1. How Extreme Can Solar Events Be ?
1st Varsiti Meeting June 7, Bulgaria　9:30-10:00 invited
How Extreme Can Solar Events Be ?
Kazunari Shibata
Kwasan and Hida Observatories,
Kyoto University, Kyoto, Japan
Collaborators : Hiroyuki Maehara, Takuya Shibayama, Yuta Notsu, Shota Notsu, Satoshi Honda, Daisaku Nogami, Takuya Takahashi, Hiroaki Isobe

2. Carrington flare (1859, Sep 1, am 11:18 ）
　The first flare that human beings observed
by Richard Carrington (England)
　white flare for 5 minutes
　very bright aurora appeared next day morning
at many places on Earth, e.g. Cuba, the Bahamas, Jamaica, El Salvador, and Hawaii.
Largest magnetic storm (> 1000 nT) in recent 200 yrs.
Telegraph systems all over Europe and
North America failed.
Telegraph pylons threw sparks and
telegraph paper spontaneously caught
Fire　（Loomis　1861）

3. Will the Carrington-class flare occur again ?
Can much bigger flares, superflares (>10^33 erg), occur on the Sun at present ?
If yes, what is frequency of superflares ?
Why and how can superflares occur on the Sun ?
To answer these questions
is the subject of my talk.

4. ？ statistics of occurrence frequency of
solar flares, microflares, nanoflares
nanoflare
microflare
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 year
solar flare ？
Largest solar flare
superflare
superflares
C M X X10 X100
C M X X10 X1000 X100000

5. How can we observe superflares on the Sun ?
If empirical statistics rule of solar flares is applied to much larger flares (superflares), then the frequency of superflares with energy 1000 times larger than the largest solar flares might occur once in years.
However, the period of modern observations of the Sun with telescope is only 400 years.
How can we observe the Sun for years ?
If we observe solar type stars (similar to our Sun) for 1 year, we can get the data similar to the data obtained from years observtions of the Sun !
Prof Sekiguchi kindly told me that the Kepler satellite is taking such data !

6. Kepler satellite (NASA)
Space mission to detect exoplanets by observing transit of exoplanets
0.95 m telescope
Observing 160,000 stars continuously (from 2009 to 2013). Among them, are solar type stars.
~30 min time cadence (public data)

7. Superflares on Solar Type Stars : Our study (Maehara et al. 2012)
Hence we searched for superflares on solar type stars using Kepler satellite data, which include data of solar type stars
Since the data are so large, we asked 1st year undergraduate students to help analyzing these stars,
　　because students have a lot of free time (2010 fall)
Surprisingly, we (they) found 365 superflares on 148 solar type stars (G-type main sequence stars)

8. Superflares on Solar type stars
Published in Nature (2012, May)
Superflares on Solar type stars
Undergraduate students
H. Maehara,
T. Shibayama, S. Notsu, Y. Notsu, T. Nagao, S. Kusaba, S. Honda, D. Nogami, K. Shibata

9. typical superflare observed by Kepler
Brightness
of a star
and a flare
Total energy
~ 10^35 erg
Time (day)
Maehara et al. (2012)

10. typical superflare observed by Kepler
What is the cause of stellar brightness variation ?
typical superflare observed by Kepler
Brightness
of a star
and a flare
Total energy
~ 10^36 erg
It is likely due to rotation of a star with a big star spot
Time (day)
Maehara et al. (2012)

11. time Model calculation of stellar brightness variation KIC6034120
model(green)
inclination = 45°
Starspot radius
0.16 R*
Stellar
brightness
2％(平均基準)
5 days
time
Notsu et al

12. time Model calculation of stellar brightness variation KIC6034120
model(green)
inclination = 45°
Starspot radius
0.16 R*
Stellar
brightness
2％(平均基準)
5 days
time
Notsu et al. 2013

13. Hudson argued that Light Curve of superflare stars is different from that of the Sun. Is this True ?
Solanki
Solar Physics (2004)
224: 197–208
60 days (Aug– Sep 2000)
Superflare Star
days

