using stereo/euvi to study active region magnetic fields · anne sandman (rice/lmsal) markus...
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Using STEREO/EUVI to Study Active Region Magnetic Fields
Anne Sandman (Rice/LMSAL)Markus Aschwanden, Jean-Pierre Wuelser, Marc DeRosa (LMSAL), David Alexander (Rice)
STEREO SWG #19Pasadena, CA, 4 February 2009
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Overview
• Objective
• Data set
• Procedure
• Findings and future work
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Feng et al. (2007, ApJ 671, L205)
Red: linear force-free field lines (MDI). Yellow: STEREO loops.
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Gary (2007)
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Data Set
• Three active regions:
• AR 10953: 23:00 UT April 30 2007.
• AR 10955: 20:30 UT May 9 2007.
• AR 10956: 12:40 UT May 19, 2007.
• EUVI images in 171, 195, and 284Å.
• High-pass filter and boxcar smoothing.
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Procedure
1. Trace loops in EUVI images.
2. Compute 3D potential magnetic field.
3. Calculate discrepancy between data and model.
4. Transform model magnetic field.
5. Recalculate misalignment angles.
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AB
Loop stereoscopy
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Procedure
1. Trace loops in EUVI images.
2. Compute 3D potential magnetic field.
3. Calculate discrepancy between data and model.
4. Transform model magnetic field.
5. Recalculate misalignment angles.
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Potential
Field
Source
Surface
Longcope (2005)
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Procedure
1. Trace loops in EUVI images.
2. Compute 3D potential magnetic field.
3. Calculate discrepancy between data and model.
4. Transform model magnetic field.
5. Recalculate misalignment angles.
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Vector tangent to observed loop
Local magnetic field vector
Magnetic field lines Angle calculation
points
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Visualizations
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EUVI loops (yellow) and MDI/PFSS field lines (pink) reveal significantly different magnetic connectivities
(courtesy of J.P.Wuelser)
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Findings:
• AR 10955 (9 May): best agreement with potential field.
• AR 10956 (19 May): worst agreement.
• NB: AR 10956 produced an eruption within 30 minutes of images.
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Procedure
1. Trace loops in EUVI images.
2. Compute 3D potential magnetic field.
3. Calculate discrepancy between data and model.
4. Transform model magnetic field.
5. Recalculate misalignment angles.
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• Gary & Alexander (1999):
• Simple radial stretching improved model’s agreement with observed loop structures.
Radial Stretching
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Radial Stretching
• Preserves divergence-free condition.
• Injects currents.
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Procedure
1. Trace loops in EUVI images.
2. Compute 3D potential magnetic field.
3. Calculate discrepancy between data and model.
4. Transform model magnetic field.
5. Recalculate misalignment angles.
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Conclusions and Future Work
• Wide variation in loop-model agreement.
• No significant change with radial stretching.
• More versatile transformations?
• Need more appropriate magnetic field observations.
• Use EUVI data to “bootstrap” a better boundary condition.
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Conclusions and Future Work
• Wide variation in loop-model agreement.
• No significant change with radial stretching.
• More versatile transformations?
• Need more appropriate magnetic field observations.
• Use EUVI data to “bootstrap” a better model.
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Results of Radial Stretching