Footpoint behavior

Hugh Hudson

UCB

Galileo science meetingNobeyama, July 12, 2002

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)

Coronal structure and conjugacy

Fletcher et al., 2001Cargill & Priest, 1995?

http://isass1.solar.isas.ac.jp/~hudson/cartoons

B. Somov, 2002

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

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

Warren & WarshallApJ 560, L87, 2001

Asai et al., Y10 proceedings,2002

Footpoint behavior for Aug. 25, 2001

(Metcalf et al. AGU poster)

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

Particle acceleration and energy release

• Neupert effect (Neupert, 1968; Hudson, 1972)

• Soft-hard-soft spectral pattern (Parks & Winckler, 1971; Benz, 1975)

RHESSI 20-25 keV (purple)

GOES 1-8 A(green)

Neuperteffect

Examples of the Neupert effect

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

Soft-hard-soft pattern

F. Farnik, 2001

Another SHS example

A

RH

ES

SI M

-clas

s flare

Broad-band spectral variation

Spectral soft-hard-soft effect

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

HX/EUV overlay for Bastille flare ribbons

Fletcher & Hudson, 2001

Ribbon evolution and photospheric B

Fletcher & Hudson, 2001

Magnetic flux swept out in ribbon motions

Fletcher & Hudson, 2001

Swept-out fluxes

Fletcher & Hudson, 2001

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

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

Final remark

• The Neupert effect and the soft-hard-soft spectral pattern are dominant, but not universal

• The exceptions are probably well worth studying