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3-dimensional Numerical Study on the Background Solar Wind

Author: YangLiPing
Tutor: FengXueShang；WeiFengSi
School: Graduate School, Chinese Academy of Sciences
Course: Space Physics
Keywords: Numerical Model CESE Scheme Solar Wind Heating Method Solar Minimum AMR
CLC: P353.8
Type: PhD thesis
Year: 2011
Downloads: 55
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Abstract

In recent years, as space weather has been proposed and developed, peopleeagerly want to understand mechanisms of space weather events, and achieve theforecast a couple of hours or a couple of days earlier. Driven by this, the numericalmodels basing on the physics and high performance computation are getting moreand more attention. In this dissertation, the SIP-CESE MHD model is improvedand implemented adaptive mesh refinement (AMR) technique. Using the model,we conduct the numerical test to see the advantages and disadvantages of thethree commonly-used heating methods. We also study the unusual propertiesduring the activity minimum between solar cycles 23 and 24.To improve the SIP-CESE MHD model, the essential focus of our numericalmodel is devoted to such aspects: (1) applying one new overset grid-six compo-nent grid. The overset grid consists of six identical component meshes to envelopea spherical surface. Each component grid is a quasi-uniform, low-latitude spher-ical mesh, and can be converted to others by the coordinate transform. This isvery convenient for coding. The use of six component grid not only avoids thesingularity and mesh convergence near the poles, but also favors to deal withthe boundary condition and the parallel computation in“(θ,φ)”direction, (2)applying an easy-to-use cleaning procedure by fast multigrid Poisson solver toremove the·B constraint error, which decreases three orders of magnitude,(3) applying Courant-number insensitive method to reduce the excess numericalviscosity because of the Courant–Friedrichs–Levy number disparity, (4) applyingthe multiple time stepping in time-integration to e?ciently quicken the computa-tion, (5) applying the time-dependent boundary condition at the subsonic regionby limiting the mass ?ux escaping through the solar surface. This boundarycondition produce the realistic structures of streamer and coronal hole. In orderto produce fast and slow plasma streams of the solar wind, we involve the topo-logical e?ect of the magnetic field expansion factor fS and the minimum angulardistanceθb (at the photosphere) between an open field foot point and its nearest coronal hole boundary. The numerical study for the 3-D steady-state backgroundsolar wind during Carrington rotation (CR) 1911 from the Sun to Earth is chosento show the above mentioned merits. Our numerical results have demonstratedoverall good agreements in the solar corona with LASCO aboard SOHO and at1 AU with WIND observations.The problem of the coronal heating and solar wind acceleration has obsessedmany physical scientists of solar and heliosphere for several decades. To producethe near-real solar wind, many ideas have been suggested by numerical modeler,among which there are three commonly-used heating methods in numerical sim-ulations: the Wentzel-Kramers-Brillouin (WKB) Alfv′en waves heating method,turbulence heating method and volumetric heating method. With Carringtonrotation 1897, we conduct simulations and compare the three types of heatingmethods. The results show that all the three heating models can basically re-produce the structures of the high-latitude fast solar wind and the low-latitudeslow solar wind as observed near the solar minimum, and that the remarkablydi?erent features are present near the Sun. The results also demonstrate thatthe major acceleration interval terminates about 4 RS for the turbulence heatingmethod and 10 RS for both the WKB Alfv′en waves heating method and the vol-umetric heating method. While the turbulence heating method achieves higherdensity, both the turbulence heating method and the volumetric heating methodcan capture the observed changing trends by the WIND satellite. However, theWKB Alfv′en wave heating method does not. This work may be helpful to de-velop numerical solar wind model which can generate the realistic solar windbackground.The observations both near the Sun and in the heliosphere during the activ-ity minimum between solar cycles 23 and 24 exhibit phenomena di?erent fromthose typical of the previous solar minima. Choosing Carrington rotation 2070,we investigate the unusual properties in this minimum. The numerical resultsare also compared with the observations from multiple satellites, such as SOHO,Ulysses, STEREO, Wind, and ACE. The result demonstrates that the first sim-ulation with the observed magnetic fields reproduces some observed peculiaritiesnear the Sun, such as relatively small polar coronal holes, the presence of mid- and low- latitude holes, a tilted and warped current sheet, and broad multiplestreamers. The numerical results also capture the inconsistency between the lo-cus of the minimum wind speed and the location of the heliospheric current sheet,and predict slightly slower and cooler polar streams with a relatively smaller lati-tudinal width, broad low-latitude intermediate-speed streams, and globally weakmagnetic field and low density in the heliosphere. The second simulation withstrengthened polar fields indicates that the weak polar fields in the current min-imum play a crucial role in determining the states of the corona and the solarwind.New extensions of our SIP-CESE MHD model to AMR implementation un-der six-component grid system have been presented. By transforming the gov-erning MHD equations from the physical space (x,y,z) to the computationalspace (ξ,η,ζ) while retaining the form of conservation, the SIP-AMR-CESEMHD model is implemented in the reference coordinates by utilizing the parallelAMR package PARAMESH. To validate the capability of this model, we presentthe preliminary results of applying the SIP-AMR-CESE MHD model for mod-eling the solar wind background of di?erent solar activity phases by comparisonwith SOHO observations and other spacecraft data from OMNI. Our numericalresults show overall good agreements in the solar corona and in interplanetaryspace with these multiple spacecraft observations.

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CLC: > Astronomy,Earth Sciences > Geophysics > Space Physics > Outer Space, the Sun-Earth relationship of physical > Interplanetary magnetic field and solar wind
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