Physics of Solar and Stellar Flares

Transcription

1 ICPP 2016 (Kaohsiung, Taiwan) July 1 (35+5 min) Physics of Solar and Stellar Flares Kazunari Shibata Kwasan and Hida Observatories, Kyoto University, Kyoto, Japan

2 contents Introduction Solar flares - What is the Mechanism? Superflares on Solar Type Stars

3 Special entertainment Kojiki and Universe ( 古事記と宇宙 ) Kitaro and Shibata Let s enjoy Various movies of Solar flares and Eruptions with Kitaro-san s Music Kojiki : Orochi (7 min) Orochi is 8 headed dragon monster

4 What is the Mechanism of Solar Flares? Shibata and Magara (2011) Living Reviews in Solar Physics

5 Simultaneous Halpha and X-ray view of a flare Hα X-ray Magnetic reconneciton

6 Plasmoid ejections are ubiquitous Unified model LDE(Long Duration Event) flares ~ 10^10 cm CMEs(Coronal Mass Ejections) from Giant arcades ~ 10^11 cm impulsive flares ~ 10^9 cm Plasmoid-Induced-Reconnection (Shibata 1999)

7 Jets from very small flares Yohkoh/SXT discovered X-ray jets from microflares (Shibata et al. 1992, Strong et al. 1992, Shimojo et al. 1996) (microflares)

8 Summary of observations of various flares flares Size (L) Lifetime microflares km Impulsive flares Long duration (LDE) flares Giant arcades (1-3) x 10 4 km (3-10)x 10 4 km km (t) sec 10 min 1 hr Alfven time (t A ) t/t A Mass ejection 1-10 sec ~100 jet/surge sec 1-10 hr sec 10 hr 2 days sec t A V A L / V (Alfven B 4 speed) ~ X-ray plasmoid/ Spray ~ X-ray plasmoid/ prom. eruption ~ CME/prom. eruption A

9 Unified model (plasmoid-induced reconnection model) (Shibata 1996, 1999) Plasmoid (a,b): large scale flares, Coronal mass ejections (c,d) :small scale flares, microflares, jets Energy release rate= de dt 2 B V 4 in L B V 4 A L 2

10 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 nanoflare solar flare microflare Largest solar flare superflare

11 Basic Puzzles 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)

12 Huge gap between micro and macro scale in solar flares Ion inertial length Ion Larmor radius Mean free path mfp Flare size 1 n r in, ion Li kt 2 e c mivc eb 2 pi cm n cm 3 1/ B T cm G K 7 cm T 6 10 K r 9 flare 10 cm n cm 1/ 2 3 1

13 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?

14 Plasmoid-Induced-Reconnection and Fractal Reconnection Shibata and Tanuma (2001) Earth, Planet, Space 53, 473 Shibata and Takasao (2016) Fractal Reconnection in Solar and Stellar Environment, In a book Magnetic Reconnection (ed. By Gonzalez and Parker, Springer)

15 Various ways of plasmoid formation in flare current sheets

16 X ray Observations show 1. Plasmoid starts to be ejected long before the impulsive phase. 2. The plasmoid acceleration occurred during impulsve phase. Ohyama & Shibata (1997) Height of plasmoid (Ohyama and Shibata 1997) HXR intensity time

17 Laboratory experiment (Ono et al )

18 What is the Role of Plasmoid Ejections? (Shibata-Tanuma 2001)

19 MHD simulations show plasmoid-induced reconnection in a fractual current sheet (Tanuma et al. 2001, Shibata and Tanuma 2001) plasmoid Reconnection rate Vin/VA See also Bhattacharjee+2009, time Tanuma et al. (2001) Loureiro+2009, Barta+ 2010, Shibayama+ 2016, many

20 Observation of hard X-rays and microwave emissions show fractal-like time variability, which may be a result of fractal plasmoid ejections (Tajima-Shibata 1997) This fractal structure enable to connect micro and macro scale structures and dynamics (Ohki 1992) Aschwanden 2002 Fractal current sheet Lazarian and Vishniac 1998 Benz and Aschwanden 1989 Zelenyi 1996, Karlicky 2004, Barta, Buechner et al Bhattacharjee+2009, Loureiro+2007, Ji and Daughton 2011

21 Multiple plasmoids are ubiquitous in the universe (Ji and Daughton 2011) Lamda = L/rho_s (ion sound gyroradius)

22 Plasmoid speed, acceleration and reconnection rate Ohyama and Shibata 1997 Observations Magara, Shibata MHD simulation Shibata and Tanuma 2001 Analytical model Qiu, Chen+ 2004, Chen,Choe Observations Acceleration Electric field Acceleration Electric field Plasmoid Acceleration ~ Reconnection electric field

23 Remaining Questions What is 3D structure of plasmoid dominated fractal current sheet? What is flare triggering mechanism? How and where particle acceleration occurs?

