Post on 01-Apr-2015
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Center for Magnetic Reconnection Studies: Final Report
Amitava Bhattacharjee Institute for the Study of Earth, Oceans, and Space
University of New HampshirePSACI PAC, Princeton, June 3 2004
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• University of New Hampshire: A. Bhattacharjee (PI and Director), N. Bessho, K. Germaschewski, J. Maron, C. S. Ng, P. Zhu
• University of Iowa: B. Chandran, L.-J. Chen, Z. W. Ma, J. Maron
• University of Chicago: R. Rosner (PI), T. Linde, L. Malyshkin, A. Siegel
• University of Texas at Austin: R. Fitzpatrick (PI), F. Waelbroeck, P. Watson
TOPS collaborators : D. Keyes, F. Dobrian, B. Smith
CMRS: Interdisciplinary group drawn from applied mathematics, astrophysics, computer science, fluid dynamics, plasma physics, and space
science communities (supported also by DOE/ASCI, NASA, NSF)
CMRS: What have we accomplished on the computing front?
Principal Computational Deliverable: Magnetic Reconnection Code (MRC)
We have developed the world’s first two-fluid (or Hall MHD) code in an Adaptive Mesh Refinement (AMR) framework. Its attributes are:
• A fully 3D code which integrates two-fluid/Hall MHD equations• Massively parallel with Adaptive Mesh Refinement (AMR)• Geometry: slab (2002) and cylindrical/toroidal (2004) (with AMR in
the radial direction)• Flexible boundary conditions: free as well as forced reconnection
studies• Options for equations of state• Modular code, with the flexibility to change algorithms if necessary• Code performs and scales well• Framework defined by FLASH---developed by active collaboration
with computer scientists• Code can be used for diverse applications, not just magnetic
reconnection
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Adaptive Mesh Refinement
Effectiveness of AMRExample: 2D MHD/Hall MHD
Efficiency of AMR
High effective resolution
Level # grids # grid points0 1 702251 83 1460802 103 2686663 153 5453164 197 10421325 404 19264656 600 1967234
Grid points in adaptive simulation: 5966118Grid points in non-adaptive simulation: 268730449Ratio 0.02
lo g
Scaling on a model reconnection problem
CMRS: High-Performance Computing Tools • Magnetic Reconnection Code (MRC), based on extended two-fluid (or
Hall MHD) equations, in a parallel AMR framework (Flash, developed at U of C).
• ExPIC, a fully electromagnetic 3D Particle-in-Cell (PIC) code
MRC is our principal workhorse and two-fluid equations capture important collisionless/kinetic effects. Why did we need a PIC code?
• Generalized Ohm’s law
• Thin current sheets, embedded in the reconnection layer, are subject to instabilities that require kinetic theory for a complete description. ExPIC is massively parallel and can do realistic electron-to-ion mass ratios.
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E + v × B =1
SJ + de
2 dJ
dt+
din
J × B −∇ •t p e( )
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What did we originally propose to do with the MRC? And what have we done so far?(√)
Applications to astrophysical, fusion, and space plasmas• Sawtooth oscillations in tokamaks(√) and magnetotail substorms (√) • Error-field studies in tokamaks (√)
• Astrophysical applications: galactic dynamo (√), transport of magnetic flux to the galactic center (√)
• Direct simulations of laboratory magnetic reconnection experiments
• RFP dynamo studiesWhile we have not completed all the tasks in our original proposal, we
have carried out a number of tasks that were not in our proposal….
What have we done so far also includes topics that are above and beyond what was proposed originally….
