The Effect of Sub-surface Fields on the Dynamic Evolution of a Model Corona

1
The Effect of Sub-surface Fields on the Dynamic
Evolution of a Model Corona

• Goals
• To predict the onset of a CME based upon
  reliable measurements of photospheric magnetic
  field
• It may be possible to obtain a
  velocity field via LCT, feature tracking,
• or spectroscopic measurements
• To model the evolution of the coronal magnetic
  field as eruptive events occur
• To test reliability of the models by simulating
  known eruptive events and comparing model data
  with existing observational data
• MURI candidate event AR8210 May 1
  1998

2
First Step

• Use dynamic sub-surface models of active region
  evolution to drive the model corona.
• Though it is possible to truly couple
  numerical models via a domain decomposition
  framework such as PARAMESH, we will not have the
  freedom to allow our coronal model to affect the
  observationally obtained photospheric boundary
  --- thus we must explore the consequences of
  driving a model corona without allowing
  feedback into the photospheric zones
• A distinct advantage of this approach
• Sub-surface code provides for self-consistent
  magnetic fields and flows throughout the boundary
  layers
• eg. The induction equation is automatically
  satisfied throughout the
• boundary layers

3
Approach

• Use a compressible MHD code (eg. Zeus3D, ZeusAMR,
  ARMS) to model the dynamic evolution of the
  magnetic field in the low-beta corona above an
  active region
• Since vector magnetograms are measures of the
  magnetic field in the photosphere, reliable
  models must
• 1. Include in the computational domain
  the geometrically thin transition layers between
  the photosphere (where Hp102km, and the plasma
  beta is of order unity) along with the low-beta
  corona.
• 2. Approximate (or treat exactly) the
  effects of optically thin radiative cooling, and
  thermal conduction in the transition layers so as
  to maintain a physical temperature and pressure
  stratification
• 3. Require that magnetic field is known
  along only a single slice in the photospheric
  layers of the simulation domain, and no
  additional sub-surface information is available
• 
  
  
  some chromospheric measurements available

4
Results1
1. From Abbett Fisher 2003 (Jan 1) ApJ (in
press)
5
Results

• The presence and distribution of boundary flows
  (particularly the component of the flow
  perpendicular to the boundary) are of great
  importance to the dynamic emergence process,
  since (in an ideal calculation) such a flow is
  necessary to transport magnetic field into the
  model corona while conserving flux.
• As the apex of a (slightly) twisted Omega-loop
  emerges into the corona, the simulations suggest
  that in most regions surrounding the emerging
  structure, the field configuration differs from a
  force-free (or potential) configuration

6
Second Step AR8210 (May 1 1998 1940)

• A significant challenge
• Only the photospheric magnetic field is known.
  Since boundary flows are important to the
  dynamics of the coronal simulation, we must
  specify a self-consistent velocity field in the
  boundary layers that (at least) satisfies the
  vertical component of the induction equation
• Options
• 1. LCT (does NOT guarantee consistency)
• 2. Feature tracking (does NOT guarantee
  consistency)
• 3. MEF method (DOES guarantee
  consistency)
• theoretical method that generates a velocity
  field given the vector magnetic field along a
  2D slice with the constraint that kinetic energy
  is minimized --- does guarantee consistency, but
  the calculated flow field is part of a family of
  solutions and may not necessarily represent the
  true flows present in AR8210.

7
MEF method (D. Longcope)
8
AR8210

• The initial atmosphere must be specified
• 1. Potential field extrapolation
• 2. Force-free model
• Initial dynamic run --- uses MEF method at the
  photospheric boundary with an initial FFF
  atmosphere provided by S. Regnier

9
Toward comparison with observational data
---Calculating temperatures and emissivities a
posteriori along model coronal loops (L.
Lundquist)
1401
1940
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2112
Yohkoh SXT 01 May 1998