Application Of Parallel Adaptive Mesh Refinement To Numerical Study Of Solar Wind Background And Fast Magnetic Reconnection

MHD simulations, which can make up the lack of observations and the limitsof analysis and can also significantly get a deeper understanding of many kindsdi-cult physical processes, have been very powerful tools to investigate the spaceplasma physics. Computational grid is critical to MHD simulations, especially forthe problems that have large and complicated space variations, so the introduceof advanced grid technique can enormously speed up the computational e-ciency.In this paper, by introducing adaptive mesh refinement (AMR) and hybrid gridtechnique into MHD simulations, we have developed a three dimensional (3D)hybrid solar wind model for the Sun-to-Earth space weather simulations andanother two and half dimensional (2.5D) AMR magnetic reconnection modelfor fast magnetic reconnections in solar coronal, by which we have studied twoimportant process that have very di-erent space and time scales, the steadysolar wind of Carrington rotation (CR) 2069 and the spontaneous fast magneticreconnection occurring in solar coronal conditions.First, we realized the 2D and 3D ideal MHD models with the help of theAMR package named PARAMESH. We use a splitting based finite volume schemewhich splits the resistive MHD equations into -uid part and magnetic inductionpart. The -uid part is solved with the second order Godunov-type central schemeand the magnetic part is handled with constrained transport (CT) approach.The results of several typical 2D and 3D ideal MHD problems show that themodels have higher-precision, are very robust, can control·B to be about10-11～10-12 and can automatically capture the abruptly changed structures,e.g., shocks, strong layers and so on.Then we developed the ideal MHD simulation to resistive MHD simula-tion and establish a 2.5D AMR RMHD magnetic reconnection model, by whichwe investigated the tempestuous evolution, the fast magnetic energy conversionand the e-ective electron acceleration process in the spontaneous fast magneticreconnection with high Lundquist number. We found that: (1) the long-thin current sheet was broken up into many small pieces because of tearing instabil-ity, the islands grown more and more big and merged with each other, finallyseveral very big islands were formed with even smaller islands being constantlyproduced and merged with them. (2) Through the shocks around the magneticreconnection -ow regions, magnetic energy were e-ectively converted into the ki-netic and thermal energy of plasma by motor e-ect, which might be responsiblefor explosive release of magnetic energy in solar -are phenomena. (3) Based onthe time-dependent electric and magnetic fields generated by the resistive MHDsimulation, we further adopted the test particle method to study the accelerationprocess of electrons. The results showed that the electron can be accelerated upto 120 MeV within about 0.3 s, during which the trapping e-ect of the magneticconfiguration was very important to maintain the electron acceleration status.(4) Moreover, taking advantage of AMR technique, the model can automaticallyresolve many fine structures, e.g., near-singular di-usion regions and very smallislands formed between two merging islands, and at the same time can signifi-cantly save computational resources, which is especially favorable for our furtherthree-dimensional (3D) magnetic reconnection studies of solar -ares.Finally, a hybrid three-dimensional (3D) MHD model for the solar windstudy is established with combined grid systems and solvers. The computa-tional domain from the Sun to Earth space is decomposed into the near-Sun(1.0 RS - 7.0 RS) and o--Sun (6.0 RS - 250 RS) domains, which are respec-tively constructed with Yin-Yang overset grid system and Cartesian adaptivemesh refinement (AMR) grid system and coupled together with a domain con-nection interface between the near-Sun and o--Sun domains. This kind of gridsystems have great advantages: (1) The Yin-Yang overset grid can avoid thewell-known singularity and polar grid convergence problems and its body-fittingproperty helps achieve high-quality resolution near the solar surface very eas-ily. (2) The block structured AMR Cartesian grid can automatically capturethe far-field plasma -ow features, such as the heliospheric current sheets (HCS)and corotating structrures and so on. Numerical study of solar wind structurefor Carrington rotation (CR) 2069 shows that the newly developed hybrid MHDsolar wind model successfully produces many realistic features of the background solar wind, in both the solar corona and interplanetary space, by the compar-isons with multiple solar and interplanetary observations ( MLSO/MK4, SOHOLASCO/C2, SOHO/EIT, Ulysses and OMNI ). The comparisons show that themodel captures a lot of peculiarities in the corona in the 2007 solar minimum, e.g.,the observed large equatorial extensions of the southern PCH and the presence ofthe isolated equatorial holes and the warp structure of the magnetic neutral line(MNL) betweenφ= 200- andφ= 330-. The solar wind parameters from thehybrid 3D MHD model also produces the corotating interaction regions (CIRs)and the solar wind parameters are roughly consistent with Ulysses observationsand with the temporal variations from OMNI data combining the measurementsby multiple spacecraft near L1 point.