(marked in red)
• Sawtooth crashes in tokamaks (using two-field, four-field, and full two-fluid equations in slab and cylindrical geometry)
• Error-field induced islands in tokamaks• Scaling of forced reconnection in the Taylor model (resistive and Hall MHD), with and
without anomalous resistivity• Discovery of compressional wave-driven bursty reconnection in the Taylor model• Magnetotail substorms: role of reconnection and ballooning instabilities at onset• 3D tearing instabilities involving nulls: discovery of the spherical tearing mode• Mathematical and computational solution of the Parker problem in coronal physics• Investigation of finite-time vortex singularity in Navier-Stokes equations (“Millenium
prize problem”)• Growth of magnetic helicity in a turbulent astrophysical dynamo• Anisotropic MHD and electron MHD turbulence
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SciDAC TOPS-CMRS collaboration
• CMRS team has provided TOPS with model 2D multicomponent MHD evolution code, and explicit solver
• TOPS has implemented fully nonlinearly implicit GMRES-MG-ILU parallel solver– in PETSc’s FormFunctionLocal format using DMMG and automatic differentiation for
Jacobian objects
• CMRS and TOPS reproduce the same dynamics on the same grids with the same time-stepping
– up to a finite-time singularity due to collapse of current sheet (further dynamics falls below present uniform mesh resolution)
• TOPS code, being implicit, can choose timesteps an order of magnitude larger, with potential for higher ratio in more physically realistic parameter regimes
– but is still slower in wall-clock time
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Interrelationship between fusion, space, and astrophysical plasmas
An example…..
Impulsive Reconnection: The Trigger Problem
Dynamics exhibits an impulsiveness, that is, a sudden change in the time-derivative of the growth rate.
The magnetic configuration evolves slowly for a long period of time, only to undergo a sudden dynamical change over a much shorter period of time. Dynamics is characterized by the formation of near-singular current and vortex sheets in finite time. ExamplesSawtooth oscillations in tokamaksMagnetospheric substormsImpulsive solar and stellar flares
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Sawtooth crash in tokamaks (Yamada et al. 1994)
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Magnetospheric Substorms
Current Disruption in the Near-Earth Magnetotail
Ohtani et al. 1992
Solar corona
Magnetic field
http://uk.cambridge.org/assets/astronomy/encyclopedias/Fig5_28.jpg
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Impulsive solar/stellar flares
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Hierarchy of collisionless reconnection models
3D Hall cylindrical MHD
Four-Field Model (Hazeltine et al. 1987, Aydemir 1992)
Variables: magnetic field B, velocity v, pressure p
Variables: magnetic potential , stream function , parallel speed v, pressure p
Two-Field Model (Porcelli et al. 1999)
Variables: magnetic potential , stream function
(with generalized Ohm's law)
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Resolving the current sheet
zoomzoom
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t
Island width
magnetic flux function
S = 109
Jz
Jz, cut
zoom
Aydemir’s four-field simulations (1992): effect of electron pressure gradient in the generalized Ohm’s law causes near-explosive nonlinear growth of m=1 island.
Sawtooth Oscillations in Tokamaks
Resistive MHDt=200 t=400 t=600
Poloidal velocity streamlines, Vz (color coded)
Hall MHDt=200 t=260 t=320
Two-fluid (or Hall MHD)
Resistive MHD
Growth rate(Time-history)
MRC Cylinder
Observations (Ohtani et al. 1992)
Hall MHD Simulation
Cluster observations: Mouikis et al., 2004
Hall MHD Ballooning Instabilities
Intermediate Regime for Instability: Compressional Stability at High
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Large but finite ky ballooning modes from initial-value studies:Towards a nonlinear theory of ballooning and tests of “detonationmodels”
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ky and Dependence
Possible Scenario of Substorm Onset:Near-Earth Ballooning Instability Induced by Current Sheet Thinning and/or Reconnection
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Error-field induced reconnection in tokamaks
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Comparison of theory and simulation for different values of resistivity and viscosity
The Spherical Tearing Mode: A fully 3D model of reconnection (with J. M. Greene and S. Hu)
• Greene (1988) and Lau and Finn (1990): in 3D, topological configuration of great interest is one that has magnetic nulls with loops composed of field lines connecting the nulls. There are two types of nulls, type A and type B, and they come in pairs.
• The null-null lines are called separators, and these are analogous to closed field lines in toroidal plasmas.
• For the magnetosphere, these ideas were already represented in the vacuum superposition models of Dungey (1963), Cowley (1973), Stern (1973). But a vacuum carries no current, and hence no spontaneous tearing instability.
Analytical 3D spherical equilibrium with spherical separator containing two
magnetic nulls
Unstable equilibrium
Equilibriumplus perturbation
Field linespenetrating thespherical tearingsurface
Breaking of the spherical tearing surface allows external field lines to penetrate into the surface
Evidence of spherical tearing in a Global General Circulation Model (GGCM) simulation with northward IMF (with J. Dorelli and J. Raeder)
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GGCM picture including solar wind open field lines draping the Earth’s magnetosphere