2 Physical Processes in Solar and Stellar Flares Eric Hilton General Exam March 17th, 2008

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Physical Processes in

Solar and Stellar Flares

Eric Hilton

General Exam

March 17th, 2008

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Outline

• Overview and Flare Observations

• Physical Processes on the Sun– Standard Two-ribbon Model – Magnetic Reconnection– Particle Acceleration

• Stellar Comparison

• Summary

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The Sun

Magnetic loopsTRACE image

Footprints

~ 109 cm

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Flare basics

• Flares are the sudden release of energy, leading to increased emission in most wavelength regimes lasting for minutes to hours.

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Light curves

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Kane et al., 1985

Time

White light

Radio

X-rays

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Moving Footprints

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Moore et. al, 2001

Sigmoid model

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Data of Sigmoid

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Moore et. al, 2001

RHESSI data

attribute

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Where does the energy come from?

A typical Solar flare emits about 1032 ergs total.

The typical size is

L ~ 3x109 cm, H ~ 2x109 cm, leading to V ~ 2x1028 cm3

Thermal energy?

In the chromosphere, the column density, col is ~0.01 g/cm2 and T~ 1x104 K.

In the corona, it’s 3x10-6 g/cm2, 3x106 K

Eth ≈ 3 colkTL2/mH ≈ 2x1029 ergs for chromosphere

≈ 2x1028 ergs for corona.

Not cutting it.

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Nuclear power?

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The corona doesn’t have the temperature or density, unless…

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No, magnetic energy

EB = VB2/(2o) so , for B = 300-1000 G, you’re at 1x1032-33 erg.

Now, how is the energy released quickly enough?

t ~ L2 o ~ 5x1011 seconds for diffusion, way too long

So, do it quickly in a current sheet

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Outline

• Overview and Flare Observations

• Physical Processes on the Sun– Standard Two-ribbon Model– Magnetic Reconnection– Particle Acceleration

• Stellar Comparison

• Summary

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“Magnetic event”(reconnection)

Two-Ribbon Flare Model

Martins & Kuin, 1990

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Flow and impaction of

current sheet

Two-Ribbon Flare Model

Gyrosynchrotron radio emission

Brehmsstrahlung hard X-ray& optical emission

Martins & Kuin, 1990

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Chromospheric

evaporation &

condensation

Two-Ribbon Flare Model

Blue-shifted UV (≈100s km/s)

Red-shifted optical (≈10s km/s)

Gyrosynchrotron radio emission

Martins & Kuin, 1990

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Two-Ribbon Flare ModelSoft X-ray

Optical

Corona become optically

thick

Martins & Kuin, 1990

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Post flare emission(quiescent)

Two-Ribbon Flare Model

Gyrosynchrotron radio emission

Optical

Martins & Kuin, 1990

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This model explains…

• the relationship to CMEs

• the Neupert effect

• Sunquakes

• Radio observations

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Outline

• Overview and Flare Observations

• Physical Processes on the Sun– Standard Two-ribbon Model– Magnetic Reconnection– Particle Acceleration

• Stellar Comparison

• Summary

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Magnetic Reconnection

Sweet-Parker

(1958,1957)

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B-field lines

vinflow

• Material flows in

• v x B gives current into the page

• called a ‘current sheet’ or ‘neutral sheet’

•current dissipation heats the plasma

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Magnetic Reconnection

Sweet-Parker

(1958,1957)

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B-field lines

vinflow

vout

Pressure is higher in the reconnection region, so flows out the ends

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Petschek mechanism

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Priest & Forbes, 2002

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Reconnection Inflow

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Narukage & Shibata, 2006

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Outline

• Overview and Flare Observations

• Physical Processes on the Sun– Standard Two-ribbon Model– Magnetic Reconnection– Particle Acceleration

• Stellar Comparison

• Summary

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Particle acceleration

1. DC from E-fields ~ 103 Vm-1 during reconnection

2. MHD shocks - accelerate more particles more slowly - can explain the main phase

3. Highly turbulent environment may give rise to stochastic acceleration - ie fast-mode Alfven-waves.

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Ion beam• 2.223 MeV is a neutron capture line - ions collide with atmosphere, producing fast neutrons.

• These neutrons thermalize for ~100 sec before being captured by Hydrogen.

• Hydrogen is turned into Deuterium, releasing a -ray

• Time profiles (with 100 sec delay) suggest beams happen at same time.

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Displaced ion and electron beams

Hurford et al.,20062003, Oct 28th flare

4th with measured gamma rays - all showing displacement between - and hard X-rays. This is first to show both footprints

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Ion and electron beam displacement•Possible displacement caused by drift of electrons and ions with different sign of charge. This effect is 2 orders of magnitude too small.

• Currently, it’s not known why there is displacement.

