Plans for Magnetic Reconnection Research

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Plans for Magnetic Reconnection Research
Masaaki Yamada Ellen Zweibel for Magnetic
Reconnection Working group
CMSO Planning Meeting at U. Chicago November
18-19, 2003
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Contents

• Current Status and Issues
• Experiments (15 min)
• Theory and Simulations (15 min)
• Immediate Plans (20 min)
• Long-range Plans (5 min)
• Discussions (30 min)

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Two competing models to explain fast reconnection
Generalized Sweet-Parker model
Petschek-type Model
Dedicated lab experiments MRX, SSX, TS-3, VTF
etc.
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Sheet thickness agrees well with Harris model

• ? is not determined by Classical Sweet-Parker
  thickness
• ---- Classical Sweet-Parker width
• 0. 35 c/?pi

Collisional regime
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The measured current sheet profiles agree well
with Harris theory
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Sheet thickness agrees well with Harris model

• ? scales with Harris model
• Demonstrate the effects of 2-fluids plasmas
• Constant normalized drift velocity

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A reconnection layer has been documented in the
magnetopause
Mozer et al., PRL 2002
POLAR satellite
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Numerical simulation can assess 2-fluids effects

• Below c/?pi electron and ion motion decouple
• electrons frozen-in
• Observed out-of-plane quadrupole fields
• Obtained a thin electron current layer of c/?pe

These results have not been verified in lab
experiments
Drake et al
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Reconnection speeds up drastically in low
collisionality regime
What causes the anomalous resistivity?
Measured resistivity
Trintchouk et al, PoP 2003
Collisionality
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Fluctuation Amplitudes Correlate with Resistivity
Enhancement
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Reconnection in MST

• Spontaneous and forced reconnection occurs
• (edge reconnection not linearly driven -
  measured)
• Current sheet widths larger than linear MHD
  prediction
• (or Sweet-Parker width)
• Hall effects important
• (Hall dynamo)

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Dynamo in a laboratory plasma
Toroidal magnetic flux
Helicity const.
Large scale magnetic field is generated in
continuous and discrete events from small scale
fluctuations From MST data
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Effects of reconnection in the lab
magnetic reconnection
toroidal magnetic flux
dynamo
heat flux (MW/m2)
energy transport
rotation (km/s)
momentum transport
ion temperature (keV)
ion heating
time (ms)
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Multiple reconnnection sites
q
radius
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Spontaneous vs Forced

• MHD predicts
• Core reconnection from linear tearing
  instabilities
• (m, n) (1,6), (1,7), (1,8)
• nonlinearly coupled
• spontaneous

• Edge reconnection is nonlinearly driven
• (1,6) (1,7) --gt (0,1)
• (1,7) (1,8) --gt (0,1)
• etc.
• forced

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m 0 mode necessary for sawtooth resets sawtooth
cycle
core
edge
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Relationship between global phenomena and local
mechanisms

• 1. Role of local reconnection on dynamo gt MST
• 2. Plasma merging MRX, SSX, and TS-3
• Global reconnection ltgt local reconnection
  dynamics

• TS-3 experiments indicated that an global driving
  force (merging speed) determined local
  reconnection rate.
• Y. Ono et al., Phys. Plasmas, 5B, 3691 (1993)

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Status- Theory/ Computation
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Plans for Magnetic Reconnection Research
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I. Overall goals

• 1) Find key relationships between the local
  physics of the reconnection layer and the
  dynamics of global plasma reconnection.
• 2) Study comprehensively the 2 fluids MHD effects
  through the generalized Ohms law in the neutral
  sheet and determine the role of turbulence in
  reconnection process,
• 3) Identify key 3-D effects on reconnection,
  whether intrinsic or due to boundary conditions.
• 4) Evaluate the role of magnetic reconnection in
  dynamos and, more generally, in magnetic
  self-organization phenomena.

