Magnetic Field of Solar Active Regions

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Magnetic Field of Solar Active Regions

• Jingxiu Wang
• National Astronomical Observatories
• Chinese Academy of Sciences

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0. Theme Outline

• Brief history of AR Magnetic Field Studies
• Outstanding Achievements
• Some New Tendencies
• Exploring New Physics
• How much we know about flux appearance (or
  flux emergence)?
• How much we know about flux disappearance?
  Is there magnetic reconnection in the lower solar
  atmosphere? What it implies in flare physics?
• What magnetic helicity diagnoses and
  constrains?

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I. Brief history

• 1859 First observations of flare (Carrington,
  Hodgson)
• 1866 Spectroscopy of split lines in spot (Lockyer
  et al.)
• 1908 Zeeman effect observed in sunspots (Hale)
• 1947 Concept of magnetic reconnection
  (Giovanelli)
• 1952 Photoelectric magnetograph (Babcocks)
• 1956 Theory of line transfer in magnetic fields
  (Unno, Stepanov)
• 1958 High resolution magnetograms (Leighton)
• 1958 Concept of magnetic neutral line (Severny)
• 1960 Vector magnetograph (Stepanov et al.)
• 1960 Loop-loop interaction flare model (Gold
  Hoyle)

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I. Brief History (Cont.)

• 1965 Small-scale nature of magnetic field
  (Sheeley)
• 1966 Early concept of standard flare model
  (Sturrock)
• 1967- Emerging flux regions (Bruzek or earlier)
• 1971 Stokes polarimeters (Cacciani Fofi, Wiehr
  )
• 1971 Moving magnetic features (Vrabec)
• 1971 First observations of CMEs (Tousey,Gosling)
• 1973 Magnetic shear (Zirin, Tanaka, Hagyard)
• 1973 Radio observation of magnetic fields
  (Gelfreikh)
• 1976 Flux tube model (Piddington, Parker,
  Vandakurov)
• 1980 Space observation of magnetic fields
  (Henze)
• 1982 Helioseismology probe of sunspot (Thomas et
  al)
• 1985 Magnetic flux cancellation (Live, Wang,
  Martin)

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II. Outstanding Achievements

• Detection of 0. "1 magnetic elements and
  structures (Talks by Berger, Carlsson et al.)
• Probe of subsurface structure and dynamics by
  helioseismology (ref. Kosovichevs talk)
• Infrared (IR) spectro-polarimetry (Mathew et al.
  2003 Khomenko et al. 2003 Penn et al. 2004
  Jennings et al.2004)
• Firm evidence of flare-associated B changes
  (Wang, Harvey, Zharkova etc.)

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Great success of IR Spectro-polarimetry

• Spectro-polarimetric observations at strong Ti
  2231 nm were made of NOAA 10008 by California
  State University Northridge National Solar
  Observatory (Penn et al.2003)
• Solar vector (Stokes IQUV) magnetograms using the
  infrared line of Mg i at 12.32 ?m (Jennings et
  al.2004)
• Tenerife Infrared Polarimeter (TIP) in two
  infrared Fe I lines at 15 648.5 Å 15 652.8 Å
  (Mathew et al.2004)

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Firm evidence of flare-associated magnetic changes

• Only very recently a few types of the definitive
  and rather universal magnetic changes are
  identified by Big Bear Solar Observatory and US
  National Solar Observatory. For example, a new
  discovery of Sudden decay of sunspot penumbrae is
  found to be universal for ?-sunspot that produce
  flares

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October 28, 2003 X17 Flare(Wang et al. 2004)
Sudden decay of sunspot senumbrae
Difference Image

14 / 32
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III. New Tendencies

• Observational MHD approach
• Synthesized analysis of vector magnetograms
  toward models of flare/CME prediction
• Very hot studies of AR magnetic helicity
  following Berger Field

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Observational MDH Approaches (Jiong Qiu et
al.2004 ApJ 565, 1335)
Macroscopic electric fields and reconnection rate
inside the reconnecting current sheet (RCS)
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Systematic investigations to extract as much
information as possible from the time-sequence of
vector magnetograms related to occurrence of
solar energetic events (Leka et al. 2003a,b,c
Falconer et al. 2002, 2003)

• Distribution of B
• Total Flux and Flux Imbalance
• Magnetic Neutral Line
• The Field Inclination
• Gradients of B, Bh, Bz

• Vertical Current Jz
• Twist Parameter, ?
• Helicity Density, Rate Pattern
• Shear Angles
• Free Magnetic Energy

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Very hot studies of helicity

• -- Measurements (Chae, Kusano, Demoulin, Berger,
  Wang, Georgoulis, Sakurai, Magara, Loncope..)
• Magnitude rate in ARs --1043 Mx2, 1039
  Mx2/s
• -- Helicity change activity (Moon, Kim, Chou,
  Zhang, Yokoyama et al.)
• -- CME helicity source (ARs are incompetent? see
  DeVore, Berger Ruzmaikin, Demoulin et al.,
  Green et al., Moon et al. Nindos Zhang, Nindos
  et al.)
• -- Cyclic evolution (Seehafer, Pevtsov,
  Abramenko, Bao Zhang, Benevolenskaya, Kleeorin,
  Kuzanyan, Sokoloff, Tian et al. )
• -- Helicity distribution annihilation (Kusano,
  Wang)