24 3D MHD simulations (Nishida+ 2013) Emission measure for X-ray images Nishida, Nishizuka, Shibata, 2013 ApJL 775, 39 (400x400x400) density temperature

25 Fragmented Current sheet Current density prominence Current sheet Multiple plasmoids are formed in a current sheet. 3D plasmoid with a finite length. Strong E-field is enhanced between plasmoids. Nishida+ 2013

26 Fractal Reconnection & Particle Acceleration by plasmoids colliding with fast shocks [Nishizuka & Shibata 2013, Phys. Rev. Let.. 110, ] Time slice image of small plasmoid ejections cf) Fermi Acceleration at the fast shock: Somov & Kosugi (1997), Tsuneta & Naito (1998) Shock at the bottom of a large plasmoid 1) Particles are trapped in a plasmoid. 2) Multiple plasmoids collide with fast shock. 3) Particles are reflected due Shock to magnetic at the loop-top mirror effect. 4) Reflection length becomes shorter and shorter. 5) Particles are accelerated by Fermi process, until Plasmoid reflection collide with length becomes comparable to Fast ion shock Larmor (M A ~1.5) radius.

27 Loop top structure: full of shocks MHD simulations by Takasao+ 2015, 2016 ApJ Density

28 Loop top structure: full of shocks MHD simulations by Takasao+ 2015, Takasao-Shibata 2016 ApJ Density Blue: Compressed regions (~ Shocks)

29 Observations of multiple plasmoids in a flare current sheet (Takasao+ 2012)

30 Superflares on Solar Type Stars Maehara et al. (2012) Nature, 483, 478

31 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 nanoflare solar flare microflare Largest solar flare superflare

32 statistics of occurrence frequency of solar flares, microflares, nanoflares nanoflare microflare 1000 in 1 year 100 in 1 year solar flare 10 in 1 year 1 in 1 year Largest solar flare 1 in 10 year superflares 1 in 100 year? 1 in 1000 year 1 in year

33 Questions Previously, it has been believed that the Sun does not produce superflares (> 10^33 erg), because the Sun is old and is slowly rotating. However, Schaefer et al. (2000) discovered 9 superflares on ordinary solar type stars with slow rotation. Schaefer et al. argued that the Sun would never produce superflares, because they believed that hot Jupiter is a necessary condition to produce superflares. Are superflares really occurring on ordinary solar type stars? Are hot Jupiters necessary condition for superflares?

34 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 1 st 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)

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

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

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

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

39 Are these indirect measurement of rotational period correct? => Need spectroscopic obs with Subaru (Notsu+2015) We performed high-dispersion spectroscopy of 50 superflare stars with Subaru telescope (34 are single stars) Photometric periods of each star are consistent with rotation velocities. Notsu et al

40 Comparison of statistics between solar flares/microflares and superflares nanoflare microflare solar flare Largest solar flare superflare?

41 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 year Superflares of 1000 times more Energetic than the largest solar flares occur once in 5000 years! nanoflare solar flare microflare Largest solar flare superflare

42 Stellar Flare duration vs. flare energy τ E 0.39±0.03 Observation: e-folding time of flare (flare energy) 0.39 Flare energy Magnetic energy volume B 2 Indirect observational evidence of Timescale of impulsive phase of flare Alfvén time scale (scale length)/(alfvén velocity) reconnection in stellar flares E L 3 B 2, τ L/v A τ E 1/3 Maehara et al. (2015)

43 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)?

44 d dt t 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) t 40 2 p B 2 2R t p Mx 10 Mx p p 1 r Hz 1 years The necessary time to generate magnetic flux of Mx that can produce superflares of erg are 40 years (<< 5000 years) (but > 11 years) only 8 years (< 11 years) to generate 2x10 23 Mx producing superflares of erg Is it possible to store such huge magnetic flux below the base of convection zone? => big challenge to dynamo theorist! => easily occur!?

