Physics of Solar and Stellar Flares

Physics of Solar and Stellar Flares

Kazunari Shibata

Kwasan and Hida Observatories,

Kyoto University

2013 May 22, Leiden, Netherlands

Cf) Shibata and Magara (2011) Living Reviews in Solar Physics Shibata et al. (2013) Publ. Astron. Soc. Japan, in press thanks to Isobe, Hillier, Choudhuri, Maehara, Shibayama, Notsu^2, Nagao, Kusaba, Honda, Nogami

contents

• Solar Flares

– Unifield view from observations

– Plasmoid-induced-Reconnection Model as a Unified Model

• Stellar Flares in EM-T diagram

– Unification of Solar and Stellar Flares

• Superflares on Solar type stars

• Can Superflares Occur on Our Sun ?

Solar flares

Solar flare

Discovered in 19c Explosive energy release That occur near sunspot

magnetic energy is the source of energy

Size ~ 109 – 1010 cm Time scale ~ 1min – 1hour Total energy ~ 1029 - 1032erg

Mechanism has been puzzling since 19c until recently

Hida Obs/Kyoto U.

Ｈα

Solar prominence eruption （biggest in the history , June 4, 1946, HAO)

Solar corona observed by Yohkoh

Solar Corona Observed By Yohkoh Soft X-ray

telescope（1keV)

Coronal

plasma 2MK-10MK

Simultaneous Halpha and X-ray view of a flare Magnetic

reconneciton Hα X-ray

Plasmoid (flux rope) ejections

impulsive flares ~ 10^9 cm

LDE(Long Duration Event) flares ~ 10^10 cm

CMEs (Giant arcades) ~ 10^11 cm

Unified model

Plasmoid-Induced-Reconnection (Shibata 1999)

• Yohkoh/SXT discovered X-ray jet (Shibata et al. 1992, Strong et al. 1992, Shimojo et al. 1996)

Jets from microflares

Anemone (Shibata et al. 1994)

Summary of “flare/CME” observations with Yohkoh

“flares” Size (L) Lifetime

(t)

Alfvén

time (tA)

t/tA

Mass

ejection

microflares 103 -

104 km

100-

1000sec

1-10 sec ~100 jet/surge

Impulsive

flares

(1-3) x

104 km

10 min –

1 hr

10-30

sec

~60-100 X-ray

plasmoid/

Spray

Long duration

(LDE) flares (3-10)x

104 km

1-10 hr 30-100

sec

~100-300 X-ray

plasmoid/

prom.

eruption

Giant

arcades

105 -

106 km

10 hr – 2

days

100-1000

sec

~100-300 CME/prom.

eruption

Unified model (plasmoid-induced

reconnection model) (Shibata 1996, 1999)

(a,b)： giant arcades, LDE/impulsive flares, CMEs

(c,d) ：impulsive flares, microflares, jets

Energy release rate＝ 2

222

2

410

4LV

BLV

B

dt

dEAin

Hinode obs (Shimojo et al. 2007)

Hinode observations of chromospheric anemone jets as evidence of ubiquitous reconnection (nanoflares)

(Shibata et al. 2007, Science)

superflare

nanoflare microflare

solar flare

statistics of occurrence frequency of solar flares, microflares, nanoflares

1000 in 1 year 100 in 1 year 10 in 1 year 1 in 1 year 1 in 10 year 1 in 100 year 1 in 1000 year 1 in 10000 year

C M X X10 X1000 X100000

？

Largest solar flare

superflares

Remaining basic questions on reconnection

• Triggering mechanism ?

• What determines the reconnection rate ?

• What is the particle acceleration mechanism ?

Two step reconnection model (Wang-Shi 1993, Chen-Shibata 2000,

Kusano et al. 2004)

Emerging flux

reconnection (cancellation) associated with emerging flux

sudden decrease in magnetic tension

expansion of flux rope more energetic reconnection

Chen-Shibata (2000) model - emerging flux triggering

• Feynmann and

Martin (1995) • Shiota et al. (2003, 2005) • Nishida et al.

(2008) • Toroek and

Schmieder (2008) • Nagashima et al.

(2007) • Kusano (2012)

(see also Lin et al. 2001)

Basic difficulty of solar reconnection (flare or coronal heating)

Diffusion time (current dissipation time)

Flare time Alfven time Magnetic Reynolds number

scmK

T

sK

T

cm

LLt

Spitzer

D

/10

10

101010/

2

2/3

6

4

2/3

6

2

9

142

st flare

32 1010

sVLt AA 10/

1/ ADm ttR

Due to Electron-ion Coulomb collision

simple Joule dissipation cannot work In the solar atmosphere

Sweet-Parker model Petschek model (1957,58) (1964)

A

mArec

t

Rtt

10010

log

A

mArec

t

Rtt

106

2/1

1010

/LVR Am

Slow reconnection Fast reconnection Sweet-Parker model with unkonwn large resistivity ? Petschek model with extremely small diffusion region ? Or other reconnection model ?

