MHD simulation of solar emerging flux: RayleighTaylor instability and threedimensional magnetic reco

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MHD simulation of solar emerging flux
Rayleigh-Taylor instability and three-dimensional
magnetic reconnection

• Hiroaki Isobe,
• Takehiro Miyagoshi,
• Kazunari Shibata
• (Kyoto University)
• Takaaki Yokoyama
• (University of Tokyo)

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Emerging flux in the Sun

• Origin of activity in solar/steller atmosphere
  magnetic field (e.g., spots, flares, jets,
  coronal heating...)
• Magnetic field emerge from convection zone into
  the upper atomosphere by Parker instability.
• New emerging flux trigger flares, jets, and
  coronal mass ejections via magnetic reconnection
  with pre-existing magnetic field in the corona.

perturbation
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Observations of emerging flux region

• Ha
• Arch Filament System dark filaments connecting
  the sunspots with opposite polarities.
• Why filamentary?
• EUV
• - Bright loops (106K) and dark loops (104K)
• - Jet reconnection with
• preexisting field

20000km
Ha(Hida)
EUV(TRACE)
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Model set up

• 3D compressible, resistive MHD to model emerging
  flux and its reconnection
• Simulation domain includes upper convection zone
  (z?-1500 km), photosphere, chromosphere, and
  corona. Realistic stratification.
• Magnetic sheet in the convection zone uniform
  oblique field. Plasma beta in the corona?0.1

x
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Model set up

• High resolution is required to resolve the thin
  current sheet created as a result of global
  evolution
• grid 800x400x600
• Anomalous resititivity
• gt fast reconnection

x
vdJ/?

• Perturbation in 70ltxlt90 to excite Parker
  instability

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The Earth Simulator

• A parallel vector computer system installed at
  the Earth Simulator Centre, in Yokohama.

• 640 Processor Nodes (PNs)
• One PN consists of 8 vector-type arithmetic
  processors (APs) and 16 GB shared memory.
• In total, 5120 APs and 10TB memory (distributed).
• 40Tflops at peak, 35.86Tflops for Linpack
  Benchmark

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Numerics

• Code Modified Lax-Wendroff verison of CANS
• Parallelization by MPI
• For 800x400x620 grids calculation, we used 20 PNs
  (160 processors) of the Earth Simulator
• It took about 28,600 sec to calculate 50,000
  steps (including data Output).
• 706 Gflops in average (56.5 of peak)

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Results
Initial perturbation nearly uniform in the y
direction except for tiny (?10-7) fluctuation.
Blue isosurface of B Side
Temperature Pink magnetic fied lines
x
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Rayleigh-Taylor instability
t70
t76
t81
t78

• Top of the EF becomes top-heavy gt unstable to
  Rayleigh-Taylor instability
• Bending the field line (kkx) is suppressed gt
  formation of filaments along B

Temporal evolution of density at the mid point of
EF.
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Density distribution similar to Ha arch filament
system
Ha AFS
isosurface of ?
Density?1012 cm-3?Temperature?10000 K Length
?10000km?Width?1000km
Our simulation propose that R-T instability is
the origin of filaments
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Filamentary current sheets in EF
J (color) and gas ?(contour) at the midpoint of
the EF
Isosurface of ?and slices of J

• Deformation of magnetic field by R-T instability
  gt formation of filamentary current sheet in the
  periphery of dense filaments.
• Nonuniform coronal heating?

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Intermittent magnetic reconnection
anomalous resistivity
B

• The rising parts of R-T instability increasing
  J and decreasing ?
• gt Anomalous resistivity locally sets in.
• gtFast reconnection starts in spatially
  intermittent way.
• Reconnection inflow enhances the evolution of RT
  ,which further enhances the reconnection rate gt
  nonlinear instability.

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Reconnection jets
isosurface of B and V (arrows)
isosurfaces of V6 and V12

• Intermittent (patchy) reconnection
• gt fine structure in jets and flares?

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Conjecture the patchy reconnection is
universal?

• Such patchy brightenings (as a result of
  reconnection) are actually ubiquitous in various
  reconnection associated events in the sun,
  magnetosphere and labratory plasmas (next slide).
  
• Intermittent, patchy reconnection due to
  interchange instability and anomalous resistivity
  can occur if
• the reconnecting current sheet is not in
  mechanical equilibrium, and
• density of the both side of current sheet is
  different.
• Also, it can be a mechanism to excite turbulence
  in the current sheet (Tajima Shibata, Lazarian)

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Evidence for patchy reconnection 1. solar flares
TRACE 195A

• Patchy bright points (kernels) in the footpoints
• Down flows above post flare loops (reconnection
  outflows?)

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Evidence for patchy reconnection 2. aurora
Reconnection in the magnetotail gt aurora in
polar regions
Note the intermittent nature of the aurora
brightenings.
movie from http//www.nasa.gov/vision/universe/sol
arsystem/aurora1110.html
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Formation of helical flux rope (plasmoid)

• Tearing instability in the current sheet gt
  formation of magnetic island (plasmoid)
• In the presense of guide field (By), the
  plasmoids form a helical flux rope.

Liu Kurokawa 2004
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Summary

• Using the Earth Simulator, we have carried out
  large scale 3D simulations of solar emerging flux
  and its interaction with pre-existing coronal
  field.
• Filamentary structure, which is very similar to
  Ha arch filament system, is formed due to the
  Rayleigh-Taylor instability in the emerging flux.
• R-T instability causes the formation of many
  filamentary current sheet in EF, which may
  explain the nonuniform heating of the corona.
• Coupling of R-T instability and anomalous
  resistivity leads to spatially intermittent
  magnetic reconnection.