1 solar stereoscopy - where we are and which developments do we require to progress? thomas...
TRANSCRIPT
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Solar stereoscopy - where we are and which developments do we require to progress?
Thomas Wiegelmann, Bernd Inhester, Li Feng, Judith de Patoul
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EUV-images from2 viewpoints
3D geometry/physics of e.g. coronal loops and polar plumes
What do we have and what we want?
How?
1. Extract curve-like objects
2. Associateobjects in both
images
3. GeometricStereoscopy
4. Estimatereconstruction
error in 3D
5. Derive physical
quantities
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1. Extract curve-like objects
Processed image: A 7x7 boxcar
smoothed image was subtracted
from original
Original Trace-image(from Aschwanden,Sol. Phys. 2008)
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1. Extract curve-like objectsa) Manual loop tracing
210 manually traced loops.Trace-image, central part(Aschwanden, 2008)
Can we teach thecomputer to trace
loops automatically?
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Ridge detection
Definition of a ridge
Image intensity I (x, y) satisfies:
Inhester et al, Sol. Phys. 2008
1. Extract curve-like objectsb) Automatic loop tracing
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1. Extract curve-like objectsb) Automatic loop tracing
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Ridge location and orientation
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Interpolation of ridge positions (ridgel)
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Ridgel connection to chains
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Polynomial fits
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Merging and cleaning
Done by hand until now
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Correspondence problem: Which two loops in the two images are the projections of the same loop in the real 3D case?
solar rotation:
two viewpoints
Two-view TRACE images mimicking the EUVI image pair
(Feng et al., Sol. Phys. 2007)
2. Associate objects in both images
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Plasma outlines magnetic field lines
Can we use coronal magnetic field modelsas a proxy for 3D plasma loops?
Magnetic field and EUV-images
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Single viewpoint: SOHO/EIT, TRACEUse B-Field for feature-recognition
(Wiegelmann et al., Sol. Phys. 2005)
• Extrapolate coronal magnetic field from photosphere.
• Project 3D field lines onto an EUV-image.
• Emissivity and gradient along projected field lines.
• Alternative-1:Compare field lines and plasma.How well are they aligned?
• Alternative-2: Extract 1-D loops out of the EUV-images and compare with projections of magnetic field lines thereafter.
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Linear force-free field with α=-0.01 [Mm-1]
3D-magnetic field lines, linear force-free α=-0.01 [Mm-1].Used in Marsch et. al. 2005 tostudy plasma flows with Sumer.
Alternative-1: Compare projected field lines direct with EUV-image
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Alternative-2: Compare projected field lines with extracted EUV-loops
Dotted lines:Projection of best-fit linear force-free field lines.
Loops extracted from EUVI-image with automaticfeature recognition tool.
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Can we extend this method to solve the correspondence problem of STEREO-images?
• A 3D magnetic field lines is unique.
• Project field lines onto EUVI-images[or loops extracted from EUVI].
• Measure distance of loops and projectedmagnetic field lines in both images.
• Loops close to the same field line inboth images are very likely associated.
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• Epipolar geometry reduces 3D-reconstructionproblem to several 2D problems.
• Coordinate system is defined by spacecraft locations and rotation axis of the Sun.
3. GeometricStereoscopy
From Inhester,ISSI 2006
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From Inhester,ISSI 2006
• Epipolar geometry provides necessary criteriumfor loop association: Related loops in bothimages must intersect with same epipolar lines.
• Epipolar range often not easy to specify,because ends of loops can be very faint=> Parts of loops not visible in one orboth images.
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Reconstruction ambiguity
Reconstruction of east-west orientation loops has two solutions, which one is correct?
Two-viewpoint images are not sufficient
3. GeometricStereoscopy
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4. Estimatereconstruction
error in 3D
• Features tangential to epipolar lines have highestreconstruction error.
• For east-west coronal loops this means that largest reconstruction errors occur at the loop top.
From Inhester,ISSI 2006
curve segment in-clined to epipolar line
curve segment par-rallel to epipolar line
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• 3D reconstruction error depends on the resolution w of EUVI-images and on the angle between projection planes.
• Projection angle is smaller or equal as angle between STEREO-A or B.
• For loop segments in the epipolarplane the projection angle is zeroand the error infinite.
4. Estimatereconstruction
error in 3D
From Inhester,ISSI 2006
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4. Estimatereconstruction
error in 3D
• Small separation angle between spacecraft:Association problem easy to solve,Large 3D reconstruction error.
• Large separation angle:Association problem difficult to solve,Small 3D reconstruction error.
