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Footpoint behavior
Hugh Hudson
UCB
Galileo science meetingNobeyama, 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., 2001Cargill & Priest, 1995?
http://isass1.solar.isas.ac.jp/~hudson/cartoons
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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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The coronal magnetic field, assuming low beta, plays two roles simultaneously:
1) The field defines the source of energy via B2/8
2) The field defines the geometry of the energy release
Comment
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Warren & WarshallApJ 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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Metcalf made a potential-field extrapolation and foundthat the separatrix structurecorrelated in interesting wayswith the in-plane motions,but not with the out-of-plane(perpendicular to B) motions.
Footpoints and separatrices
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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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RHESSI 20-25 keV (purple)
GOES 1-8 A(green)
Neuperteffect
Examples of the Neupert effect
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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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F. Farnik, 2001
Another SHS example
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A
RH
ES
SI M
-clas
s 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 don’t reflect photospheric field
• Ribbon motions and swept-out field don’t match preconceptions
• In the Bastille 2000 flare, the hard X-ray footpoint brightnesses don’t 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 don’t 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