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SH11B-1918: Numerical Modeling of the Heliosphere Using Interplanetary Scintillation Data as Boundary Conditions Tae K. Kim 1,2 ([email protected]), Sergey Borovikov 2 ([email protected]), Keiji Hayashi 3 ([email protected]), Nikolai Pogorelov 1,2 ([email protected]) 1 Department of Physics, The University of Alabama in Huntsville, 2 Center for Space Plasma and Aeronomic Research, The University of Alabama in Huntsville, 3 W.W. Hansen Experimental Physics Lab., Stanford University ● Abstract Radio waves from distant, astronomical sources are scattered as they propagate through turbulent medium, such as the solar wind. The resulting random fluctuation in intensity observed is called interplanetary scintillation (IPS), which can be used in determining the solar wind properties, such as density and velocity. To numerically determine the plasma properties in the outer-heliosphere, such as the heliosheath where the two Voyager spacecraft are, we need the boundary conditions at some fixed distance from the Sun that are time-dependent and three-dimensional (3-D). Since IPS measurements cover a wide range of radial distances and latitudes, they fit the profile very well as such boundary conditions. The Center for Space Plasma and Aeronomic Research (CSPAR) at the University of Alabama in Huntsville has developed a package of numerical codes called Multi-Scale Fluid- Kinetic Simulation Suite (MS-FLUKSS) that are designed for solving ideal magnetohydrodynamics (MHD) equations with multiple discontinuities and simulating the complex flow of partially-ionized plasma in the outer heliosphere. In this study, we implement the IPS data from the Solar-Terrestrial Environment Laboratory (STEL) at Nagoya University in Japan as boundary conditions in MS-FLUKSS and compare the results with in situ measurements by Voyager 1 and 2 spacecraft for accuracy. 2. IPS Data 4. Simulation Results 5. Conclusions and Discussions 3. Computational Tools 1. Introduction ● References ● Acknowledgments The structure of the heliosphere and the Voyager 1 and 2 trajectories (Image Credit: NASA) As Voyager 1 and 2 continue their journey past the termination shock toward the heliospheric boundary with the local interstellar medium (LISM), they are gathering invaluable data about the previously unexplored regions of the outer-heliosphere, which is a very interesting place characterized by the interaction between the solar wind and the LISM. The figure on the left shows the structure of the outer part of the heliosphere. At the termination shock, the solar wind slows down to subsonic speeds; the solar wind is further decelerated by interaction with the LISM in the heliosheath just beyond the termination shock. Then, the heliopause, defined by a tangential discontinuity produced by collisions between the two streams of solar wind and LISM, marks the theoretical boundary between the heliosphere and the LISM. There may also lie a bow shock within the LISM beyond the heliopause. The solar wind data gathered by Voyager 1 and 2 in the upcoming years - as the two spacecraft are currently probing the inner heliosheath at approximately 119 astronomical units (AU) and 97 AU from the Sun, respectively - would further enhance our understanding of the complex physical processes occurring in the outer-heliosphere. In contrast to in situ measurements by spacecraft that provide the solar wind parameters directly, IPS measurements are integrated along the line of sight from the observer to the source in order to extract the corresponding solar wind parameters; therefore, its accuracy can become an issue. However, since IPS measurements cover a wide range of radial distances and latitudes, whereas in situ measurements are limited to a particular location in space and time, IPS measurements of the solar wind can help us construct more comprehensive, realistic boundary conditions to be used in our time-dependent 3-D MHD codes. UHF Radio Telescope at Fuji Station in Japan (Image Credit: STEL) The figures in the left column show the solar wind number density and the radial component of velocity at the distance of 5 AU from the Sun on January 01, 2001 (near the most recent solar maximum). The figures in the right column show the same parameters for December 31, 2009 (around the current solar minimum). We use MS-FLUKSS, which is a publicly available package of