Changqing Xiang

THREE-DIMENSIONAL SOLAR WIND MODELING FROM THE SUN TO EARTH BY A SIP-CESE MHD MODEL WITH A SIX-COMPONENT GRID

The objective of this paper is to explore the application of a six-component overset grid to solar wind simulation with a three-dimensional (3D) Solar-InterPlanetary Conservation Element/Solution Element MHD model. The essential focus of our numerical model is devoted to dealing with: (1) the singularity and mesh convergence near the poles via the use of the six-component grid system, (2) the ∇ · B constraint error via an easy-to-use cleaning procedure by a fast multigrid Poisson solver, (3) the Courant-Friedrichs-Levy number disparity via the Courant-number insensitive method, (4) the time integration by multiple time stepping, and (5) the time-dependent boundary condition at the subsonic region by limiting the mass flux escaping through the solar surface. In order to produce fast and slow plasma streams of the solar wind, we include the volumetric heating source terms and momentum addition by involving the topological effect of the magnetic field expansion factor fS and the minimum angular distance θ b (at the photosphere) between an open field foot point and its nearest coronal hole boundary. These considerations can help us easily code the existing program, conveniently carry out the parallel implementation, efficiently shorten the computation time, greatly enhance the accuracy of the numerical solution, and reasonably produce the structured solar wind. The numerical study for the 3D steady-state background solar wind during Carrington rotation 1911 from the Sun to Earth is chosen to show the above-mentioned merits. Our numerical results have demonstrated overall good agreements in the solar corona with the Large Angle and Spectrometric Coronagraph on board the Solar and Heliospheric Observatory satellite and at 1 AU with WIND observations.

A Hybrid Solar Wind Model of the CESE+HLL Method with a Yin-Yang Overset Grid and an AMR Grid

A hybrid three-dimensional (3D) MHD model for solar wind study is proposed in the present paper with combined grid systems and solvers. The computational domain from the Sun to Earth space is decomposed into the near-Sun and off-Sun domains, which are respectively constructed with a Yin-Yang overset grid system and a Cartesian adaptive mesh refinement (AMR) grid system and coupled with a domain connection interface in the overlapping region between the near-Sun and off-Sun domains. The space-time conservation element and solution element method is used in the near-Sun domain, while the Harten-Lax-Leer method is employed in the off-Sun domain. The Yin-Yang overset grid can avoid well-known singularity and polar grid convergence problems and its body-fitting property helps achieve high-quality resolution near the solar surface. The block structured AMR Cartesian grid can automatically capture far-field plasma flow features, such as heliospheric current sheets and shock waves, and at the same time, it can save significant computational resources compared to the uniformly structured Cartesian grid. A numerical study of the solar wind structure for Carrington rotation 2069 shows that the newly developed hybrid MHD solar wind model successfully produces many realistic features of the background solar wind, in both the solar corona and interplanetary space, by comparisons with multiple solar and interplanetary observations.

Energy transfer during intense geomagnetic storms driven by interplanetary coronal mass ejections and their sheath regions

The interaction of the solar wind and Earths magnetosphere is complex, and the phenomenology of the interaction is very different for interplanetary coronal mass ejections (ICMEs) compared to their sheath regions. In this paper, a total of 71 intense (Dst <= -100 nT) geomagnetic storm events in 1996-2006, of which 51 are driven by ICMEs and 20 by sheath regions, are examined to demonstrate similarities and differences in the energy transfer. Using superposed epoch analysis, the evolution of solar wind energy input and dissipation is investigated. The solar wind-magnetosphere coupling functions and geomagnetic indices show a more gradual increase and recovery during the ICME-driven storms than they do during the sheath-driven storms. However, the sheath-driven storms have larger peak values. In general, solar wind energy input (the epsilon parameter) and dissipation show similar trends as the coupling functions. The trends of ion precipitation and the ratio of ion precipitation to the total (ion and electron) are quite different for both classes of events. There are more precipitating ions during the peak of sheath-driven storms. However, a quantitative assessment of the relative importance of the different energy dissipation branches shows that the means of input energy and auroral precipitation are significantly different for both classes of events, whereas Joule heating, ring current, and total output energy display no distinguishable differences. The means of electron precipitation are significantly different for both classes of events. However, ion precipitation exhibits no distinguishable differences. The energy efficiency bears no distinguishable difference between these two classes of events. Ionospheric processes account for the vast majority of the energy, with the ring current only being 12%-14% of the total. Moreover, the energy partitioning for both classes of events is similar.