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II Major current issues

• The most important issue, both in laboratory and
  astrophysical plasmas, is to characterize the
  relationship between the macroscopic (global
  structure) and microscopic (local) scale
  reconnection physics. Our important key issues
  are
• 1) Reconnection rate It is widely known that
  most observations in laboratory and astrophysical
  plasmas show much faster reconnection rate than
  the Sweet-Parker rate. It is important to find a
  new model to explain the observed data.
• 2) Local reconnection dynamics It is necessary
  to develop a comprehensive understanding of the
  mechanisms by which large scale systems generate
  the local reconnection structures, through the
  formation of current sheets, either arising in
  situ or forced by boundary conditions. It is also
  crucial to assess 2 fluids MHD effects through
  the generalized Ohms law in thin current sheets
  which have been already demonstrated in
  laboratories.
• 3) Large scale effects of reconnection
  Reconnection influences the energy balance and
  dynamics of a global plasma structure. Some
  depend on the details of the reconnection process
  itself possibly others do not. It is important
  to find the extent to which large scale processes
  are sensitive to the small scale physics of
  reconnection, and develop macroscopic diagnostics
  of the reconnection process.
• 4) Dynamics of spontaneous and driven
  reconnection in laboratory and astrophysical
  contexts. It is important to know when and how
  reconnection is initiated, the effect of the
  boundary conditions and rational surfaces.

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III. Immediate research objectives

• 1) Investigate theoretically, computationally
  and experimentally the local dynamics in the
  vicinity of the neutral sheet (reconnection
  layer), to assess 2-fluids MHD effects, such as
  Hall and turbulence effects. Potential roles of
  electron diffusion region will be assessed.
  Evaluate how these 2-fluids effects can be
  implemented into the MHD description which
  applies on large scales. We will attempt to
  identify criteria for the transition from the
  one-fluid MHD to the non MHD regime. Possible
  effects include the relative thicknesses of the
  Sweet-Parker layer and the ion skin depth, the
  amplitude of the initial perturbation, and the
  nature of forcing. We will continue to develop
  the theory of reconnection in weakly ionized
  systems. There are preliminary indications that
  the Hall regime is pushed to larger length scales
  because ion-neutral friction increases the
  effective ion inertia.
• 2) Explore the relationship between anomalous
  ion heating and reconnection events in both
  laboratory and astrophysical plasmas and to
  investigate why Ti is generally higher than Te.
  Magnetic reconnection and anomalous ion heating
  are observed to be closely associated in both
  laboratory and astrophysical plasmas. We will
  address this problem collaboratively in our
  center experiments and theoretical/computational
  studies. The effects of the energetic ions on
  reconnection will be also studied in linear and
  non-linear stages.
• 3) Investigate how the local reconnection
  process is related to global reconnection and
  dynamo activities. Study flux conversion process
  and the effects of turbulent EMF along the mean
  magnetic field during magnetic self-organization.

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III-A. Specific experimental work plans

• 1) We will investigate quantitative relationship
  between the observed enhanced resistivity and all
  fluctuations of broad spectrum. Our proposed
  research includes experimental study of the role
  of magnetic fluctuations in observed in MRX and
  SSX where reconnection layer is well identified,
  and also in MST and SSPX where reconnection layer
  exists in multiple places.
• 2) An attempt will be made to identify a dominant
  reconnection layer in MST and SSPX. In these
  laboratory plasmas, reconnection can occur
  simultaneously at a number of rational surfaces.
  In solar flares, energy appears to be released
  within a large volume. A turbulent medium may
  have many reconnection sites in close proximity.
  We will investigate the effects of interaction
  between multiple reconnection sites on the
  reconnection rate.
• 3) We will investigate how the local
  reconnection process is related to global dynamo
  activities or flux conversion process in MST.
  More specifically, it includes accurate
  measurements of time evolution of magnetic and
  flow topology around reconnection sites. This
  task requires support from theory and simulation
  linear, quasi-linear, and possibly nonlinear
  theories of dominant instabilities (e.g. tearing
  modes) in realistic 3-D geometry. New theories
  and simulations using 2-fluid models are likely
  required to explain experimental data.
• 4) Joint development of diagnostics tools will be
  made in the area of spectroscopic measurements
  (IDSP), and local measurements of magnetic field
  fluctuations among all four devices.
• 5) We attempt to find a scaling of reconnection
  rate with respect to local plasma parameters in
  MRX, SSX, MST and SSPX.

