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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 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 corona observed by Yohkoh
Solar Corona Observed By Yohkoh Soft X-ray
telescope(1keV)
Coronal
plasma 2MK-10MK
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)
Remaining basic questions on reconnection
• Triggering mechanism ?
• What determines the reconnection rate ?
• What is the particle acceleration mechanism ?
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
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
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
Remaining basic questions on reconnection
• Triggering mechanism ?
• What determines the reconnection rate ?
• What is the particle acceleration mechanism ?
New Particle Acceleration Mechanism in Solar Flares (Nishizuka and Shibata (2013)
Phys. Rev. Lett. 110, 051101)
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
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
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
Maehara et al. (2011)
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
Maehara et al. (2011)
superflare
nanoflare
microflare
solar flare
Comparison of statistics between solar flares/microflares and superflares
?
Largest solar flare
superflare
nanoflare
microflare
solar flare
Comparison of statistics between solar flares/microflares and superflares
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 C M X X10 X1000 X100000
Largest solar flare
superflare
nanoflare
microflare
solar flare
Comparison of statistics between solar flares/microflares and superflares
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 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
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)
Flare energy vs sunspot area (magnetic flux)
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
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
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
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
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 !
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*
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