The Solar Corona

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The Solar Corona

• B. C. Low
• High Altitude Observatory
• National Center for Atmospheric Research

The National Center for Atmospheric Research is
operated by the University Corporation for
Atmospheric Research under sponsorship of the
National Science Foundation. An Equal
Opportunity/Affirmative Action Employer.
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The White-Light Corona
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Mark IV CoronameterMauna Loa Solar Observatory
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The Magnetic Sun
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The Magnetic Corona
Activity Maximum 1980
Activity Minimum 1994
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A CME out to 32 R_Sun
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A Shower of MeV Protons
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The 2-3 Million-Degree Corona
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Solar EUV Output
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Orbit Height of the SMM Satellite
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Coronal Drivers of Space Weather

• Variable heating of the Earths Upper Atmosphere
• Episodes of CME-Magnetospheric Interaction
• High-energy particles
• Evolution of the corona-heliosphere over the
  11-year solar cycle

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The Solar Corona as a Hydromagnetic Atmosphere

• Maintained at million-degree temperatures
• cooling time 1 day
• dissipative heating
• A nearly perfect conductor of heat
• solar-wind expansion
• A nearly perfect conductor of electricity
• low-beta plasma atmosphere dominated by
  a
• 10G global magnetic field reversing
  in
• cycles of 11 years

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Time-Dependent Ideal MHD
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Magnetic Helicity

• The magnetic vector potential
• Magnetic helicity
• Helicity transport

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Magnetic Helicity Linkage Numbers
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The Hydromagnetic Induction Equation

• The magnetic field is frozen into the embedding
  plasma with perfect electrical conductivity. The
  perfect conductor is a singular limit of the
  weakly resistive conductor being nearly perfect
  is not the same as just being perfect.

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The Surprisingly Dissipative Corona

• Quiescent heating flares heating by a
  turbulent dissipation of spontaneous current
  sheets (Parker 1994)

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Petschek Reconnection
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A Good Question

• If magnetic reconnection under conditions of high
  electrical conductivity makes a plasma readily
  dissipative, what are we to say about its
  canonical properties of being an excellent
  electrical conductor? Can magnetic reconnection
  short away all the electric currents in a
  magnetized plasma under conditions of
  ?

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Limits on Magnetic Reconnection under

• Longevity of astronomical-scale magnetic flux,
  e.g., potential fields as minimum-energy ground
  states very hard to get rid of magnetic flux.
• Conservation of (relative) magnetic helicity
  within sufficiently large magnetic structures
  (Taylor 1974, Berger 1984) very hard to get rid
  of magnetic twist.

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Petschek Reconnection
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Coalescence of Two Ropes of Twisted Fields
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The Ideal and Dissipative Nature of
High-Temperature Plasmas

• Magnetic reconnection under
  does not destroy but transfer magnetic flux
  and helicity among subsystems of flux.
• Despite its dissipative nature, there is a limit
  to how much magnetic energy magnetic reconnection
  can liberate. The approximate conservation of
  magnetic helicity stores magnetic energy against
  flaring origin of long-lived coronal structures.

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Emergence of a Twisted Magnetic Field(The
Magnetic End-Product of a Confined Flare)
Manchester et al. 2004
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Magnetic Flux Ropes in the Solar Atmosphere
Potential State with Zero Helicity
Minimum-Energy State with a conserved Net Helicity
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Sigmoidal Plasma Structures and Magnetic Flux
Ropes
Fan Gibson (2003)

• The sigmoid separatrix flux surface (Parker
  1994,Titov Demoulin 1999, Low Berger 2003)

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Preferred Sigmoidal X-ray Plasma Structures in
the two Hemispheres
North

• Left- and right-handed twisted flux ropes are
  preferred in the northern and southern
  hemispheres respectively (Canfield, Petstov,
  Rust, ..).
• Helicity Rule holds for all solar cycles.

South
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A Role of CMEs in Coronal Evolution

• Large-scale expulsion of coronal mass
  clear out into interplanetary space
• Is there a collective effect of the CMEs on the
  solar corona over an 11-year solar cycle? Is
  energy release the only consequence of the CMEs
  for the corona?

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Magnetic-flux Emergence and the Complementary
Roles of Flares and CMEs

• Magnetic reconnection as flares serves to shed
  excess energy and simplify field topologies but
  cannot destroy the large-scale magnetic flux
  threading across the solar photosphere.
• Under its approximate conservation law, the
  magnetic-helicity emerging into the corona can be
  removed either by the mutual cancellation of
  opposite helicities or by an outward transport
  into interplanetary space, in order to avoid an
  unbounded accumulation in the corona the global
  helicity rule identifies the latter mechanism
  with the CMEs.

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Creation Removal of Magnetic Flux Across a
Geometric Surface
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CMEs and Coronal Magnetic-Field Reversals

• CMEs are episodes of hydromagnetic expulsions of
  the magnetic flux and helicity of the old cycle
  out into the interplanetary solar wind, to make
  room for the new-cycle flux of the opposite
  polarity (Low Zhang 2004).
• SMM LASCO observations suggest a direct
  association between the progress of a field
  reversal at a solar pole and the rates of CMEs
  taking off near that (Gopalswamy et al. 2003).

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Gopalswamy et al. 2003
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The Solar-Heliospheric Outflow of Magnetic Flux
and Helicity

• There is a global transport of magnetic flux
  system from the solar interior out into the solar
  wind, obeying the Helicity Rule. Complementary
  roles for flares prominence and CME eruptions.
• Sub-photospheric origin of atmospheric magnetic
  helicityare we seeing clear through into the
  interior dynamo?
• The hydromagnetic interplay between dissipative
  (flares) and ideal (CMEs) processes is the basic
  drama of solar activity that is the origin of
  space weather (Zhang Low 2005, Ann. Rev. Astron
  Astrophys.).

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References

• Hundhausen, A. J. 1997, in Cosmic Winds and the
  Heliosphere, ed. J. R.Jokipii, C. P. Sonnett, \
  M. S. Giampapa, U. of Arizona Press, p. 259
• Low, B. C. 2001, JGR 106, 25141 references
  therein
• Zhang, M., \ B. C. Low 2005, ARAA vol. 43, in
  press, download /toshi/ftp/pub/zhm/ZhangLow.pdf
  references therein.