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III-B. Specific theoretical work plans

• 1) Develop asymptotic formulae for the scaling
  of the reconnection rate in systems in which the
  reconnection layer is many orders of magnitude
  less than the size of the system. These formulae
  can be tested, but not replaced by, numerical
  simulations. The appropriate model may be a two
  stage scenario which addresses the development
  of small scales followed by rapid dissipation.
• 2) Reconnection is often posited as an energy
  source in astrophysical settings, but there are
  few quantitative models of the energetic effects
  of reconnection which could be compared with
  observations. We will compute radiative
  signatures of plasma heating, particle
  acceleration, and generation of bulk flows by
  reconnection in particular systems.
• 3) Computations of large scale dynamics cannot
  simultaneously include
  
  the reconnection scale. We will use
  detailed models of the reconnection region
  itself, and the coupling between the large and
  small scales, to develop a parameterization of
  reconnection which can be used in large scale
  computations. Optimally, this parameterization
  would give a good approximation to the energetic
  and dynamical effects of reconnection.
• 4) MHD analysis of reconnection will continue,
  especially with regard to the effects of
  turbulence and 3D geometry. The generalized
  Sweet-Parker model and the Kulsrud model based on
  1-fluid MHD will be tested by FLASH code as well
  as in MRX and SSX.

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III-C. Specific computational work plans

• We anticipate the need for simulations, combined
  with theory, to complement many of the
  experimental initiatives. Not all computational
  capabilities can be realized within a single
  code, so we expect to use multiple codes, with
  overlapping regimes of validity. This will permit
  internal benchmarking studies. Important features
  will be
• 1) Implicit time stepping so that both fast
  dynamical phenomena and slow resistive phenomena
  can be studied in the same problem. NIMROD and
  DEBS are implicit. FLASH is being made implicit.
  The IRC (anachronistic term for the code
  developed at U. Iowa) is implicit in 2D and
  explicit in 3D.
• 2) Ability to treat time dependent problems in 3
  space dimensions, for a greater degree of
  physical realism in both lab and astrophysical
  plasmas. NIMROD, DEBS, FLASH, and IRC all have
  this capability.
• 3) Flexibility in the boundary conditions,
  including periodic, line tied, and open, to suit
  different physical problems. For example,
  reconnection problems in stellar coronae require
  line field boundary conditions.
• 4) Physics beyond MHD. Hall effects and possibly
  anisotropic electron pressure must be included to
  model laboratory measurements of the reconnection
  layer. In astrophysical problems (and lab
  applications where energetic ions are present)
  species such as dust grains and cosmic rays
  should be treated as particles even when the
  background plasma can be modeled as two fluids.
• 5) Collaborations involving codes written outside
  the Center. The IRC is a particularly attractive
  possibility, as it has a detailed treatment of
  non-MHD physics, was written with space plasma
  applications in mind, and there is some overlap
  of personnel. Once the codes are in place we will
  use them to model layer physics, including the
  electron pressure term, and to develop a simple
  parameterization of layer physics that can be
  used for larger scale computations in MHD codes.

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IV. Long range plans

• 1) Inject 1MW NBI in MST and MRX to investigate
  the role of magnetic reconnection in dynamo
  phenomena. Within the next two years, the
  effects of injected hot ions on dynamo and
  reconnection phenomena will be studied in MST.
  After that the 20kV, the NBI beam will be
  injected into MRX to study dynamo effects of
  injected high-energy ions as well as the effects
  of hot ions in the reconnection region. These
  two studies will be carried out collaboratively.
• 2) Investigate the effects of turbulence and
  magnetic chaos on reconnection. We will
  investigate the effect of a variety of
  perturbations on reconnection in laboratory
  plasmas and develop a theory for the interaction
  between multiple reconnection sites.
• 3) Evaluate the roles of magnetic reconnection
  in other self-organization phenomena,
  particularly in dynamos and ion heating. In
  addition we expect strong connection with
  magnetic chaos and momentum transport phenomena.

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Comparison the Sweet-Parker and Ion Inertia
Lengths
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How can we apply lab results to astrophysics?

• 1. Can we find a Taylor state in space plasmas?
• Do we see magnetic helicity conservation?
• -- There is no defined boundary in most space
  plasmas
• 2. Can we identify a reconnection layer? (-gt May
  be Yes)
• Observed in the magnetosphere
• Solanskis data in the Sun