Physics of Solar and Stellar Flares

Transcription

1 2013 May 22, Leiden, Netherlands Physics of Solar and Stellar Flares Kazunari Shibata Kwasan and Hida Observatories, Kyoto University 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

2 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?

3 Solar flares

4 Hida Obs/Kyoto U. Solar flare Hα Discovered in 19c Explosive energy release That occur near sunspot magnetic energy is the source of energy Size ~ cm Time scale ~ 1min 1hour Total energy ~ erg Mechanism has been puzzling since 19c until recently

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

6 Solar corona observed by Yohkoh Solar Corona Observed By Yohkoh Soft X-ray telescope (1keV) Coronal plasma 2MK-10MK

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

8 Plasmoid (flux rope) ejections Unified model LDE(Long Duration Event) flares ~ 10^10 cm CMEs (Giant arcades) ~ 10^11 cm impulsive flares ~ 10^9 cm Plasmoid-Induced-Reconnection (Shibata 1999)

9 Jets from microflares Yohkoh/SXT discovered X-ray jet (Shibata et al. 1992, Strong et al. 1992, Shimojo et al. 1996) Anemone (Shibata et al. 1994)

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

11 Unified model (plasmoid-induced reconnection model) (Shibata 1996, 1999) Hinode obs (Shimojo et al. 2007) (a,b): giant arcades, LDE/impulsive flares, CMEs (c,d) :impulsive flares, microflares, jets Energy release rate= de dt 2 B V 4 in L B V 4 A L 2

12 Hinode observations of chromospheric anemone jets as evidence of ubiquitous reconnection (nanoflares) (Shibata et al. 2007, Science)

13 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 year nanoflare solar flare microflare Largest solar flare superflare superflares? C M X X10 X1000 X100000

14 Remaining basic questions on reconnection Triggering mechanism? What determines the reconnection rate? What is the particle acceleration mechanism?

15 Two step reconnection model (Wang-Shi 1993, Chen-Shibata 2000, Kusano et al. 2004) reconnection (cancellation) associated with emerging flux sudden decrease in magnetic tension expansion of flux rope more energetic reconnection Emerging flux

16 Feynmann and Martin (1995) Shiota et al. (2003, 2005) Nishida et al. (2008) Toroek and Schmieder (2008) Nagashima et al. (2007) Kusano (2012) Chen-Shibata (2000) model - emerging flux triggering (see also Lin et al. 2001)

17 Remaining basic questions on reconnection Triggering mechanism? What determines the reconnection rate? What is the particle acceleration mechanism?

18 Basic difficulty of solar reconnection (flare or coronal heating) Diffusion time (current dissipation time) Flare time Alfven time Magnetic Reynolds number t D L 2 / 10 Spitzer t flare ta L/ VA 10 L 9 10 cm 4 T 6 10 K s s 2 T 6 10 K 3/ 2 cm simple Joule dissipation R cannot t / t 1 work m D A In the solar atmosphere 2 / 3/ 2 s s Due to Electron-ion Coulomb collision

19 Sweet-Parker model Petschek model (1957,58) (1964) t rec t A 6 R 1/ 2 m t A m A t rec t A log R t Sweet-Parker model with unkonwn large resistivity? Slow reconnection Fast reconnection Petschek model with extremely small diffusion region? Or other reconnection R model V L?/ m A

20 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)

21 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

22 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?

23 A Hint from solar observations Plasmoid-Induced-Reconnection And Fractal Reconnection Shibata and Tanuma (2001) Earth, Planets, and Space

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

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

26 Plasmoid (flux rope) ejections Unified model LDE(Long Duration Event) flares ~ 10^10 cm CMEs (Giant arcades) ~ 10^11 cm impulsive flares ~ 10^9 cm Plasmoid-Induced-Reconnection (Shibata 1999)

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

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

29 Laboratory experiment (Ono et al )

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

31 MHD simulations show plasmoid-induced reconnection in a fractal current sheet (Tanuma et al. 2001, Shibata and Tanuma 2001) plasmoid Reconnection rate Vin/VA time Tanuma et al. (2001)

32 Observation of hard X-rays and microwave emissions show fractal-like time variability, which may be a result of fractal plasmoid ejections This fractal structure enable to connect micro and macro scale structures and dynamics (Tajima-Shibata 1997) (Ohki 1992) Fractal current sheet Aschwanden 2002 Benz and Aschwanden 1989 Zelenyi 1996, Karlicky 2004 Lazarian and Vishniac 1999

33 tearing cascade in reconnection Bárta, Büchner & Karlický 2009, 2010, 2011 NIshida 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)

34 Remaining basic questions on reconnection Triggering mechanism? What determines the reconnection rate? What is the particle acceleration mechanism?

