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Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 1
Solar coronal magnetic fields:Source of Space weather
Thomas Wiegelmann, Julia Thalmann, Bernd Inhester, Li Feng, Peng Ruan
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 2
Usually we see the solar corona only during an eclipse
11. August 1999 total solar eclipse
• Solar corona is about 200times hotter as photosphere.
• But only 10-12 as dense.• Corona is very faint and
produces only about 10-6 the visible brightness compared with photosphere.
• Observations from spaceallow us to observe thecorona all the time in different wavelength.
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 3
Observation fromSpace with SOHO/EIT
Coronalactivity
is driven by magnetic
fields!
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 4
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 5
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 6
Space weather
-Solar Storms
-Charged particlesimpact Earth
-Aurora Question:Origin of coronal
eruptions
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 7
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 8
Coronal mass ejectionsand flares are assumed to occur due toinstabilities in thecoronal magnetic field configuration.
It is importantto investigatethe coronal
magnetic field
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 9
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 10
Solar magneticfield measured
routinely only inphotosphere
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 11
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 12
Effect of magnetic field visible in corona
• We cannot measure the 3D-coronal magnetic field accurately direct.
• Measurements of photospheric B-field.(Line-of-sight and vector magnetograms).
• Extrapolate photospheric field into corona.
• Extrapolation method depends on assumptions about coronal plasma.
Magnetic ‘field lines’ observed with TRACE
=> Coronal magnetic field models
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 13
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 14
Model Mathematics Observationsneeded
Validity
Potential
Fields
Line of sightmagnetogram
(Global) currentfree regions,quiet sun
Linear
Force-Free
LOS mag.
+ observations ofplasma structures
Local in activeregions, low-betaplasma
Nonlinear Force-Free
Vectormag.
(Noise, ambiguity
Preprocessing)
Active regions,low beta plasmain low corona
MHS
Equilibrium
Vectormag.
+ Tomography
Helmet streamer,finite beta plasma,full solar corona
0
)()( 0
B
pBB
0
)(
B
BrB
0
B
BB 0
0
B
B
Coronal magnetic field models
Better Models More Challenging
Computational + Observational
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 15
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 16
Mathematicalsimple and requiresonly Line-of-sightmagnetograms
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 17
-Minimize L numerically.-Boundary condition: Photospheric B-field vector.-Noise and inconsistencies in vector magnetograms require preprocessing.
NLFFF-consortium:Most accurate and
fastest method.
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 18
Ground based Vector magnetographSolar Flare Telescope, Tokyo, (Sakurai et al.1995)
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 19
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 20
CoronalMagnetic Field
Nonlinear Force-free code
Preprocessing tool
Vectormagnetogram
H-AlphaImage
ChromosphericMagnetic Field
Optional
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 21
Flare seen in Trace and H-Alpha, Jan. 20. 2004,(from Goff, van Driel-Gesztelyi, Demoulin, Sol. Phys. 2007)
Evolution of Flaring Active Region NOAA 10540 (Thalmann and Wiegelmann, A&A 2008, in press)
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 22
SOHO/MDINAO/SFT, Time cadence about 1 day
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 23
Evolution of AR10540
For movie the spatial resolution was reduced (factor 2) and from 4 magnetograms with a time cadence of (28h, 22h, 21h) we interpolated to15 min [movie is only illustrative and not for data analysis].
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 24
Flaring Active Region Quiet Active Region
Solar X-ray flux. Vertical blue lines: vector magnetograms available
Magnetic field extrapolationsfrom Solar Flare telescope
Extrapolated from SOLISvector magnetograph
M6.1 FlareMagneticenergy builds
up and isreleases during
flareFuture plans:Study flaring
ARs with highertime cadence
with Solar Dynamics Observatory.
M6.1 Flare
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 25
Future: Combine magnetic field extrapolation with vector tomographic methods based on coronal
Faraday and Hanle measurements.(feasibility study by Kramar and Inhester 2006)
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 26
Coronal plasma and magnetic field
Lorentz force
pressure gradient
gravity
B
for vertical scales < 0.1 RS
(gravitational scale height) => Pressure constant on field lines
Plasma outlinesmagnetic field lines
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 27
Force-free equilibria and coronal plasma
smallsmall
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 28
Coronal magnetic field Plasma in EUV
SOHO/EIT, Traceetc. show 2D projections of
coronal plasma
3D Structure?
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 29
The STEREO-mission within the NASA/STP program
Two almost identical spacecrafts observe the Sun and heliosphere
• Launched October 2006• Aim: 3D view of
Solar corona.• Spacecraft have
different orbits due to swing-by at moon.
• The two STEREO spacecraft seperate about 44o every year.
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 30
Stereoscopic Reconstruction of 3D coronal loop structures
(Li Feng et al. 2007)
EUVI_B EUVI_AJune 8, 2007Angle 11.8o
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 31
Coronal loops in NOAA 10960
EUVI_AEUVI_BLoops identified from unsharp mask filtered images.
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 32
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
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 34
Loop reconstruction
3D loop is the intersection of two back-projection surfaces generated from two view directions.
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 35
3D Loop reconstruction
Yellow: reconstructed 3D loops
Red: best fit magnetic field lines
view from STEREO_A Northeast of AR
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 36
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.
Still to do:Reconstruct
density, temperature,plasma flowalong loops
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 37
Outlook: Self-consistentB-Field and Plasma model
Force-free code
Vectormagnetogram
MHS code
Force-freemagnetic field
-Length+shape of loops-Magnetic field strength -Electric current density
com
pare
STEREOimages
Plasma along magnetic
loops
Scaling laws
Artificialimages
LOS-integration
Tomography
Self-consistentequilibrium
Newark, 31.03.2008 Wiegelmann et al.: Coronal magnetic fields 38