Footpoint behavior

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Footpoint behavior

• Hugh Hudson
• UCB

Galileo science meeting Nobeyama, July 12, 2002
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What can footpoints teach us?

• How do we understand the symbiosis of energy
  release and particle acceleration?
• What is the nature of the geometrical evolution
  of the corona in the impulsive phase of a flare
  (or the acceleration phase of CME)?
• Recall work of Sakao (Yoyogi conference) Saita
  (unfinished thesis) Asai (ongoing)

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Coronal structure and conjugacy
Fletcher et al., 2001
Cargill Priest, 1995?
http//isass1.solar.isas.ac.jp/hudson/cartoons
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Somov cartoon
B. Somov, 2002
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Coronal separatrix structure

• The separatrix surfaces deform during an
  energy-release event
• The flare ribbons in the chromosphere should map
  into these separatrices
• Ribbon brightening not only reveals the energy,
  but also describes the coronal restructuring

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Comment

• The coronal magnetic field, assuming low beta,
• plays two roles simultaneously
• The field defines the source of energy via B2/8p
• The field defines the geometry of the energy
• release

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Examples of conjugate behavior
Warren Warshall ApJ 560, L87, 2001
Asai et al., Y10 proceedings, 2002
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Footpoint behavior for Aug. 25, 2001 (Metcalf et
al. AGU poster)
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Footpoints and separatrices
Metcalf made a potential- field extrapolation and
found that the separatrix structure correlated in
interesting ways with the in-plane motions, but
not with the out-of-plane (perpendicular to B)
motions.
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Particle acceleration and energy release

• Neupert effect (Neupert, 1968 Hudson, 1972)
• Soft-hard-soft spectral pattern (Parks
  Winckler, 1971 Benz, 1975)

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Examples of the Neupert effect
Neupert effect
RHESSI 20-25 keV (purple) GOES 1-8 A (green)
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Lessons from the Neupert effect

• The energy release that fills coronal loops with
  hot plasma has a direct relationship with
  particle acceleration
• To a first approximation, this relationship is
  independent of the scale or intensity of the
  energy release

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Soft-hard-soft pattern
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Another SHS example
F. Farnik, 2001
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A RHESSI M-class flare
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Broad-band spectral variation
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Spectral soft-hard-soft effect
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Lessons from soft-hard-soft

• Non-thermal time scales are usually not
  determined by trapping
• The spectral evolution at high energies is an
  intrinsic property of the acceleration mechanism
• Understanding hard X-ray spectral morphology will
  be a major RHESSI goal

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HX/EUV overlay for Bastille flare ribbons
Fletcher Hudson, 2001
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Ribbon evolution and photospheric B
Fletcher Hudson, 2001
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Magnetic flux swept out in ribbon motions
Fletcher Hudson, 2001
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Swept-out fluxes
Fletcher Hudson, 2001
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Lessons from ribbons

• Ribbon shapes dont reflect photospheric field
• Ribbon motions and swept-out field dont match
  preconceptions
• In the Bastille 2000 flare, the hard X-ray
  footpoint brightnesses dont match the
  expectation from mirroring

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Conclusions

• The footpoint sources (hard X-ray or other) can
  teach us a lot about coronal restructuring
  (flares and CMEs)
• The lessons learned thus far dont agree
  particularly well with expectations from simple
  models

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Final remark

• The Neupert effect and the soft-hard-soft
  spectral pattern are dominant, but not universal
• The exceptions are probably well worth studying