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Gamma-ray movie

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Soft X-Rays Hard X-Rays Gamma Rays

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New model for particle acceleration

Fletcher & Hudson, 2008 (RHESSI Nugget #68, Feb 4th, 2008)

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Outline

• Overview and Flare Observations

• Physical Processes on the Sun– Standard Two-ribbon Model– Magnetic Reconnection– Particle Acceleration

• Stellar Comparison

• Summary

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Stellar comparison

• When we look at a star, we lose all spatial resolution, lots of photons, and continuous monitoring.

• We can’t observe hard X-rays, and only observe limited soft X-rays

• We gain new regimes of temperature, magnetic field generation and configuration, plasma density, etc.

• We can adopt the Solar analogy, but is it valid? What observations can we make?

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Osten et al., 2005

Stellar Flares

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Big stellar flares

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Hawley & Pettersen, 1991

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Flare - quietData courtesy of Marcel Agüeros

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X-ray/microwave ratio

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Benz & Gudel, 1994

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The Sun is not a Flare Star!

• Although some parts of the analogy clearly hold, we would not see flares on the Sun if it were further away.

• Are the flares we see fundamentally different?

• We are biased to detecting only the largest flares, so must be cautious about extrapolating to rates of smaller flares.

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Solar vs. StellarAschwanden, 2007

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Mullan et al.,2006

Magnetic loop lengths

V-I

L/R

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EUVE Flare rates

Audard et al., 2000

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My Thesis

• I will make hundreds of hours of new observations of M dwarfs to determine flare rates

• I am creating model galaxy simulations to predict flare rates on a Galactic scale that includes spectral type and activity level. We can ‘observe’ this model to predict what LSST will see.

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Summary of Solar Flares

• Magnetic loops become entangled by motions of the footprints, storing magnetic energy

• This energy is released through rapid magnetic reconnection that accelerates particles.

• Flares emit in all wavelength regimes.• The general theory is well-established, but

the details continue to be very complex.

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The Sun, in closing

“Coronal dynamics remains an active research area. Details of the eruption process including how magnetic energy is stored, how eruptions onset, and how the stored energy is converted to other forms are still open questions.”

- Cassak, Mullan, & Shaypublished March 3rd, 2008

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Summary of Stellar Flares

• Many aspects of the Solar model seem to be true on stars as well.

• Observations have revealed inconsistencies that have not yet been resolved.

• Flares are the coolest!

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Thanks

• Thanks to my committee, esp. Mihalis for coming all the way from Ireland on St. Patrick’s Day.

• Thanks to my fellow grad students for feedback on my practice talk.

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The End

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Radio Flares Osten et al.,2005

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X-ray flares

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Stats of RHESSI flares

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Su, Gan, & Li, 2006

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Stats con’t

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Su, Gan, & Li, 2006

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Stats con’td

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Su, Gan, & Li, 2006

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Stats con’td

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Su, Gan, & Li, 2006

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Statistical motivation for Avalanche

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Charbonneau et al., 2001

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Downward motion of centroid

Sui, Homan, & Dennis, 2004

6-12keV *0.5

25-50keV

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Disproving nano-flare heating

Aschwanden, 2008

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Reconnection & X-ray flux

Jing et al., 2005

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Correlations

Jing et al., 2005

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CMEs

Jing et al., 2005

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Neupert Effect

Veronig et al., 2005

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Gamma-ray spectroscopy

Smith et al., 2003

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Movie-time

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Loop lengths in active stars

Mullan et al.,2006

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Avalanche

Such models, although a priori far removed from the physics of magnetic reconnection and magnetohydrodynamical evolution of coronal structures, nonetheless reproduce quite well the observed statistical distribution of flare characteristics. - Belanger, Vincent, & Charbonneau, 2007

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The model

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Model results

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Charbonneau et al., 2001

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Model results

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Charbonneau et al., 2001

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SOC-Cascades of Loops

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Hughes, et al.,2003

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Cassak et. al, 2008

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Peak values during the flares

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Benz & Gudel, 1994

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Sunquakes

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More complicated reconnection

• Petschek is just a special case of almost-uniform reconnection

• There are also non-uniform models with separatrix jets.

• In some cases, the sheet tears, and enters the regime of impulsive bursty reconnection

• The 3D models are very complicated 3D MHD.

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More sigmoid

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Moore et. al, 2001

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Types of Magnetic Emission

Flaring: strong, impulsive emission that decays rapidly (minutes to hours), both line and continuum flux may be detected (L ≈ 10-3 - 102 Lbol)

Quiescent: steady emission that persists over long periods, typically line flux only in optical (L ≈ 10-6 - 10-3 Lbol)

Liebert et al. (1999)

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Twisted field lines

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Priest & Forbes, 2002

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Solar to stellar - scaling laws

Aschwanden, 2007

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Veronig et al., 2005