35 New Particle Acceleration Mechanism in Solar Flares (Nishizuka and Shibata (2013) Phys. Rev. Lett. 110, )

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

37 Stellar Flares

38 Observations of Stellar Flares Solar Flare X-ray Intensity (3-24keV) 10 hours X-ray Intensity (2-10 kev) Protostellar Flare of YLW15 time Stellar Flare of Prox Cen X-ray Intensity (~ 1 kev) time time

39 Can stellar and protostellar flares be explained by magnetic reconnection Yes! mechanism? Indirect evidence has been found in empirical correlation between Emission Measure ( EM n 2 L 3 ) and Temperature (Shibata and Yokoyama 1999, 2002)

40 Emission Measure (EM=n 2 V) of Solar and Stellar Flares increases with Temperature (T) (n:electron density V: volume)(feldman et al. 1995)

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

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

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

44 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

45 Flare Temperature Scaling Law Reconnection heating=conduction cooling (Yokoyama and Shibata 1998) 2 7/ 2 B V / 4 T / 2L A T B 6/7 L 2/7

46 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

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

48 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)

49 Total energy of stellar flares Stellar flares Solar flares Solar microflares Superflares Their energy = 10-10^6 times that of the largest solar flares

50 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

51 reconnection model of protostellar flare and jets (Hayashi, Shibata, Matsumoto 1996)

52 Why young stars produce superflares? Answer: because young stars are fast rotators. T-Tauri (Pallavicini et al. 1981)

53 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?

54 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)

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

56 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)

57 typical superflare observed by Kepler Brightness of a star and a flare Total energy ~ 10^35 erg Time (day) Maehara et al. (2011)

58 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. (2011)

59 Flare energy vs rotational period There is no hot Jupiter in these superflare stars Stars with period longer than 10 days cf solar rot period ~ 25days Maehara et al. (2011)

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

61 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 nanoflare solar flare microflare Largest solar flare superflare C M X X10 X1000 X100000

62 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 C M X X10 X1000 X100000

63 Stellar Brightness variation Most active Sun-like star 7 superflares in 500 days! Shibayama Et al 2012 day

64 What is Solar/Stellar Cycle dependence of Flare frequency? Mx Mx Mx nanoflare Solar maximum 2001 solar flare Solar minimum 2008 microflare Most active Sun-like stars Largest solar flare superflare

65 Can Superflares Occur on Our Sun? Shibata et al. (2013) PASJ, in press (June)

66 1) Big Sunspots are Necessary Condition for Superflares

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

68 Flare energy vs sunspot area (magnetic flux) Brightness variation (in unit of average stellar brightness)

69 E flare fe mag f B 2 L 3 B 2 A 3/ 2 Flare energy spot 8 8 vs sunspot area f B [ erg] G f 2 A 310 spot 19 3/ 2 (magnetic flux) 2 cm Superflares on solar type stars Solar flares

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

71 How to make big star spot? Rotation is slow near poles Rotation is fast near equator rot B t t rot ( V B) rot ( r V r days 2 ( ) 10 rot rot 6 2r rot 6 Hz ( in Hz our paper) B) B z( d / dz) p [ Hz]

72 Bp Rp Δr Δz Base of convection zone

73 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

74 Solar differential rotation (SOHO/MDI, Schou et al. 1998) Ω/2π

75 d dt t t 40 How to make big star spot? B t 2 t p rot B 2 ( V t p Mx 10 Mx p B) 2R p 1 r rot ( r Hz 1 Rotation is slow near poles Rotation is fast near equator years B) B z( d / dz) p 7 [ Hz]

76 Poloidal magnetic flux p B polarch R polarch (0.3R ) Mx B polarch 10 Gauss R polarch 0.3R cm

77 極磁場の観測

78 Necessary time to generate magnetic flux producing superflares d dt t 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!?

79 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)

80 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 times that of the most energetic flare of the Sun is once in years. We cannot deny the possibility that superflares would occur on our present Sun!

81 Backup slides

82 Spectroscopic Observations of Solar type stars causing superflares will be extremely important Okayama 3.8m New Technology Telescope Project of Kyoto Univ New Technology Making Mirrors with Grinding Segmented mirror Ultra Light mounting Will be completed ~ 2015 High speed photometric And spectrometric Observation of transient objects Gamma ray bursts Stellar flares (superflares) courtesy of Prof. Nagata (Department of Astronomy, Kyoto University)

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

84 Flare energy vs sunspot area (magnetic flux)

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

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

87 5. Case of Rapidly Rotating Stars

88 Flare frequency vs rotation speed d dt t 2 B p S If f dynamo d dt f flare f If the flare frequency ( flare ) is correlated with magnetic flux generation rate, then we can relate the observed flare frequency with the rotation speed

89 Observed superflare frequency vs rotation period f flare

90 Activity-Rotation relationship (Guedel 2004 AApRev 12, 71) Ro = P/tau (Rossby number) P= rotation period Tau = convection turn over time