analysis techniques. Although the direct measuremet of the interstellar flow is not the primary science goal of the IBEX mission, it will provide a valuable new dataset which can be used as input for heliospheric modelers. Recent calibration measurements of IBEX-LO at the University of Bern have shown that the signal from the helium component of the LISM will be stronger than expected. Here we use the existing measurements interstellar neutral parameters as well as calibration results to predict the signal seen by IBEX-LO from the interstellar neutral flow.
SH14A-1697
The Boundary Conditions of the Heliosphere: Photoionization Models Constrained by In Situ and Interstellar Data
The Sun is immersed in a small, warm, and low density interstellar cloud, known as the Local Interstellar Cloud (LIC). The LIC thickness, less than about 100,000 AU, and low density, n~0.25 cm-3 at the Sun, yield a LIC that is transparent to helium-ionizing radiation while having significant optical depth to hydrogen-ionizing radiation. As a result, the heliosphere boundary conditions depend on the radiative transfer properties of the surrounding LIC. At the heliosphere, elements with first ionization potential of 13.6-25 eV, e.g. H, He, N, O, Ne, and Ar, are partially ionized with fractional ionizations of about 0.2 to 0.7. We present results on the boundary conditions of the heliosphere that are based on models that include the radiation field gradient, and the thermal balance and photoionization of the surrounding interstellar material. The model constraints are data on interstellar material inside of the heliosphere, such as pickup ions, He, N, O, Ar, Ne, and anomalous cosmic rays, as well as astronomical observations of interstellar gas towards the star eta CMa. The interstellar radiation field is made up of stellar FUV and EUV, as well as diffuse soft X-ray emission from the hot gas of the Local Bubble and the thermally conductive boundary between the LIC and the Local Bubble. We find that we can satisfy the observational constraints, including the temperature and density of neutral He in the heliosphere, for a range of model parameters that affect the radiation field. Despite the wide range of possible input parameters, the output values for quantities important for shaping the heliosphere are confined to a fairly small range: n(H°)=0.19 -- 0.20 cm-3, n(H+)=0.04 -- 0.07 cm-3. The best models indicate that the ISM creating the heliosphere boundary conditions is low density and partially ionized, n(H°)=0.19 cm-3 and n(H+)=0.05 cm-3. About 38% of the helium is ionized, and about 22% of the hydrogen is ionized. At the heliosphere edge, the neutral hydrogen density is always be higher than ten times the neutral helium density, or n(H°)>0.151 cm-3, because of the radiation field hardness. Our results favor a reference abundance pattern for the LIC that is comparable to the protosolar abundances. The depletion pattern of refractory elements onto interstellar dust grains is characteric of warm low density interstellar material. The LIC gas-to-dust mass ratio derived from astronomical data depends strongly on the assumed reference abundance set, and ranges from 140 to 490. In contrast, in situ observations of interstellar dust in the heliosphere yield a value 116 to 124.
SH14A-1698
Observing Interstellar Neutral Helium from the Ground
Cool neutral Helium is now being observed near the Sun. Routinely from sensitive IR spectropolarimetry data using SOLARC at Haleakala and from eclipses, observations reveal scattered photospheric light at 1083nm from He I far above the solar limb. Here we report on observations of this transient coronal constituent that help to understand whether this comes from the coronal "inner source" or from the interstellar Helium wind.
SH14A-1699
High-latitudinal Activity in Solar Cycle 23
Cycle 23 is going to be finished. What is the new we know about the nature of the solar cyclicity on the base of the modern observations in space and on the ground? How deep is our knowledge about the high-(mid)- latitudinal activity on the Sun? And, it is the next question. Is our undestanding of the solar activity enough to predict the next cycle and the behavior of the solar activity in the future. All these questions come from the past and are still an important. The Solar and Heliospheric Observatory provides us an important data of coronal and magnetic activity since 1996 (the beginning of the cycle 23). Due to these data we can look at the EUV corona on the solar disk and compare it with the magnetic energy release, directly. We can observe the changing of the magnetic pattern during the solar cycle from small-scale magnetic elements ('knots', or facular) to large-scale (prominances, sunspots) both at the photosphere and the corona. The results of investigations of the current cycle made by different authors show the complicated picture of the solar cycle while the main features like Hale's and Joe's laws, 'butterfly' distibutions, polar magnetic field reversals are kept. Here, I represent the results of the investigation of the high-latitudinal activity and its relation to the mid-latitude of the photoshere and the corona for the current cycle 23 using the SOHO/MDI, SOHO/EIT and WSO solar data. The observational dynamic behavior of the solar magnetic features is discussed with the relation to the dynamo theories.
