Chromospheric flares in the modern era

H. HudsonSpace Sciences Lab, UC Berkeley

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Chronology of flare physics

• 19th century. The photosphere (Carrington; Trouvelot)

• Early 20th century. The chromosphere (spectroscopic observations, H)

• Current. The corona (X-rays, CMEs). Major theoretical ideas formulated

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Hirayama 1974

Reference model of coronal magnetic reconnection

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Three areas of current interest for chromospheric flares

1. Energetics: Can the chromosphere play a significant role in the energetics of a flare? No

2. Footpoints & Ribbons: The impulsive-phase excitation of the lower solar atmosphere remains opaque (to us)

3. Structure: Can the chromosphere provide clues to the important physics? Yes; Kopp & Poletto (1984) et seq

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Topic 1: Energetics

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Can the chromospheric provide the energy for a solar flare?

• Mass 1016 g• Gravitational energy 1031 ergs (to infinity)• Gravitational energy 1029 ergs (2” altitude)• Magnetic energy 1030 ergs• Ionization energy 1029 ergs• Thermal energy 1029 ergs• Energy in flows 1026 ergs

No. There is insufficient energyor stress in the chromosphere to power a major flare

Enough for a CME…

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Distribution of magnetic energy in the corona

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Topic 2: Ribbons and footpoints

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How do we understand the flaring chromosphere?

• Observationally, via imaging spectroscopy

• Theoretically, in steps (don’t need to pay attention if you heard Carlsson’s talk)

- Structure in a stratified atmosphere

- Non-LTE radiative transfer

- Fluid approximations (“radiation hydrodynamics”)

- Fluid approximation for the fields (MHD)

- Correct plasma microphysics

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Flare gradual phase

• Accepted scenario of radiative/conductive cooling of coronal loops with initially high gas pressures

• Ablation (“evaporation”) of the chromosphere directly observed by XUV imaging spectroscopy

• A lot of the physics is well developed

Kane & Anderson, 1970

• Long-Decay Event (LDE)• The Neupert effect• Loop prominence systems

Bruzek 1964

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Flare impulsive phase

• The chromospheric and UV observations show intense intermittent excitation of unresolved features

• The existing imaging spectroscopy is totally inadequate

• The physics is completely unclear

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The “thick target” model

• The ionospheric response showed us that the lower solar atmosphere - the UV source - dominated flare energetics

• After (2006 - 1859 = 147 years), we still don’t know how the Sun concentrates so much energy in such a trivial layer

• Hence the “thick target” model of particle beams

Najita & Orrall, 1971 Svestka, 1970

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Model spectra of WL/UV continuum(Fletcher et al. 2006)

TRACE 1700

TRACE WL

Allred et al. 2005

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The biggest problem in flare chromospheres is understanding the visible/UV spectrum formed in the impulsive phase

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Topic 3: Structure

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Intermittency: consecutive TRACE images

24 July 2004, C4.832 x 68 arc s frames

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• Flare radiant energy appears in the form of compact, intense patches

• The high-energy footpoint excitation moves rapidly, with a crossing time of order <30 sec (Schrijver et al. 2006)

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M9.1 flare, time in sec,TRACE WL X10 flare, 1.56 (Xu et al., 2004)

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What is not understood?• How the bulk of flare energy can be focused into these tiny

chromospheric structures

• How important flare effects can appear as deep as the opacity minimum

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Coronal restructuring inferred from footpoint behavior (Kopp & Poletto 1984 et seq.)

Fletcher & Hudson 2001

• Footpoint motions measure magnetic flux transfer rate • Hard X-rays identify site of energy release

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A modern “chromosphere” problem:511 keV line formation (e+ + e- -> 2)

Share et al., ApJ 615, L169 (2004)

A column depth of a few gm/cm2 at transition-regiontemperatures, stable for many tens of seconds, appearsto be required

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Conclusions

• The concept of a static chromosphere is inappropriate for flare conditions

• Concentration of flare energy into

compact flare elements is puzzling

• There are many theoretical issues

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De Pontieu et al., 2003

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A chromospheric contribution to the problem of flare energy?

• Gravitational potential energy in partially ionized flux tubes can stress the coronal field (energy buildup)

• Downward motions can provide energy (flare energy release)

• “Dipped” flux tubes should happen naturally near sunspots and in regions of flux emergence

• There is little relevant theoretical work at present

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Does the chromosphere have enough gravitationalpotential energy to make a flare?

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NoCalculation based on Maltby et al. (1986) model E says…

Sturrock et al. (1996)

Chromospheric massstresses the field, drivingcoronal current systems*

*poorly illustrated here

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Loop-top WL emission seen by TRACE

Event of 2002 Nov. 12, 17:58 UT

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WL versus UV (1700A)

TRACE WL WL difference 1700 A

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Kane & Donnelly, ApJ 154, 171 (1971)