The Yohkoh observations of solar flares

Hugh Hudson

UCB

The Yohkoh observations

• Structure in soft X-rays

• Dynamics in soft X-rays

• Footpoint behavior

• Coronal hard X-ray sources

• Microflares/nanoflares

• Waves

Yohkoh discoveries• Large-scale arcades• The Masuda phenomenon• Dimming (3 kinds?)• Sigmoids and CMEs• Foot-point “motions”• Coronal hard X-ray sources• X-ray detection of waves

More discoveries• TILs• Hard X-ray ribbons• Jets• Coronal-hole channels• Loop-top features• Cusps

All of the preceding images came from the Yohkoh science nuggets, to be found at

http://solar.physics.montana.edu/nuggets

What are some meaty problems?

• How do flares launch global waves? • 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)?

Topics

1. Coronal structure and conjugacy

2. Fine structure in the corona

3. Particle acceleration

4. Global waves

5. Extraordinary events

1. Coronal structure and conjugacy

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

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

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

B. Somov, 2002

Warren & WarshallApJ 560, L87, 2001

Asai et al., Y10 proceedings,2002

2. Fine structure in the coronaHigher-temperature things in the coronalook fuzzier than lower-temperaturethings (eg, “yellow line” vs “red line”)

TRACE/Yohkoh comparison from Warren et al, ApJ 572, 121 (1999)

Observations of spatial fine structure for coronalnon-thermal source (White et al., preprint 2002)

Another example of fine structure at high energies(White et al., ApJ 384, 656, 1992).

Hard X-ray footpoints systematically trace out fine-scalefeatures (T. Metcalf, fall AGU meeting 2001)

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.

3. Particle acceleration and energy release

• Neupert effect

• Soft-hard-soft vs soft-hard-harder

RHESSI 20-25 keV (purple)

GOES 1-8 A(green)

Neuperteffect

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

F. Farnik, 2001

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

Comments

• The flares that exhibit departures from the Neupert effect or from soft-hard-soft spectral morphology are the most interesting

• There is more non-thermal physics in the corona than is evident from the impulsive (CME acceleration) phase alone

4. Global waves

QuickTime™ and aGIF decompressor

are needed to see this picture.

Thompson et al., Solar Phys. 193, 161, 2000

Hudson et al., submitted 2002

Lessons from global waves

• The Uchida model (weak fast-mode shock, as a blast wave) works well

• The X-rays show the initiation of the disturbance close to the flare core, and we may learn something fundamental about the restructuring from this

5. Extraordinary events

• April 18, 2001: a major X-class flare two days behind the west limb

Lessons from this extraordinary event

• Tail of electron distribution function

(>20 keV) contained >0.2% of the total population

• Non-thermal particles may be the dominant source of gas pressure in a CME interior (speculation!)

Conclusions for FASR - I

• The FASR spectral domain offers the best chance to track the coronal restructuring responsible for flare/CME energy

• Clues to the restructuring may come from global waves

• The FASR frequency agility may be essential for studying the “invisible hand” at work in the restructuring

Conclusions for FASR - II

• The Yohkoh data confirm and extend our view that particle acceleration must be considered as an integral part of the energy release

• Interpretation of FASR will require modeling the evolution of distribution function and geometry self-consistently

• The frequency agility will be a key to success