45 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 Budget for operation Is still lacking. Please support us! Will be completed ~ 2017 High speed photometric and spectroscopic observation of Transient objects Gamma ray bursts Exoplanets Stellar flares (superflares) courtesy of Prof. Nagata (Department of Astronomy, Kyoto University)

46 Summary Recent observations show unified view of solar flares, prominence eruption, coronal mass ejections, microflares, jets, and nanoflares (Shibata and Magara 2011). Plasmoid-induced-reconnection and fractal reconnection have been proposed (Shibata and Tanuma 2001), which explain coupling between micro and macro plasma dynamics and particle acceleration (Nishizuka and Shibata 2013). Kepler data revealed superflares of 10^34-10^35 erg occur on Sun-like stars with frequency of once in years (Maehara et al. 2012). Hence there is a possibility that superflares of 10^34 10^35 erg might occur on our present Sun with similar frequency (Shibata et al. 2013). => dangerous for our civilization and society Thank you for your attention

47 Backup Slides

48 Stellar Flares

49 Observations of Stellar Flares In 1924, Hertzsprung first noticed a stellar flare in Carina After that, many flares have been observed on M-type red dwarfs (flare stars) using visible light photometric observations (e.g., Luyten (1949), Joy and Humason (1949), see review by Gershberg (2005)). Similar to white light flare on the Sun 10min a flare of AD Leo (Hawley and Petterson 1991)

50 X-ray Observations of Stellar Flares Solar Flare X-ray Intensity (3-24keV) 10 hours time X-ray Intensity (2-10 kev) Protostellar Flare of YLW15 (Monmerle,Tsuboi ) Stellar Flare of Prox Cen (Haisch et al. 1983) X-ray Intensity (~ 1 kev) time time

51 Can stellar flares be explained by magnetic reconnection? Yes! Indirect evidence has been found in empirical correlation between Emission Measure ( EM n 2 L 3 ) and Temperature from soft X-ray obs (Shibata and Yokoyama 1999, 2002)

52 Emission Measure (EM=n 2 V) of Solar and Stellar Flares increases with Temperature (T) (n:electron density V: volume)(feldman et al. 1995) soft X-ray observations

53 EM-T relation of Solar and Stellar Flares Log-log plot of Feldman etal (1995) s figure Shibata and Yokoyama, 1999, 2002

54 EM-T relation of Solar and Stellar Flares microflare (Shimizu 1995) Shibata and Yokoyama, 1999, 2002

55 young-star and protostellar flares Tsuboi (1998) Pallavicini (2001) Koyama (1996) Class I protostar Shibata and Yokoyama (1999) ApJ 526, L49

56 2D MHD Simulation of Reconnection with Heat Conduction and Chromospheric Evaporation T B 6/7 L 2/7 Yokoyama and Shibata (1998) ApJ 494, L (2001) ApJ 549, 1160

57 What determines Flare Temperature? Reconnection heating=conduction cooling (Yokoyama and Shibata 1998) (radiative cooling time is much longer) 2 7/ 2 B V / 4 T / 2L A T B 6/7 L 2/7

58 Flare Emission Measure (Shibata and Yokoyama 1999) Emission Measure EM n 2 L 3 Dynamical equilibrium (evaporated plasma must be confined in a loop) 2nkT B Using Flare Temperature scaling law, we have 2 /8 EM B 5 T 17/ 2

59 EM-T correlation for solar/stellar flares Shibata and Yokoyama (1999, 2002)

60 Magnetic field strength(b)=constant EM B 5 T 17/ 2 Magnetic field strengths of solar and stellar flares are comparable ~ G Shibata and Yokoyama (1999, 2002)

61 Total energy of stellar flares Stellar flares Solar flares Solar microflares Superflares Their energy = 10-10^6 times that of the largest solar flares Their host stars are young stars and binary stars with fast rotation

62 Q: What determines flare total energy? A: loop length (because magnetic field strength is roughly constant) The reason why stellar flares are hot => loop lengths of stellar flares are large Shibata and Yokoyama (2002) Cf Isobe et al. 2003, Aulanier et al. 2013

63 Aged stars (Sun)) Why young stars produce superflares? Answer:young star s rotation is fast (so dynamo is active and total magnetic flux is large => loop is large) Stellar X-ray Luminosity Young stars Young stars Aged stars (Sun) Stellar rotational velocity

64 Relation between flare energy and spot size

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

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

67 Flare energy vs sunspot area (magnetic flux) Stellar flares? E flare fe mag f 2 3 B L 8 [ erg] f 2 B A 8 3/ 2 spot f B G 2 A 310 spot 19 cm 2 3/ 2 Solar flares Shibata et al. (2013)

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

69 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 i Slow rotators narrow line profile Line of sight Rotation Axis Measurement methods Takeda et al.(2008etc)

70 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) v sin i [km s -1 ] Pole-on view i Sun: Vrot~2 [km s -1 ] pole-on view Velocity estimated from brightness variation period (Vlc[km s -1 ]) v lc Line of Sight Rotation Axis 2 R P star

71 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)

72 Strong magnetic field area around starspots show strong Ca II emission!! The Sun with visible light The Sun with Ca II K line (BigBear Solar Observatory data) We can indirectly investigate the existence of large starspots by using the intensity of Ca II lines.