Basic Puzzle of Reconnection

1. What determines the Reconnection Rate ?

Recent magnetospheric observations and collisionless plasma theory suggest that fast reconnection occurs if the current sheet thickness becomes comparable with ion Larmor radius or ion inertial length (either with anomalous resistivity or collisionless conductivity)

Huge gap between micro and macro scale in solar flares

• Ion inertial length

• Ion Larmor radius

• Mean free path

• Flare size

2/1

310,10

300

cm

ncm

c

pi

ionin

2/1

6

1

1010100

K

T

G

Bcm

eB

vcmr iLi

1

310

2

6

7

2

2 101010

1

cm

n

K

Tcm

e

kT

nmfp

cmrflare

910

Basic Puzzle of Reconnection

1. What determines the Reconnection Rate ?

Recent magnetospheric observations and collisionless plasma theory suggest that fast reconnection occurs if the current sheet thickness becomes comparable with ion Larmor radius or ion inertial length (either with anomalous resistivity or collisionless conductivity)

2. How can we reach such small scale to lead to anomalous resistivity or collisionless reconnection in solar flares ?

A Hint from solar observations

Plasmoid-Induced-Reconnection And Fractal Reconnection

Shibata and Tanuma (2001) Earth, Planets, and Space

Shibata and Tanuma (2001) Earth, Planet, Space 53, 473

Citation History of Shibata and Tanuma (2001) paper “Plasmoid-Induced-Reconnection and

Fractal Reconnection”

Plasmoid (flux rope) ejections

impulsive flares ~ 10^9 cm

LDE(Long Duration Event) flares ~ 10^10 cm

CMEs (Giant arcades) ~ 10^11 cm

Unified model

Plasmoid-Induced-Reconnection (Shibata 1999)

Ohyama & Shibata (1997)

1. Plasmoid starts to be ejected long before the impulsive phase.

2. The plasmoid acceleration occurred during impulsive phase.

(Ohyama and Shibata 1997)

X ray Observations of An impulsive flare

Height of plasmoid

time

HXR intensity

CME (coronal mass ejection) acceleration during rise phase (Zhang et al. 2001)

Laboratory experiment (Ono et al. 2000 )

What is the Role of

Plasmoid Ejections ?

(Shibata-Tanuma

2001)

MHD simulations show plasmoid-induced reconnection

in a fractal current sheet (Tanuma et al. 2001, Shibata and Tanuma 2001)

Tanuma et al. (2001)

Vin/VA

plasmoid

Reconnection rate

time

Observation of hard X-rays and microwave emissions show fractal-like time variability, which may be a result of fractal

plasmoid ejections

(Ohki 1992)

(Tajima-Shibata 1997)

Benz and Aschwanden 1989 Zelenyi 1996, Karlicky 2004 Aschwanden 2002

This fractal structure enable to connect micro and macro scale structures and dynamics

Fractal current sheet Lazarian and Vishniac 1999

tearing cascade in reconnection

Bárta, Büchner & Karlický 2009, 2010, 2011

Related recent studies: Loureiro 2007, Huang-Bhattacharjee 2010, Uzdenski et al. 2010, Dawson 2011, Mozer and Bellan 2012 associated particle acceleration (Drake et al 2006, Nishizuka and Shibata 2012)

NIshida

New Particle Acceleration Mechanism in Solar Flares (Nishizuka and Shibata (2013)

Phys. Rev. Lett. 110, 051101)

Nishizuka and Shibata (2013) Phys. Rev. Lett. 110, 051101

Stellar Flares

Observations of Stellar Flares Solar Flare X-ray Intensity (3-24keV)

time

10 hours

Stellar Flare of Prox Cen

Protostellar Flare of YLW15

time

X-ray Intensity (~ 1 keV)

time

X-ray Intensity (2-10 keV)

Can stellar and protostellar flares be explained by magnetic reconnection

mechanism ?

• Yes !