numerical codes developed at CSPAR with support from NASA and NSF. In a nutshell, MS- FLUKSS consists of an adaptive mesh refinement module, multi-fluid and kinetic modules, and a cosmic ray transport module that are designed for solving ideal MHD equations with multiple discontinuities and simulating the complex flow of partially-ionized plasma in the outer heliosphere (Pogorelov et al., 2009). Although it is particularly challenging to fit numerical results to measured values in the outer- heliosphere, which is characterized by interaction between ions and neutral atoms, MS-FLUKSS has Block scheme of MS-FLUKSS (Pogorelov et al., 2011) Hayashi, K., Kojima, M., Tokumaru, M., Fujiki, K.: MHD Tomography Using Interplanetary Scintillation Measurement, J. Geophys.Res., 108, A3, 1102, doi: 10.1029/2002JA009567, 2003. Pogorelov, N. V., Borovikov, S. N., Florinski, V., Heerikhuisen, J., Kryukov, I. A., Zank, G. P. (2009), in Astronomical Society of the Pacific Conf. Ser. 406, Numerical Modeling of Space Plasma Flows: ASTRONUM-2008, ed. N. V. Pogorelov, E. Audit, P. Colella, & G. P. Zank, 149- 159 (San Francisco: ASP) Pogorelov, N. V., Borovikov, S. N, Heerikhuisen, J., Kim, T., Kryukov, I. A., Zank, G. P. (2011), in Astronomical Society of the Pacific Conf. Ser. 444, Numerical Modeling of Space Plasma Flows: ASTRONUM-2010, ed. N.V. Pogorelov, 130-136 (San Francisco: ASP) Kraken (top) and Lonestar (bottom) proven to be capable of handling such processes very well. The above figure shows a simple block scheme of MS-FLUKSS. The solar wind parameters at 5 AU, such as those shown in the previous section, were obtained numerically from MHD tomography analysis (see Hayashi et al., 2003) using the daily IPS measurements at STEL. The MHD tomography data contains the solar plasma number density (count/cc), temperature (K), and velocity (km/s) and magnetic field (nT) components at 2,112 grid points structured on a 5 AU sphere in the Heliographic Inertial (HGI) coordinate system. After obtaining a steady state solution, we used the MHD tomography data from January 01, 2001 to December 31, 2009 as the inner boundary conditions in our 3-D time- varying simulation, in which we employed a two fluid model consisting of solar wind protons and pickup ions. The computations were performed on both Kraken and Lonestar that are shown in the images on the left. The simulation results in the previous section show the time variation of number density (/cc) and radial component of velocity (km/s) between 2001 and 2009. Utilizing MS-FLUKSS's capability of extracting the solar wind parameters along the trajectories of Voyager 1 and 2, we compare the simulation data with the spacecraft data as well. The termination shock distances in our simulation are around 86 and 81 AU along the Voyager 1 and 2 trajectories, respectively; these values are smaller than the observed termination shock distances of 94 and 83.6 AU and need further improvement. The figures in the bottom row of the results section show comparisons between the measured data and the simulation data scaled around the termination shock, and there seems to be a good correlation at least with the Voyager 2 data. As for Voyager 1, only the radial component of velocity in the inner heliosheath is available for comparison. However, given the relatively large difference between the simulated and observed termination shock distances, the data comparison with the Voyager 1 measurements is probably not very meaningful after all. It is possible that the MHD tomography data used as the inner boundary conditions may contain errors, although there could be other reasons. Our next step would be to construct additional boundary conditions by combining IPS speed and density data at 50 solar radii from the Sun obtained from STEL with other observational data, and continue our data-driven simulation. The Voyager 1 and Voyager 2 data are courtesy of R. A. Decker and J. D. Richardson. This research was supported by an allocation of advanced computing resources provided by the National Science Foundation. The computations were performed on Kraken at the National Institute for Computational Sciences (http://www.nics.tennessee.edu) and on Lonestar at the Texas Advanced Computing Center (http://www.tacc.utexas.edu). This research was also supported by the NASA EPSCoR project titled The dynamical Inner Heliosphere and the Space Radiation Environment .
