SH12B-01 INVITED
The Interstellar Boundary Explorer (IBEX) -- Half a Year to Launch!
*On behalf of the IBEX Science and Mission Teams Recent observations by Voyagers 1&2 in the vicinity of the termination shock and inner heliosheath, along with renewed vigor in theoretical and computational work, make this an incredibly exciting time in the study of the heliosphere's interaction with the local interstellar medium. By their very nature, however, Voyager observations measure only the local properties in the vicinity of the two spacecraft, often raising as many questions about the global configuration and structure of the heliospheric interaction as they answer. Fortunately, the Interstellar Boundary Explorer (IBEX) mission will launch in mid-2008 and provide the first global views of the interstellar interactions at the edge of our heliosphere. IBEX will make these complementary global observations using two ultra-high sensitivity single pixel energetic neutral atom (ENA) cameras that image ENAs from 10 eV -- 6 keV in 14 energy bins. The IBEX spacecraft is a simple sun-pointed spinner, which allows the ENA cameras to sweep out full sky views of the heliospheric interaction every six months. IBEX will also be the first spacecraft to achieve a very high apogee (~50 RE) Earth orbit starting with a standard Pegasus launcher. IBEX's highly elliptical Earth orbit allows viewing of the outer heliosphere from beyond the Earth's relatively bright magnetospheric ENA emissions. This talk provides an overview of the IBEX mission, the science data products and their access, and updates the community on our status only half a year out from launch! http://ibex.swri.edu
SH12B-02 INVITED
Imaging the Heliosphere With ENA's From a Non-thermal Heliosheath
With the launch of NASA's Interstellar Boundary Explorer (IBEX) in less than a year, energetic neutral atoms (ENA's) are destined to become one of the leading tools in understanding the global structure of the heliosheath. Although little is known about the nature of the heliosheath, it seems likely that the plasma is not in thermal equilibrium. Instead, it is probably better described by a kappa distribution, with a relatively cool core and a power law tail. I will present simulated ENA flux maps from a coupled MHD-plasma/kinetic-neutral code, in which we have approximated the heliosheath plasma as having a kappa distribution. I will show some key features of the simulated heliosheath which are reflected in the ENA maps, and describe how IBEX can be used to infer such features in the real heliosheath. A brief overview of future challenges in heliospheric modeling and ENA imaging will also be given.
SH12B-03
The Orientation of the Local Interstellar Magnetic Field and Induced Asymmetries of the Heliosphere: Neutrals-MHD model
We present the results of a 3D Neutral-MHD model of the heliosphere. The neutrals are treated in a multi-fluid approach coupled to the ionized component by charge exchange. Comparisons are made with previous studies that showed that the local interstellar magnetic field introduces asymmetries in the heliosphere that are consistent with Voyager 1 and 2 observations of radio emissions and energetic particle streaming (Opher et al. Science 2007; Opher et al. ApJL 2006). We present, additionally, preliminary results of a 3D Kinetic-MHD model. The main advantage of this model is a rigorous kinetic description of interstellar H atoms, especially at the Bow Shock, Heliopause and Termination Shock interfaces. Differences of kinetic and multi-fluid approaches are discussed. The new model should provide refined estimates of the strength and direction of the local interstellar field and of the resulting distortions of the shape of the heliosphere.
SH12B-04
Heliospheric structure due to magnetic fields and neutral particles from the three- dimensional MHD-kinetic model
Neutral particles play a major role in determining the structure of the outer heliosphere. This is primarily due to the low degree of ionization of the local interstellar medium (LISM). Charge exchange and collisions among atoms and solar wind (SW) ions results in the deceleration of the latter. It is responsible for the birth of pickup ions (PUIs). Secondary neutrals can propagate far upstream of the LISM and modify its parameters. By decreasing the ratio of the SW and LISM ram pressures neutral particles decrease the distance of the heliopause to the Sun, so affecting the location and strength of the SW termination shock (TS). We have shown recently that charge exchange considerably decreases the effect of the interstellar magnetic field (ISMF) on the TS asymmetry -- the phenomenon very well known from ideal MHD simulations performed over the last decade. Since the mean free-path of neutral hydrogen, which is the most important atomic component of the LISM, is comparable with the characteristic lengths (the distances between major discontinuities) of the solar wind (SW) interaction with the LISM, a physically-consistent way to model its behavior is based on the solution of the kinetic Boltzmann equation. An efficient way to solve the Boltzmann equation is provided by stochastic simulations with a direct simulation Monte Carlo method. We apply the Riverside MHD-kinetic model of the heliospheric interface, incorporated into a parallel, adaptive mesh refinement code MS-FLUKSS (Multi-Scale FLUid-Kinetic Simulation Suite) to investigate the the structure of the heliosphere under the assumption that the plane formed by the ISMF and the LISM velocity vectors belongs to the hydrogen deflection plane (HDP) identified in the recent observations from the SOHO SWAN experiment. The actual deflection of the neutral hydrogen flow from its original orientation in the LISM is determined in the presence of the interplanetary magnetic field and ISMF. We present the results in the form of distributions of the plasma and magnetic field quantities in the directions of the Voyager 1 and Voyager 2 spacecraft. Connection of different points on the Voyager trajectories to the TS is investigated. We also analyze the regions of favorable 2-3 kHz radio emission occurring in the outer heliosheath due to charge exchange of the LISM plasma and hot secondary H atoms born in the inner heliosheath.