• Indirect evidence has been found in empirical correlation between

Emission Measure ( )

and Temperature

(Shibata and Yokoyama 1999, 2002)

32LnEM

Emission Measure （ＥＭ＝ｎ２Ｖ） of Solar and Stellar Flares increases with Temperature （Ｔ）

（n：electron density、Ｖ： volume）(Feldman et al. 1995)

ＥＭ－Ｔ relation of Solar and Stellar Flares

Log-log plot

of Feldman

etal (1995)’s

figure

Shibata and Yokoyama, 1999, 2002

ＥＭ－Ｔ relation of Solar and Stellar Flares

microflare

(Shimizu 1995)

Shibata and Yokoyama, 1999, 2002

young-star and protostellar flares

Tsuboi (1998) Pallavicini (2001)

Koyama (1996)

Class I protostar

Shibata and Yokoyama (1999) ApJ 526, L49

2D MHD Simulation of Reconnection with Heat

Conduction and Chromospheric Evaporation

7/27/6 LBT

Yokoyama and Shibata (1998) ApJ 494, L113 ------------------------------- (2001) ApJ 549, 1160

Flare Temperature Scaling Law

• Reconnection heating＝conduction cooling （Yokoyama and Shibata 1998)

LTVB A 2/4/ 2/72

7/27/6 LBT

• Emission Measure

• Dynamical equilibrium (evaporated plasma must be confined in a loop)

• Using Flare Temperature scaling law, we have

Flare Emission Measure (Shibata and Yokoyama 1999)

32LnEM

2/175TBEM

8/2 2BnkT

EM-T correlation for solar/stellar flares

Shibata and Yokoyama (1999, 2002)

Magnetic field strength（Ｂ）＝constant

Magnetic field strengths of solar and stellar flares are comparable ~ 50-100 G

2/175TBEM

Shibata and Yokoyama (1999, 2002)

Total energy of stellar flares

Superflares Their energy = 10-10^6 times that of the largest solar flares

Solar flares

Solar microflares

Stellar flares

Q: What determines the total energy of flares ? A: It is the loop length.

The reason why stellar flares are hot => loop lengths of stellar flares are large

Shibata and Yokoyama (2002)

Cf Isobe et al. 2003, Kawamiti et al. 2008

reconnection model of protostellar flare

and jets （Hayashi, Shibata, Matsumoto 1996)

Why young stars produce superflares ?

• Answer: because young stars are fast rotators.

T-Tauri

(Pallavicini et al. 1981）

Is there a possibility that superflares would occur on our present Sun ?

Immediate Answer: Superflares would not occur on our present Sun because the Sun is not young so is slow rotator

But amazingly, Schaefer et al. (2000) discovered 9 superflares on ordinary solar type stars with slow rotation, though they concluded superflares would not occur on our Sun, because there is no hot Jupiter in our Sun.

Is this true ? Are superflares really occurring on solar type stars with slow rotation ?

Our study (Maehara et al. 2012)

• we searched for superflares on solar type stars using Kepler satellite data

• Surprisingly, we found 365 superflares on 148 solar type stars (G-type main sequence stars)

Superflares on Solar type stars

H. Maehara, T. Shibayama, S. Notsu, Y. Notsu, T. Nagao, S.

Kusaba, S. Honda, D. Nogami, K. Shibata

Published in Nature (2012, May)

Kepler satellite

• Space mission to detect exoplanets by observing transit of exoplanets

• 0.95 m telescope

• Observing 150,000 stars for a few months

• ~30 min time cadence (public data)

typical superflare observed by Kepler

Brightness of a star and a flare

Time (day)

Total energy ~ 10^35 erg

Maehara et al. (2011)

typical superflare observed by Kepler

Brightness of a star and a flare

Time (day)

Total energy ~ 10^36 erg


What is the cause of stellar brightness variation ?

It is likely due to rotation of a star with a big star spot

Flare energy vs rotational period

Stars with period longer than 10 days cf solar rot period ~ 25days

There is no hot Jupiter in these superflare stars


superflare

nanoflare

microflare

solar flare

Comparison of statistics between solar flares/microflares and superflares

？

Largest solar flare

superflare

nanoflare

microflare

solar flare



C M X X10 X1000 X100000 C M X X10 X1000 X100000

Largest solar flare

superflare

nanoflare

microflare

solar flare



C M X X10 X1000 X100000

Largest solar flare

Superflares of 1000 times more Energetic than the largest solar flares occur once in 5000 years !

Most active Sun-like star

7 superflares in 500 days ! Stellar Brightness variation

day

Shibayama Et al 2012

superflare

nanoflare

microflare

solar flare

What is Solar/Stellar Cycle dependence of Flare frequency ?

Largest solar flare

Most active Sun-like stars

Solar minimum 2008

Solar maximum 2001

1023 Mx

1024 Mx 1025 Mx

Can Superflares Occur on Our Sun ?

