Introductory Lectures on Solar Magnetism

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Introductory Lectures onSolar Magnetism
Activity

• Jingxiu Wang
• National Astronomical Observatories, CAS

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• All in all, the Sun becomes our great physics
  laboratory in the sky. All other stars are too
  distant for detailed observation, and we expect
  that most of them exotic in the same way as the
  Sun, so the Sun becomes the gateway to the stars.
• ---Eugene N. Parker

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Outline

• Overview of Solar Magnetism
• Measurements of Solar Vector Magnetic Field
• Studies Based on Vector Magnetic Field
  Measurements
• Solar Magnetic Activity in Term of Magnetic Field
  Behavior
• Selected Topics for Graduate Studies

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1. Overview of Solar Magnetism

• 1.1 Scientific Opportunities
• The complexities of the Sun its internal
  structure, rotation and convection, and the
  resulting cyclic and random generation of its
  magnetic fields and the magntoactive, hot,
  explosive, extended solar atmosphere and solar
  wind are fascinating and challenging.
• The Suns variable output radiation,
  particles, and fields is controlled by the
  structure and evolution of solar magnetic field.
  

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• Solar magnetism and magnetic activity are one of
  the most exciting and challenging disciplines in
  solar physics and astrophysics.
• The magnetic Sun is a laboratory to investigate
  the dynamic behavior of cosmic magnetic fields.
• A key for understanding and predicting the
  impacts of the Sun on the Earths global changes
  and space weather in our living environment.
• A basis for understanding the only known system
  in the Universe in which the intelligent life has
  been created and flourishing.

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• ????????

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Solar Effects on Life and Society
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1.2 Morphology Classification of Solar Magnetic
Field

• 1.2.1 Active region field
• Sunspot strong field was diagnosed by Hale
  (1908), which marked the beginning of
  astrophysics
• Plage enhanced magnetic network, bright areas
  surrounding sunspots and in decayed active
  regions
• EFR emerging flux region, the basic brick to
  build solar active regions
• MMF moving magnetic feature, intriguing
  properties of sunspot, and a puzzling phenomenon

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Spatial resolution of 0.2 arcsec
Granule
Penumbra
?Umbra
?
Lightbridge
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• AR9077
• Filtergram
• VMG
• Lighter
• Barker
• Linelength
• ?B?
• Alignment
• B

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Vector Magnetogram
Positive?
?Moving magnetic Feature
Transverse fields?
?Magnetic Neutral line
Strong magnetic shear ?
?Negative
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Magnetic evolution leading to Bastille event on
July 14 2000
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• H?Filtergram

?filament
?plage
?fibrils
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MDI magne-togram for NOAA9077
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MDI Synoptic magnetic chart
?AR9077
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Plage is referred to extended emission feature of
an active region seen from the first magnetic
flux emergence until the scattered remnant.
Magnetic fields are more intense and tempratures
are higher.
Plage?
Plage?
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Plage how they come from decayed active region
and appear as enhanced network
Quiet
Enhanced
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There are 3 major EFRs in AR8100 identified on
Nov.3 1997
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Arch Filament System?
?
?Bright Plage
?
?
?
?
?
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Flux emergence is marked by arch filament
systems, surge activities, and flarings in H?
filtergrams, and growing and separating of
opposite polarity magnetic fields in the
magnetograms.
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• Episodes of flux emergence in the form of
  moving magnetic features may trigger the
  homologous flare/CME events, though MMFs are
  rather small-scale phenomena.

Homologous CME flares
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• It is not known the nature and the generation
  mechanism of MMFs. But they do not represent
  sunspot decay.
• However, it is clear that they have rather strong
  magnetic fields.

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• 1.2.2 Ubiquitous small-scale magnetic field on
  the quiet Sun
• Network magnetic field quiet network
• First defined from chromospheric
  observations CaII k H? brightness pattern at
  the borders of supergranulation.
• Intranetwork magnetic field
• The weakest component of solar magnetism
• Ephemeral (active) regions
• Small scale bipoles in both quiet and active
  Sun . Hageneer (2001) estimated 5?1023Mxd-1 in
  the form of ephemeral regions

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Enhanced Network
Quiet Network
Plage Regions
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• Network seen at H? central line (right) offband
  (left)
• At the network boundaries, there are always
  small-scale magnetic activity, such as the
  mini-filament eruption, macrospicules, and
  microflaring shown as network bright points

?
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CaII k filtergram (left) magnetogram (right).
In the marked areas there appeared intranetwork
field very small-scale brightpoints (Sivaraman
et al.2000)
?Network elements
?Network elements
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• BBSO
• Caltech
• June 4,
• 1992.
• Studied
• by wang
• et al.95,
• Zhang et
• el.98a,b,c

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Summary on quiet sun fields

• At any given time, more
• than 20 of total flux on
• the quiet Sun is in the
• form of intranetwork
• elements which have a
• peak flux distribution
• at for network, at
• The lifetime of intranetwork elements is about
• 2 hours while network elements, 50 hours.
• They contribute 1024 Mx flux each day.