SH12B-05
Hybrid simulations of the interaction of a current sheet with the termination shock
We address the physics of the interaction of a current sheet with the solar-wind termination shock by utilizing self- consistent hybrid simulations. The characteristics of the region near the shock and the intersection point with a current sheet depends largely on the orientation of the field. For example, if the magnetic field changes sign across the current sheet in such a way that the drift of charged particles is directed away from the shock, a large region of hot, low density plasma, known as a hot-flow anomaly forms near the shock. This may be relevant to our understanding of observations made by the Voyager spacecraft near the termination shock. Voyager 1 observed a heliospheric current sheet sector boundary just prior to its crossing of the shock. We find from our simulations that the size of this region is at least a few million km in diameter; thus, it could take several hours for a Voyager spacecraft to cross through this region and observe its characteristics. In addition, the interaction of current sheets with shocks leads to, among other things, efficient particle acceleration, and turbulent downstream plasmas which may be relevant to Voyager observations of the heliosheath. Our new numerical simulations address the formation of these regions, their spatial sizes, and how they are influenced by large-scale upstream turbulence, which exists in the regions between current sheets. We also examine the influence on various ionic species. Implications for spacecraft observations will be discussed.
SH12B-06
Acceleration at the termination shocks: the pitch-angle perspective
We use a recently developed anisotropic transport model to investigate observed pitch-angle distributions (PADs) of energetic protons near the solar wind termination shock. Voyager data is represented in the form of expansion in spherical harmonics in the frame aligned with the measured mean magnetic field, permitting a direct comparison with the model. We find that observed PADs are consistent with model predictions for particle acceleration at a highly oblique shock wave. The termination shock obliquity angle is derived from first-order anisotropy measurements. Additionally, second-order anisotropy is used to obtain energy dependence of the particle mean free paths. We demonstrate that analyzing second-order anisotropy is crucial for interpreting data correctly. It is also shown that certain measured distributions characterized by anomalously large second moments can be interpreted as oppositely propagating beams in regions multiply-connected to the spacecraft.
SH12B-07
Modulation of galactic cosmic rays by global merged interaction region near the solar wind termination shock and in the heliosheath
Measurements from Voyager spacecraft at distant heliosphere show that cosmic rays are still modulated near the termination shock and even in the heliosheath. The September 2005 solar event propagated to the outer heliosphere, resulting in a very noticeable global merged interaction region (GMIR) at both Voyager 2 (still in the solar wind slightly insider the termination shock) and Voyager 1 (in the heliosheath). Decreases of galactic cosmic ray fluxes were observed in correlation with the GMIR. After the passage of GMIR, galactic cosmic ray fluxes have been recovering towards the solar minimum level, but cosmic ray flux at Voyager 2 is changing at a faster rate than at Voyager 1. The cosmic ray decreases in 2006 are definitely produced by the GMIR, but the cause of different rates of cosmic ray recovery after the GMIR is less certain. We use stochastic simulation of cosmic ray transport to investigate how the particle drift pattern and the GMIR, as a propagating diffusion barrier, can affect cosmic ray modulation in outer heliosphere. In this paper, we present calculation results of cosmic ray flux and its radial gradients when a GMIR passes the spacecraft near the termination shock or in the heliosheath. The results will be compared with observations to find out major causes of modulation under the current conditions in the outer heliosphere. We will also demonstrate how sensitive the cosmic ray modulation level and their recovery at the Voyager locations depend on the properties of GMIR in the heliosheath.
SH12B-08
Time dependent Solar Modulation of GCRs using a Monte-Carlo simulation
In characterising solar modulation of galactic cosmic rays, an often used parameter is the so called modulation parameter φ, whose value vary by a factor of more than 2 from the solar maximum to the solar minimum, reflecting the effects of different levels of solar activities on the modulation. Simple and elegant, this approach however, is based on the assumption that the current of GCRs is small and can be taken to be zero at all times, leading to a steady state solution of the transport equation. Recently, Wiedenbeck et al, using data from CRIS/ACE and ground neutron monitor, found that, for the declining phase of the solar cycle 23, the neutron monitor counting rate has been at a level of 4 percents higher than during the period when φ had comparable values during the rising phase of solar cycle 23. In this work, we examine this differences of neutron monitor counting rates (or alternatively, the difference of φ themselves) between the rising phase and declining phase of solar cycle. We show that the modulation of low energy GCRs is more sensitive to the phase of the solar cycle than high energy GCRs. We present in this work, the first time, a formalism to solve a time dependent transport equation using a stochastic method.