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IV. Exploring New Physic

• How much we understand the flux emergence, or
  more general, the flux appearance?
• Emerging flux regions (EFRs) are
  elementary building bricks of magnetic fields of
  ARs, play a central role in explosive activity,
  manifest the sub-surface dynamics and dynamo
  action.
• -- Simple bipolar region
• -- Complicated ?-sunspots
• -- Peculiar flux emergence in sunspot
  periphery
• -- Moving magnetic features (MMFs)
• -- Sympathetic emergence?
• -- Hot spots or active longitudes

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3D MHD simulations of twisted, O-shaped tube from
convection zone by Fan (2001)
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?-Sunspots
Emergence Rapid breakdown of twisted flux rope
model for strong flare (Kurokawa et al. 2002 )
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VMG AR10486
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0430 UT Oct.28 2003
Current helicity density
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MMFs peculiar flux emergence in the periphery
of huge monopolar sunspots
Often flare/CME associated
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EFRs in sunspot periphery
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Are MMFs part of U-loops emanating from sunspots
magnetic canopy? (Zhang, Solanki Wang 2003
Zhang et al. 2004, see observations by Yurchyshyn)
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What are MMFs ?

• Why huge monopolar sunspots with MMFs are often
  flare/CME associated?

(Zhang Wang, 2002)
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Are MMFs current carrying? (Wang et al. 2004)
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We are far from having a basic understanding
about flux emergence

• We do not know if the model by Tanaka (1991)
  about ?-sunspot is truly correct and the only
  solution. Helicity diagnosis is of helps.
• We have no idea about what happen in the
  periphery of some monopolar sunspot.
• We can not be sure what are MMFs?
• Super activity often involves a few ARs. Is their
  emergence connected?
• How dynamo theories can be of helps in predicting
  the hot spots ?

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IV. Exploring New Physic (cont.)

• What we know about flux disappearance? What are
  the fate of new EFRs? What is the flux
  cancellation? Is there magnetic reconnection in
  the lower solar atmosphere? What this
  reconnection implies in flare/CME process?
• -- Observed flux cancellation between EFRs
  pre-existing flux, inseparable elementary process
• -- Correlation of flux cancellation to
  activity
• -- Vector magnetogram diagnosis
• -- Coupling of photosphere to corona

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Early Work on Flux Cancellation

• First detailed descriptions
• On the quiet Sun Livi, Wang, Martin, 1985
  In an active region Martin, Livi, Wang, 1985
• Magnetic flux cancellation is described as the
  mutual flux disappearance in closely-spaced
  magnetic fields of opposite polarities. It takes
  place everywhere on the Sun, on quiet Sun, in
  active regions, and in coronal holes.

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Note While active searching for evidence of
reconnection in the corona to meet the so-called
standard flare model has been making without
success, careful measurements of the rate of
inflow flux and flux disappearing are available
in the photosphere. Moreover vector field
structure and flux evolution history reveal the
field connectivity clearly in most cases.

• Inflow velocity -- 0.3 0.5 km/s
• Flux change rate -- -1018 Mx/h (quiet)
• -- -1019 Mx/h
  (ARs)
• Transverse fields -- Discontinuous sheared

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Need more considerations about conductivity in
photosphere (even in corona see Gary 2003)
(Wang, 1993)
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The reconnection in the lower atmosphere should
be ubiquitous. This has been proved by
theoretical consideration, seen by TRACE, by IR
polarimeter observations.

• At the flare trigger site, opposite polarity
  fields of 2700 and 1000 G occurred within a
  single 2" resolution element, implying an
  extremely high field strength gradient ,5 G/km,
  prior to the flare. This is the largest gradient
  ever observed suggests reconnection at or near
  the temperature-minimum height. Vector (Stokes
  IQUV) magnetograms by the infrared line of MgI at
  12.32 ?m (Jennings et al.2002 ApJ 568 1043)

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Reconnection can take place even below the
photosphere, resulting peculiar flux appearance,
say an EFR with hidden positive pole, but
enhanced transverse field (Wang Shi, 93)
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• Why a new EFR could appear as having no positive
  polarity, but only an enhanced bundle of
  transverse field and continuous increase of
  negative flux? Flare activity was seen to
  correlate with this EFR.

Magnetic neutral line
Photosphere
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Before Flare After Flare
A finding by Haimin Wang et al. 2004
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Correlation with Activities
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B vector evolution of AR8100
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Evolution of two EFRs
Magnetic Interface
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Current
Helicity
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Helicity pattern of CME-associated ARs

• For 9 sampling ARs, contrary to the helicity
  charging picture, we find evidence that the new
  emerging flux often brings up the helicity with
  sign opposite to the dominant helicity of the
  ARs. Moreover the flare/CME initiation site was
  characterized by the close contacting of magnetic
  flux with opposite sign helicity in coincidence
  with observed flux cancellation. This support the
  paradigm that interaction of topology-independent
  flux systems is a key ingredient in flare/CME
  magnetism (Wang, Zhou Zhang, 2004)

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Concluding Remark

• Trying hard to not widen but narrow the gaps
  between theories and observations. Without the
  knowledge of photospheric magnetic fields, we
  have no way to understand the physics of solar
  activity.
• Trying hard to see new physics. When the
  mathematics becomes too much complicated it seems
  time to stop to finding new physics when the
  observation goes into too many details it seems
  time to stop to thinking whats the physics we
  are working for.
• Thanks to Harvey, Haimin Wang, Sakurai, Shibata
  for providing materials and suggestions for this
  talk.