2. Associateobjects in both
images
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Artificial STEREO-images from a model Active Region. (Wiegelmann & Inhester, Solar Phys. 2006)
Magnetic stereoscopy
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a) Original exact solution b) Pure geometric stereoscopic 3D-reconstruction.
The reconstructed 3D loops have ambiguities.
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Magnetic modeling with different models (c) potential,d) linear force-free, e) nonlinear force-free). Yellowdotted lines show the original exact loops. We used (artificial) EUV-images from two different viewpoints to constrain the magnetic loops.
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Geometric stereoscopy. We removed ambiguities (multiplesolutions) in the stereoscopic reconstruction by choosing thesolution which is closest to a given magnetic field model.(Here we used the worst model, a potential field, which was,however, sufficient to remove the ambiguities.)
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Magnetic stereoscopy Wiegelmann&Inhester 2006
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• Rigorous Test of B-field models.
• Obtain free model parameters.
• Test scaling laws.
• Get plasma parameters along loops- Temperature- Density- Pressure
Compare real + artificial images,
Loops + B-lines
Modeling,Tomography
5. Derive physical quantities
30EUVI_AEUVI_B
Example-1: Active Region loops, Feng et al., ApJL 2007
NOAA 10960
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EUVI_AEUVI_B
Loops identified from unsharp mask filtered images.
1. Extract curve-like objects
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MDI : 2007-06-08T03:12:00 UT 2007-06-08T03:12:00 UT
Linear force-free extrapolation
3D magnetic field lines :
a guide to the loop correspondence
Loop correspondence
2. Associateobjects in both
images
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correspondence in the northern active region
Loop correspondence
2. Associateobjects in both
images
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Loop reconstruction
Yellow: reconstructed 3D loops
Red: best fit magnetic field lines
view from STEREO_A Northeast of AR
3. GeometricStereoscopy
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4. Estimatereconstruction
error in 3D
Yellow: Reconstructed loop with error bars.Red: Best fit linear force-free field line.
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Loop parameters
1. The linear force-free assumption is often not adequate.
2. Most of the loops cannot be approximated by planar curve segments .
3. Most of the loops are not circular.
5. Derive physicalquantities
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0 1 2 3
4
0 1 2 34
EUVI_A
EUVI_B
Example-2: Polar Plumes (Feng et al., ApJ, submitted)
1. Extract curve-like objects.Here: straight lines,intensity maximum.
2. Associateobjects in both images.
Easy for smallseparation angle.
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dotted lines: 3D reconstruction results
solid lines: extrapolations back to the solar surface.
solar limb as seen from STEREO A Side View
3. GeometricStereoscopy
4. Estimatereconstruction
error in 3D
Small separation angle: Correspondenceproblem easy to solve, no reconstruction ambiguity.But: Large 3D reconstruction error.
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Project 3D plumes onSUMER observations
(1)Doppler Shift map: No obvious outflows detected in plume regions.
(2) Ne
measured from density sensitive Si VIII line ratio
(3) Te
measured from Mg IX temperature sensitiveline pair.
5. Derive quantitiesDensity, Temperature,
Plasma flow
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• Outflow velocity along plumes is to small to make it a dominant contributor to the fast solar wind.
• 3D plumes are more horizontal than a dipole field.• Plumes are in hydrostatic equilibrium. • Temperature derived from the density scale height is
higher than electron temperature.
5. Derive quantitiesDensity, Temperature,
Plasma flow
Polar Plumes, Feng et al. 2009
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Stereoscopy vs. coronal field extrapolation
Hinode FOV
From DeRosa et al. 2009: Blue lines are stereoscopic reconstructed loops (Aschwanden et al 2008), Red lines nonlinear force-freeextrapolated field lines from Hinode/SOT.
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Stereoscopy vs. coronal field extrapolation
• Vector magnetogram data (Hinode/SOT) areessential for nonlinear force-free field modeling.
• Unfortunately Hinode-FOV covered only a smallfraction (about 10%) of area spanned by loopsreconstructed from STEREO-SECCHI images.
• Quantitative comparison was unsatisfactory.• Plan: Compare magnetic field extrapolations
from SDO/HMI and stereoscopy with STEREO/SECCHI and SDO/AIA.
• Can we combine extrapolations from photosphericmeasurements with stereoscopy?
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Self-consistentequilibrium
Artificialimages
LOS-integration
Where to go in stereoscopy?Where to go in corona modeling?
Force-free code
SDO/HMImagnetogram
MHS code
3D Force-freemagnetic field
3D fieldlines
com
pare
Plasma along magnetic
loops
Scaling laws Tomograp
hy
Stereoscopy STEREOimages
3D EUVloops
consistent?