Shibata et al. (2013) PASJ, in press (June)

1) Big Sunspots are Necessary Condition for Superflares

Mechanism of superflare occurrence

Big starspot is necessary

32

3

32

2/3232

04.0101.0][10

88

R

L

G

Bferg

AB

fLB

ffEE

spot

spotmagflare

ergE

RLIf

flare

spot

3534 1010

,4.02.0

][1010 24232MxBLspot

Basic mechanism of superflare is the same as that of solar flares (i.e. reconnection) because MHD (magneto-hydrodynamics) is scale free

Flare energy vs sunspot area (magnetic flux)

Brightness variation (in unit of average stellar brightness)


2/3

219

2

3

32

2/3232

103101.0][107

88

cm

A

G

Bferg

AB

fLB

ffEE

spot

spotmagflare

Solar flares

Superflares on solar type stars

2) Generation of Magnetic Flux at the Base of the Convection Zone

How to make big star spot ?

pt BBrrotBVrot

t

B

)()(

)/( dzdz

][106.5

2.0

7 Hz

Rotation is fast near equator

Rotation is slow near poles

rot

rrV

2

)(108.22

10)9.24.2(2

3025

6

6

paperourinHz

Hz

days

rot

rot

rot

Δr Δz Rp

Bp

Base of convection zone

e.g., Flux Transport Model (Dikpati, Choudhuri, ,,) Contours: toroidal fields at CZ base Gray-shades: surface radial fields

Observed NSO map of longitude-averaged photospheric fields

Dikpati, de Toma, Gilman, Arge & White, 2004, ApJ, 601, 1136

Solar differential rotation (SOHO/MDI, Schou et al. 1998)

Ω/２π

How to make big star spot ?

pt BBrrotBVrot

t

B

)()(

yearsHzMxMx

t

rRBdt

d

pt

pppt

1

7

1

2224 106.5101040

222

)/( dzdz

][106.5 7 Hz

Rotation is fast near equator

Rotation is slow near poles

Poloidal magnetic flux

MxR

RB polarCHpolarCHp

222

2

101)3.0(310

cmRR

GaussB

polarCH

polarCH

101023.0

10

極磁場の観測

Necessary time to generate magnetic flux producing superflares

yearsHzMxMx

t

rRBdt

d

pt

pppt

1

7

1

2224 106.5101040

222

only 8 years (< 11 years) to generate 2x1023 Mx producing superflares of 1034 erg

The necessary time to generate magnetic flux of 1024 Mx that can produce superflares of 1035 erg are 40 years （<< 5000 years) (but > 11 years)

Is it possible to store such huge magnetic flux below the base of convection zone ? => big challenge to dynamo theorist !

=> easily occur !?

Evidence of superflare ?

Corresponding to 10^35 erg superflare If this is due to a solar flare (Miyake et al. Nature , 2012, June, 486, 240)

Summary

• Recent solar observations show various evidence of reconnection in solar flares, microflares, and nanoflares, leading to a unified model of solar flares.

• Plasmoid ejections are ubiquitous in flares and flare-like events, which may play a key role to induce fast reconnection in a fractal (turbulent) current sheet.

• Stellar flares can also be unified with a reconnection model, if we use the emission measure – temperature diagram.

• Using Kepler data, we found that superflares can occur on Sun-like stars (so on our Sun ?) and the occurrence frequency of the superflare whose energy is 100-1000 times that of the most energetic flare of the Sun is once in 800-5000 years.

• We cannot deny the possibility that superflares would occur on our present Sun !

Backup slides

Okayama 3.8m New Technology Telescope Project of Kyoto Univ

courtesy of Prof. Nagata (Department of Astronomy , Kyoto University)

High speed photometric And spectrometric Observation of transient objects Gamma ray bursts Stellar flares (superflares)

Will be completed ~ 2015

Spectroscopic Observations of Solar type stars causing superflares will be extremely important

New Technology Making Mirrors with Grinding Segmented mirror Ultra Light mounting

Variation Amplitude

10^35 erg X10000

su

10^34 erg X1000

10^33 erg X100

10^32 erg X10

10^31 erg X

10^30 erg M

10^29 erg C

0.0001 0.001

0.01

Flare energy vs sunspot area

Once in 1000 years

Once in 10 years

Once in 100 years

Once in 1 year

10 in 1 year

100 in 1 year

1000 in 1 year

Sunspot area (in unit of solar Surface area)

Solar flare

Superflares on solar type stars

Sammis et al. 2000

Model calculation of stellar brightness variation KIC6034120

2％(

平均基準)

model(green) inclination = 45° Starspot radius 0.16 R*

5 days Notsu et al.

KIC6034120

2％(

平均基準) 5 days

Notsu et al.

Model calculation of stellar brightness variation

model(green) inclination = 45° Starspot radius 0.16 R*

5. Case of Rapidly Rotating Stars

Flare frequency vs rotation speed

• If

SBdt

dp

t

2

dt

dfdynamo

If the flare frequency ( ) is correlated with magnetic flux generation rate, then we can relate the observed flare frequency with the rotation speed

flaref

flaref

Observed superflare frequency vs rotation period

flaref

Activity-Rotation relationship (Guedel 2004 AApRev 12, 71)

Ro = P/tau (Rossby number) P= rotation period Tau = convection turn over time

Physics of Solar and Stellar Flares

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