73 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! Superflare stars Superflare stars Superflare stars Large starspots The core depth becomes shallow. 18Sco (Solar-twin)

74 Starspot coverage vs Ca II 8542 intensity r0: Normalized intensity of Ca II 8542 r 0 (8542) Sun 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.

75 NotsuY+ (2015)

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

77 Two Sun-like Superflare Stars Rotating as Slow as the Sun (Nogami et al. 2014) Using spectroscopic observations, the rotational period of two superflare stars (Eflare = 10^34 erg) has been determined as KIC P_rot = 21.8 d KIC P_rot = 25.3 d => very similar to the Sun!

78 Can Superflares Occur on Our Sun? Shibata et al. (2013) PASJ, 65, 49 (theoretical paper)

79 Mechanism of superflare occurrence Basic mechanism of superflare is the same as that of solar flares (i.e. reconnection) because MHD (magnetohydrodynamics) is scale free Big starspot is necessary E flare fe 10 mag 32 If E f [ erg] L 2 B L 8 spot flare 2 BL spot 3 f 2 B A 8 f B G / 2 spot 2 R 10 L 0.04 erg 24, spot R [ Mx] 3

80 How to make big star spot? Rotation is slow near poles Rotation is fast near equator B t t rot ( V B) rot ( r B) B p z( d / dz) [ Hz]

81 d dt t Necessary time to generate magnetic flux producing superflares t 40 2 p B 2 2R t p Mx 10 Mx p p 1 r Hz 1 years The necessary time to generate magnetic flux of Mx that can produce superflares of erg are 40 years (<< 5000 years) (but > 11 years) only 8 years (< 11 years) to generate 2x10 23 Mx producing superflares of erg Is it possible to store such huge magnetic flux below the base of convection zone? => big challenge to dynamo theorist! => easily occur!?

82 Evidence of superflares on the Sun

83 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)

84 Another evidence? AD775 From Miyake et al. (2013) Nature Communications 2783 Another event! AD993 Miyake 2012 Miyake 2013

85 Short time cadense data of superflares observed by Kepler Maehara et al. (2015) Earth, Planets, and Space

86 Superflares (short cadence data) Maehara et al., EPS 67, 59 (2015)

87 Superflares (short cadence data) Maehara et al., EPS 67, 59 (2015)

88 Flare frequency vs. flare energy Maehara et al. (2015) Superflares on Sun-like stars (P>10d, T eff = K) ~ 1 in 800 years ~ 1 in 5000 years ~ 1 in 100 years

89 Flare energy vs. area of starspots Flare energy is consistent with the magnetic energy stored near the starspots. ->Large starspots are necessary. superflares Flares above the line may occur on the stars with low-inclination angle (or stars with polar spots?) E A spot 3/2 Solar flares f=0.1, B=3000G f=0.1, B=1000G Maehara et al. (2015)

90 Stellar Flare duration vs. flare energy τ E 0.39±0.03 Observation: e-folding time of flare (flare energy) 0.39 Flare energy Magnetic energy volume B 2 Indirect observational evidence of Timescale of impulsive phase of flare Alfvén time scale (scale length)/(alfvén velocity) reconnection in stellar flares E L 3 B 2, τ L/v A τ E 1/3 Maehara et al. (2015)

91 Flare Triggering Mechanism

92 Flare Triggering Mechanism Break Out Model (Antiochos) Tether-Cutting Model (Moore) Catastrophe Model (Forbes, Lin) Antiochos et al breakout Moore and Roumeliotis 1992 Tether-cutting Two-step Reconnection Model (Wang-Shi, Chen-Shibata, Kusano)

93 Two step reconnection model (Wang-Shi 1993, Chen-Shibata 2000, Kusano et al. 2012) reconnection (cancellation) associated with emerging flux sudden decrease in magnetic tension expansion of flux rope more energetic reconnection X Emerging flux

94 emerging flux triggering model Feynmann and Martin (1995) --Obs Chen-Shibata (2000) D Lin+ (2001) -- analytic Toroek and Schmieder (2008) 3D Nagashima et al. (2007) -- Obs Kusano+ (2012) 3D

95 Multi-step reconnection as Triggering mechanism of flares Janvier, Kishimoto, Li (2011) PRL Structure Driven Nonlinear Instability in Resistive Double Tearing Mode Extension of Ishii et al. (2002)

96 Other slides

97 Lamda = L/rho_s (ion sound gyroradius)

98

99 Plasmoid-dominated reconnection becomes fast (rec rate ~ 0.01) for S > 10^4 Bhattacharjee et al Loureiro et al. (2012) Phys Plasma 19,

100 abstract