SH14A-1700
Ulysses Transition into the Polar Coronal Holes
The Ulysses spacecraft has now half-completed its third polar orbit around the Sun. Like the first polar orbit, in 1992-1998, it is occurring at declining to minimum solar activity. Yet the two transitions into the hole are remarkably different. The first transition was substantially more regular than the second one is. This may be understood in the context of the current sheet orientation, which was generally flatter during the first transition, but is more strongly warped during the second. We model the global distribution of slow and fast solar wind streams starting from the Wilcox Solar Observatory maps of the solar magnetic field at the source surface. The model assumes slow solar wind to emanate from the vicinity of the current sheet with a speed that increases as a function of angular distance from the CS, i.e., with magnetic latitude, as it has been found on Helios. Fast solar wind of constant speed is assumed to emanate from the coronal holes and to expand superradially so as to fill the entire solid angle above a certain magnetic latitude. Thus we obtain a model solar wind speed at the position of Ulysses, and comparing to the observed solar wind speed there we can optimize the model to finally obtain the angular width of the belt of slow solar wind around the heliospheric current sheet.
SH14A-1701
Recent Ulysses Fast latitude Scan: Magnetic Field Observations
In January 2006, the Ulysses spacecraft passed completely above the Heliospheric Current Sheet (HCS) at latitude -37 degrees and entered the fast wind. It reached maximum southern latitude of - 80 degrees in February 2007. Continuous field measurements provide a second opportunity to investigate the 3D heliosphere at solar minimum under significantly changed circumstances, a reversal in the magnetic field polarity and a polar cap field smaller by a factor of about 2. As in 1994-1995, observations of r2 Br are independent of latitude. However, the average value appears to have decreased with a marked change in the non-Gaussian probability distribution functions indicating a systematic change in the large amplitude magnetic fluctuations. February 2007 began the Fast Latitude Scan (FLS) when Ulysses sweeps between polar caps in only 10 months reducing the influence of temporal variations. This interval is also optimal for investigating north-south asymmetries. Ulysses traveled northward leaving high- speed high-latitude wind and crossing the HCS at -30 degrees. In August, Ulysses crossed the ecliptic plane at nearly the same longitude as Earth and the network of near-Earth spacecraft. The FLS ends in November. These new magnetic field observations will be presented and compared with those at the previous solar minimum.
SH14A-1702
Voyager 2 in the Termination Foreshock: Application of Mission Independent Software to Energetic Particle Observations
The Low Energy Charged Particle (LECP) instruments on the Voyager 1 and 2 (V1 and V2) spacecraft have been returning unique scientific measurements since launching in 1977, most notably observations from the historic tour of the giant planets. As these spacecraft continue on their exit trajectories from the Solar system they have become an interstellar mission and have begun to probe the boundary between the heliosphere and the interstellar cloud. In December 2004, V1 crossed the termination shock in the northern heliosphere, entering a region of sub-sonic solar wind flow caused by the pressure of the interstellar material and magnetic field flowing at 26 km/s with respect to the Sun. Now V2 is in the termination foreshock and appears to be very near the termination shock in the South. As the mission changed from one focused on discrete encounters to an open ended search for heliospheric boundaries and transitory disturbances, the positions and timing of which are not known, the data processing needs have changed. Open data policies and the push to draw data under the umbrella of emerging Virtual Observatories have added a data sharing component that was not a part of the original mission plans. We will present our work in utilizing new, reusable software analysis tools to access legacy data in a way that leverages pre-existing data analysis techniques. We will take an existing Applied Physics Laboratory application, Mission Independent Data Layer (MIDL) -- developed originally under a NASA Applied Information Research Program (AISRP) and subsequently used with data from Geotail, Cassini, IMP-8, ACE, and New Horizons -- and apply it to Voyager data. The MIDL codebase will also be used to generate standard data products such as daily summary plots and associated tabulated data that increase our ability to monitor the heliospheric environment on a regular basis. These data products will be publicly available and updated automatically. We will illustrate the usefulness of these techniques of data processing and analysis by investigating some of the recent results from Voyager 2 as it approaches (or crosses) the termination shock.