14. Flare energy vs rotational period
Solar Rotation
Fast rotation
(young)
Slow rotation
(old)
Stars with
period longer
than 10 days
cf solar rot period ~ 25days
There is no hot Jupiter in these
superflare stars against previous
prediction (Schaefer+ 2000)
25days
Maehara+(2012),　Notsu+ (2013)

15. Flare energy vs sunspot area
Superflares on solar type stars
Once in 1000
years
10^35 erg X10000
Once in years
10^34 erg X1000
Once in years
10^33 erg X100
Once in 1 year
10^32 erg X10
0.01 su
Sunspot area (in unit of solar surface area)
10 in 1 year
10^31 erg 　 X
100 in 1 year
10^30 erg M
Solar flare
1000 in 1 year
10^29 erg 　 C
Sammis et al.
2000
0.0001
0.001

16. Flare energy vs sunspot area
Superflares on solar type stars
Once in 1000
years
10^35 erg X10000
Once in years
10^34 erg X1000
Once in years
10^33 erg X100
Once in 1 year
10^32 erg X10
0.01
Solar flares su
Sunspot area
(in unit of solar
Surface area)
10 in 1 year
10^31 erg 　 X
100 in 1 year
10^30 erg M
Solar flare
1000 in 1 year
10^29 erg 　 C
Sammis et al.
2000
0.0001
0.001

17. Flare energy vs sunspot area (magnetic flux)?
Stellar flares
Solar flares
Shibata et al. (2013)

18. Indirect Measurement of Rotational Period and Spot Area are True
Indirect Measurement of Rotational Period and Spot Area are True ? => Spectroscopic Observations of Superflare Stars
Notsu et al. (2015) PASJ

19. Projected rotation velocity (v sin i)
We can estimate projected rotation velocity (v sin i) from the Doppler broadening of absorption lines.
Fast rotators ⇒ wide line profile
Slow rotators ⇒ narrow line profile
Line of sight
Rotation
Axis
We explain this in a bit detail in this slide.
We can estimate projected rotation velocity (v sin i) from the broadening of absorption lines in our spectroscopic data.
This figure is an example of our spectroscopic data of superflare stars.
As shown here, fast rotating stars have wide line profiles, while slowly rotaing stars like the Sun have narrow line profiles.
We estimated projected rotation velocity (v sin i) of the target superflare stars in this way,
but we must note here the velocity estimated here is only “projected” rotation velocity
since we cannot know the inclination angle between rotaion axis and line of sight.
※Measurement methods Takeda et al.(2008etc)

20. Rotation Period ⇔Brightness variation period ?
Most of the data points locate below the line of i=90°
⇒“Brightness variation≒Rotation” is OK!!
Edge-on view(sin i =1)
Velocity estimated from brightness
variation period (Vlc[km s-1])
v sin i
[km s-1]
Pole-on view
Line of Sight
Rotation
Axis
Then, in this figure, we compared the projected rotation velocity with the brightness variation of Kepler data.
This figure is a bit complex, so here I try to explain in detail.
The vertical axis of this figure is stellar projected rotation velocity (v sin i), estimated from the line broadening of our spectroscopic data.
The horizontal axis is velocity estimated from the period of the brightness variation by using this equation.
If the brightness variation completely corresponds to the rotation, the values of horizontal axis and vertical axis become equal.
But, some data points in this figure have differences between these two values.
We consider such differences are explained by the inclination angle effect, in other words, the effect of sin I in the vertical axis.
In this figure we plot four lines corresponding to other inclination angle values.
As a result, this figure shows two important results.
First, most of the data points locate below the line of inclination angle I is 90 degree.
In other words, for almost all the stars, v sin i in the vertical axis is less than the velocity in the horizontal axis.
This is consistent with our assumption that the brightness variation is caused by the rotation since the inclination angle effect, in other words, the effect of sin I in the vertical axis can cause the data points located below .the line of i = 90 degree..
 In addition, stars distributed in the lower right side of this figure are expected to have
small inclination angles such as I (inclination)=10 degree, and to be nearly pole-on stars.
As a result of this figure, the rotation velocity from spectroscopic data is consistent with the rotation velocity from the brightness variation period.
This fact supports the assumption that the brightness variation of superflare stars is caused by the rotation.
pole-on view
※Sun: Vrot～2 [km s-1]