A DATA-DRIVEN MODEL FOR THE GLOBAL CORONAL EVOLUTION

This work is devoted to the construction of a data-driven model for the study of the dynamic evolution of the global corona that can respond continuously to the changing of the photospheric magnetic field. The data-driven model consists of a surface flux transport (SFT) model and a global three-dimensional (3D) magnetohydrodynamic (MHD) coronal model. The SFT model is employed to produce the global time-varying and self-consistent synchronic snapshots of the photospheric magnetic field as the input to drive our 3D numerical global coronal AMR-CESE-MHD model on an overset grid of Yin-Yang overlapping structure. The SFT model and the 3D global coronal model are coupled through the boundary condition of the projected characteristic method. Numerical results of the coronal evolution from 1996 September 4 to October 29 provide a good comparison with multiply observed coronal images.

Three-dimensional MHD simulation of CMEs in three-dimensional background solar wind with the self-consistent structure on the source surface as input: Numerical simulation of the January 1997 Sun-Earth connection event

A three-dimensional time-dependent, numerical magnetohydrodynamic (MHD) model is used to investigate the propagation of coronal mass ejections (CMEs) in the nonhomogenous background solar wind flow. On the basis of the observations of the solar magnetic field and K-coronal brightness, the self-consistent structure on the source surface of 2.5 Rs is established with the help of MHD equations. Using the self-consistent source surface structures as initial-boundary conditions, we develop a three-dimensional MHD regional combination numerical model code to obtain the background solar wind from the source surface of 2.5 Rs to the Earths orbit ( 215 Rs) and beyond. This model considers solar rotation and volumetric heating. Time-dependent variations of the pressure and velocity configured from a CME model at the inner boundary are applied to generate transient structures. The dynamical interaction of a CME with the background solar wind flow between 2.5 and 215 Rs ( 1 AU) is then investigated. We have chosen the well-defined halo-CME event of 6 - 12 January 1997 as a test case. Because detailed observations of this disturbance at 1 AU ( by WIND spacecraft) are available, this event gives us an excellent opportunity to verify our MHD methodology and to learn about the physical processes of the Sun-Earth connection. In this study, we find that this three-dimensional MHD model, with the self-consistent structures on the source surface as input, provides a relatively satisfactory comparison with the WIND spacecraft observations.

Time‐dependent MHD modeling of the global solar corona for year 2007: Driven by daily‐updated magnetic field synoptic data

In this paper, we develop a time-dependent MHD model driven by the daily-updated synoptic magnetograms (MHD-DUSM) to study the dynamic evolution of the global corona with the help of the 3D Solar-Interplanetary (SIP) adaptive mesh refinement (AMR) space-time conservation element and solution element (CESE) MHD model (SIP-AMR-CESE MHD Model). To accommodate the observations, the tangential component of the electric field at the lower boundary is specified to allow the flux evolution to match the observed changes of magnetic field. Meanwhile, the time-dependent solar surface boundary conditions derived from the method of characteristics and the mass flux limit are incorporated to couple the observation and the 3D MHD model. The simulated evolution of the global coronal structure during 2007 is compared with solar observations and solar wind measurements from both Ulysses and spacecrafts near the Earth. The MHD-DUSM model is also validated by comparisons with the standard potential field source surface (PFSS) model, the newly improved Wang-Sheeley-Arge (WSA) empirical formula, and the MHD simulation with a monthly synoptic magnetogram (MHD-MSM). Comparisons show that the MHD-DUSM results have good overall agreement with coronal and interplanetary structures, including the sizes and distributions of coronal holes, the positions and shapes of the streamer belts, and the transitions of the solar wind speeds and magnetic field polarities. The MHD-DUSM results also display many features different from those of the PFSS, the WSA, and the MHD-MSM models.