Wang et al.1995
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1.3 Intrinsic Properties

• 1.3.1 Concept of strong elementary flux tubes
• Since the early of 1970s, an idea has been
  widely accepted that more than 90 of Suns
  magnetic flux is in the form of strong flux tubes
  with field strength gt1kG, and diameter lt150 km.
  Convective collapse is the known interpretation.
  There has been debates on this the nature of
  magnetic elements on the Sun strong or weak?
  New facts and idea emerged in the middle of
  1990s.
• 1.3.2 Weak magnetic field on the Sun
• Several key works to re-activate this field
  are Keller et al.(1994), Wang et al.(1995),
  Lin(1995), indicating, indirectly or directly,
  the weakness of IN fields.

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Report of the panel on Solar Astronomy from
American National Research Council (2001,
pp246-247)

• Indeed, hints of a weak magnetic field
  component that covers the entire Sun have
  been discovered in several recent observations.
  This global phenomenon may be of crucial
  importance for the magnetic cycle and
  variability. The AST is the ideal tool for
  quantitative measure-ments of these weak
  turbulent fields.

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Importance of the Weak Field

• Significant amount of Sun flux is in the form
  of intrinsically weak magnetic element. Meunier
  et al.(1998) confirmed based on infrared spectral
  diagnosis, The relative amounts of flux in
  weak and strong fields is also similar to the
  relative amounts of flux in intranetwork and
  network fields deduced by Wang et al. (1995).
  
• The interaction of IN and network fields may
  provide enough energy to heat the corona and
  accelerate the solar wind (Zhang et al.1998).
• Theoretically, another type of solar dynamo may
  operate in the solar surface layer.

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1.4 Large-scale pattern

• 1.4.1 Active Complex (activity Nests)
• It consists of one or more large and complex
  active regions, persists for several rotations,
  (even years) by additional region forming as
  earlier ones decay. The foci of super active
  regions and major solar events. Stellar spots in
  stellar astrophysics?
• 1.4.2 Coronal hole
• An extended region of the corona with low
  density and assciated with dominantly unipolar
  phtospheric regions having open field topology.
  They are the source of high-speed solar wind.
  Coronal hole are darker in X-ray, but brighter in
  HeI 10830å images.

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Synoptic magnetic charts are useful to learn
the large-scale pattern of solar magnetic fields.
This is an area Chines Solar astronomers have no
much work and enough knowledges.
?Active Nests
Polar coronal hole
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)
(
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? Coronal Hole
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1.5 Two observed modes of magnetic field evolution

• 1.5.1 Flux emergence
• Emerging flux regions (EFRs) and
  Ephemeral regions in the form of ? loops. A
  dynamo theory should be able to explain why
  always like this?
• Moving magnetic features (MMFs) from the
  border of sunspot -- in small bipole pairs? What
  they are?
• Intranetwork elements in cluster of mixed
  polarities.
• U-loop emergence? How about the
  subsurface connection?

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Flux emergence in the form of ?-loops

• It should be understood why
• the appearance of new flux
• to solar surface is mostly
• in the form of ?-loops even
• for smallest ephemeral
• regions. Buoyancy instability?
• If we can simulate an active
• region from very beginning to
• the end of its life. How about
• magnetic fields in other stars?

?
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• Is there U-loop emergence?
• Spruit et al. (1987) use this model to
  interpret the magnetic flux cancellation and the
  intra- network fields

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• The newly emerging magnetic flux (EFR) plays a
  decisive role in almost all the forms of solar
  activity and the heating of solar corona. EFR
  seems the driver of solar activity. To identify
  an EFR, to reveal its manifestations, to find the
  physical link of EFR to the energy storage and
  explosive release appear to be a key task in both
  observational and theoretical studies. Since the
  first detection (Bruzek, 1967 Martres et al.,
  1968 Zirin, 1972) work on EFR has never stopped
  in solar research.

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1.5.2 Magnetic flux cancellation

• In 1985, magnetic flux cancellation was first
  described by using high resolution Big Bear
  magnetograms (Livi, Wang, Martin, 1995 Martin,
  Livi, Wang, 1995 Wang, Zirin, Shi, 1995). By
  definition, flux cancellation is the mutual flux
  disappearance of closely spaced magnetic fields
  of opposite polarities. It has been identified to
  be the most important mode of flux disappearance
  on the Sun. It is more likely the magnetic
  reconnection in the lower solar atmosphere.

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2
(
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1
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Obvious discontinuity of transverse field in a
canceling feature
(By courtesy of Dr. Bruce Lites)
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Observed Characteristics ofFlux Cancellation

• Discontinuity of transverse field in canceling
  magnetic features. This indicates the interaction
  between topology-independent flux systems.
• Upward chromospheric flows found in the
  interface between canceling flux patches
• All indicate a reconnection scenario
• Flux disappearance rate is well measured
• approaching velocity
• canceling rate

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Correlation with Activities