21. Flare energy vs. area of starspots
Spectroscopic rotational velocity　(Subaru obs)
Stellar flares
low inclination angle
Photometric rotational velocity
Solar flares
Notsu, Y. et al. (2015)

22. (BigBear Solar Observatory data)
Strong magnetic field area around starspots show strong Ca II emission!!
This figure in the left side is an example of photometric image of the Sun,
and this figure in the right side is a map of Ca II line intensity of the same date.
This figure shows strong magnetic field area around starspots show strong Ca II emission
and reflects Ca II line is a well-known indicator of stellar magnetic activity of solar-type stars.
Then we can indirectly investigate the existence of large starspots by using the intensity of Ca II lines.
The Sun with Ca II K line
The Sun with visible light
(BigBear Solar Observatory data)
We can indirectly investigate the existence of large starspots by using the intensity of Ca II lines.

23. Indirect estimation of starspot coverage with Ca II lines
As the magnetic activity enhanced, the core depth become shallow because of the greater amount of the emission from the chromosphere.
Chromospheric activity ⇒These stars have large starspots !
The core depth becomes shallow.
Large
starspots
Superflare stars
Superflare stars
Superflare stars
18Sco (Solar-twin)
This figure is an example of Ca II 8542 lines of superflare stars and the Sun.
As the magnetic activity enhanced, the core depth become shallow because of the greater amount of the emission from the chromosphere.
The core depth of Ca II lines of superflare stars shown here is shallower than that of the Sun,
and as a result, these stars have large starspots compared to the Sun.

24. Starspot coverage vs Ca II 8542 intensity
↑ Sun
r0 (8542)
r0: Normalized intensity of Ca II 8542
Next, in this figure, we compared Ca II intensity with the brightness variation amplitude of Kepler data.
The vertical axis of this figure is normalized intensity of the line center of Ca II 8542 line.
The upper side value correspond to the existence of large starspots on the basis of spectroscopic data.
The horizontal axis is the amplitude of the brightness variation of Kepler data.
We can see a rough correlation in this figure between Ca II intensity and the amplitude of brightness variation.
If we assume that the brightness variation is caused by the rotation, the brightness variation amplitude in the horizontal axis corresponds to the starspot coverage of these stars.
Then, we can say that there is a rough positive correlation between the starspot coverage on the basis of Kepler data and Ca II intensity in the vertical axis.
(下部表示)
This rough correlation shows us all the target stars that are expected to have large starspots on the basis of their large amplitude of the brightness variation in the horizontal axis also show the existence of large starspots on the basis of spectroscopic data in the vertical axis.
In other words, as for the viewpoint of starspot sizes,
results from Kepler data are consistent with the spectroscopic results.
Brightness Variation Amplitude ≒ starspot coverage
All the stars that are expected to have large starspots (from large brightness variation amplitude) show high (Ca II) magnetic activities.

25. Superflare stars have large spots
Hypothetical image
In visible light
(photosphere)
Hypothetical image
In CaII line
(chromosphere)

26. Comparison of statistics between solar flares/microflares and superflares
Shibata et al. 2013
nanoflare
microflare
solar flare ？
Largest solar flare
superflare
C M X X10 X100

27. Comparison of statistics between solar flares/microflares and superflares
X10000 flare
Shibata et al. 2013
Superflares of 1000 times more
Energetic than the largest solar flares occur once in 5000 years !
nanoflare
microflare
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 year
solar flare
Largest solar flare
superflare
C M X X10 X100
C M X X10 X1000 X100000