Three-dimensional MHD simulation of the evolution of the April 2000 CME event and its induced shocks using a magnetized plasma blob model

A three-dimensional (3-D) time-dependent, numerical magnetohydrodynamic (MHD) model with asynchronous and parallel time-marching method is used to investigate the propagation of coronal mass ejections (CMEs) in the nonhomogenous background solar wind flow. The background solar wind is constructed based on the self-consistent source surface with observed line-of-sight of magnetic field and density from the source surface of 2.5 R-s to the Earths orbit (215 Rs) and beyond. The CMEs are simulated by means of a very simple flux rope model: a high-density, high-velocity, and high-temperature magnetized plasma blob is superimposed on a steady state background solar wind with an initial launch direction. The dynamical interaction of a CME with the background solar wind flow between 2.5 and 220 Rs is investigated. The evolution of the physical parameters at the cobpoint, which is located at the shock front region magnetically connected to ACE spacecraft, is also investigated. We have chosen the well-defined halo-CME event of 4-6 April 2000 as a test case. In this validation study we find that this 3-D MHD model, with the asynchronous and parallel time-marching method, the self-consistent source surface as initial boundary conditions, and the simple flux rope as CME model, provide a relatively satisfactory comparison with the ACE spacecraft observations at the L1 point.

Using a 3‐D spherical plasmoid to interpret the Sun‐to‐Earth propagation of the 4 November 1997 coronal mass ejection event

We present the time-dependent propagation of a Sun-Earth connection event that occurred on 4 November 1997 using a three-dimensional (3-D) numerical magnetohydrodynamics (MHD) simulation. A global steady state solar wind for this event is obtained by a 3-D SIP-CESE MHD model with Parkers 1-D solar wind solution and measured photospheric magnetic fields as the initial values. Then, superposed on the quiet background solar wind, a spherical plasmoid is used to mimic the 4 November 1997 coronal mass ejection (CME) event. The CME is assumed to arise from the evolution of a spheromak magnetic structure with high-speed, high-pressure, and high-plasma-density plasmoid near the Sun. Moreover, the axis of the initial simulated CME is put at S14W34 to conform to the observed location of this flare/ CME event. The result has provided us with a relatively satisfactory comparison with the Wind spacecraft observations, such as southward interplanetary magnetic field and large-scale smooth rotation of the magnetic field associated with the CME.

RECONSTRUCTION OF THE CORONAL MAGNETIC FIELD USING THE CESE-MHD METHOD

We present a new implementation of the MHD relaxation method for reconstruction of the nearly force-free coronal magnetic field from a photospheric vector magnetogram. A new numerical MHD scheme is proposed to solve the full MHD equations by using the spacetime conservation-element and solution-element method. The bottom boundary condition is prescribed in a similar way as in the stress-and-relax method, by changing the transverse field incrementally to match the magnetogram, and other boundaries of the computational box are set by the nonreflecting boundary conditions. Applications to the well-known benchmarks for nonlinear force-free-field reconstruction, the Low & Lou force-free equilibria, validate the method and confirm its capability for future practical application, with observed magnetograms as inputs.

A NEW CODE FOR NONLINEAR FORCE-FREE FIELD EXTRAPOLATION OF THE GLOBAL CORONA

Reliable measurements of the solar magnetic field are still restricted to the photosphere, and our present knowledge of the three-dimensional coronal magnetic field is largely based on extrapolations from photospheric magnetograms using physical models, e. g., the nonlinear force-free field (NLFFF) model that is usually adopted. Most of the currently available NLFFF codes have been developed with computational volume such as a Cartesian box or a spherical wedge, while a global full-sphere extrapolation is still under development. A high-performance global extrapolation code is in particular urgently needed considering that the Solar Dynamics Observatory can provide a full-disk magnetogram with resolution up to 4096 x 4096. In this work, we present a new parallelized code for global NLFFF extrapolation with the photosphere magnetogram as input. The method is based on the magnetohydrodynamics relaxation approach, the CESE-MHD numerical scheme, and a Yin-Yang spherical grid that is used to overcome the polar problems of the standard spherical grid. The code is validated by two full-sphere force-free solutions from Low & Lous semi-analytic force-free field model. The code shows high accuracy and fast convergence, and can be ready for future practical application if combined with an adaptive mesh refinement technique.