28. Comparison of statistics between solar flares/microflares and superflares
X1000 flare
Shibata et al. 2013
Superflares of 100 times more
Energetic than the largest solar flares occur once in 800 years !
nanoflare
microflare
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 year
solar flare
Largest solar flare
superflare
C M X X10 X100
C M X X10 X1000 X100000

29. Comparison of statistics between solar flares/microflares and superflares
X100 flare
Maehara+ 2015
Superflares of 10 times more
Energetic than the largest solar flares occur once in 500 years !
nanoflare
microflare
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 year
solar flare
Largest solar flare
superflare
Maehara
et al. 2015
C M X X10 X100
C M X X10 X1000 X100000

30. Fundamental Question
Why and how can superflares occur on Sun-like stars (i.e., present Sun) ?
Superflares occur because of the presence of large spots.
=>
Why and how can large spots be generated on Sun-like stars (i.e., present Sun) ?

31. What is the spot area statistics ? Teff=5500-6000 K
Many stars without superflares
Show evidence of large spots !
What is the spot area statistics ? Teff= K
Solar Rotation
Amplitude
(in unit of total area of a star)
Spot Area
Maehara+ (2016)
Candelaresi+ (2014)
Period (day)

32. Statistics of Spot Area on the Sun
Courtesy of Ishii (2015)

33. Statistics of Spot Area on the Sun and Sun-like Stars
Large spots
exist in many
Sun-like stars
though frequency
Is small
Maehara+ (2016)

34. Necessary time to generate magnetic flux producing superflares (Shibata et al. 2013)
Why and how can large spots be generated on the present Sun ? (Shibata et al. 2013)
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)
only 8 years (< 11 years) to generate 2x1023 Mx producing superflares of 1034 erg
=> easily
occur !?
Is it possible to store such huge magnetic flux below the base of convection zone ?
=> big challenge to dynamo theorist !

35. Most active Sun-like star
4 superflares in 500 days !
Shibayama
et al 2013
Stellar
Brightness
variation
day
KIC (rot period 15 days)

36. What is Solar/Stellar Cycle dependence of
Flare frequency ?
Shibayama
et al 2013
1024 Mx
1025 Mx
1023 Mx
nanoflare
microflare
Most active
Sun-like stars
solar flare
Solar maximum
2001
Largest solar flare
superflare
Solar minimum
2008

37. Evidence of a superflare ?
Corresponding to 10^34 erg superflare
If this is due to a solar flare
(Miyake et al. Nature ,
2012, June, 486, 240)

38. Okayama 3.8m New Technology Telescope of Kyoto Univ
Spectroscopic Observations of Solar type stars causing superflares will be extremely important
Okayama 3.8m New Technology Telescope of Kyoto Univ
(under construction)
New Technology
1. Making Mirrors with Grinding
2. Segmented mirror
3. Ultra Light mounting
High speed photometric
and spectroscopic
observation of
Transient objects
Gamma ray bursts
Exoplanets
Stellar flares (superflares)
Budget for operation
Is still lacking.
Please support us !
Will be completed ~ 2017
courtesy of Prof. Nagata (Department of Astronomy , Kyoto University)

39. Summary
Using Kepler data, we found 365 superflares (10^33-10^36 erg) on 148 solar type stars (among stars) during 120 days (Maehara+ 2012). => 1547 superflares from 279 solar type stars during 500 days (Shibayama+ 2013).
Superflares occur on Sun-like stars ( K and slow rotation) with frequency such that superflares with energy 10^33-10^35 erg (X100-X solar flare) occur once in years
These stars have large star spot (Notsu+ 2013).
Rotational velocity and large star spot of 50 superflare stars has been confirmed by spectroscopic observations (Notsu+ 2015)
Hence we cannot reject the possibility that superflares of 10^33 – 10^35 erg would occur once in 500 – 5000 years on the present Sun (Shibata+ 2013, Nogami+ 2014, Maehara+ 2015)
Evidence of superflares on the Sun years ago ? (Miyake+ 2012, 13)

40. Thank you for your attention