# 2 Physical Processes in Solar and Stellar Flares Eric Hilton General Exam March 17th, 2008.

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2 Physical Processes in Solar and Stellar Flares Eric Hilton General Exam March 17th, 2008

3 Outline Overview and Flare Observations Physical Processes on the Sun –Standard Two-ribbon Model –Magnetic Reconnection –Particle Acceleration Stellar Comparison Summary

4 The Sun

Magnetic loops TRACE image Footprints ~ 10 9 cm

6 Flare basics Flares are the sudden release of energy, leading to increased emission in most wavelength regimes lasting for minutes to hours.

7 Light curves Kane et al., 1985 Time White light Radio X-rays

8 Moving Footprints

9 Moore et. al, 2001 Sigmoid model

10 Data of Sigmoid Moore et. al, 2001 RHESSI data attribute

11

12

13 Where does the energy come from? A typical Solar flare emits about 10 32 ergs total. The typical size is L ~ 3x10 9 cm, H ~ 2x10 9 cm, leading to V ~ 2x10 28 cm 3 Thermal energy? In the chromosphere, the column density,  col is ~0.01 g/cm 2 and T~ 1x10 4 K. In the corona, it’s 3x10 -6 g/cm 2, 3x10 6 K E th ≈ 3  col kTL 2 /m H ≈ 2x10 29 ergs for chromosphere ≈ 2x10 28 ergs for corona. Not cutting it.

14 Nuclear power? The corona doesn’t have the temperature or density, unless…

15 No, magnetic energy E B = VB 2 /(2  o ) so, for B = 300-1000 G, you’re at 1x10 32-33 erg. Now, how is the energy released quickly enough? t ~ L 2  o  ~ 5x10 11 seconds for diffusion, way too long So, do it quickly in a current sheet

16 Outline Overview and Flare Observations Physical Processes on the Sun –Standard Two-ribbon Model –Magnetic Reconnection –Particle Acceleration Stellar Comparison Summary

17 “Magnetic event” (reconnection) Two-Ribbon Flare Model Martins & Kuin, 1990

18 Flow and impaction of current sheet Two-Ribbon Flare Model Gyrosynchrotron radio emission Brehmsstrahlung hard X-ray & optical emission Martins & Kuin, 1990

19 Chromospheric evaporation & condensation Two-Ribbon Flare Model Blue-shifted UV (≈100s km/s) Red-shifted optical (≈10s km/s) Gyrosynchrotron radio emission Martins & Kuin, 1990

20 Two-Ribbon Flare Model Soft X-ray Optical Corona become optically thick Martins & Kuin, 1990

21 Post flare emission (quiescent) Two-Ribbon Flare Model Gyrosynchrotron radio emission Optical Martins & Kuin, 1990

22 This model explains… the relationship to CMEs the Neupert effect Sunquakes Radio observations

23 Outline Overview and Flare Observations Physical Processes on the Sun –Standard Two-ribbon Model –Magnetic Reconnection –Particle Acceleration Stellar Comparison Summary

24 Magnetic Reconnection Sweet-Parker (1958,1957) B-field lines v inflow Material flows in v x B gives current into the page called a ‘current sheet’ or ‘neutral sheet’ current dissipation heats the plasma

25 Magnetic Reconnection Sweet-Parker (1958,1957) B-field lines v inflow v out Pressure is higher in the reconnection region, so flows out the ends

26 Petschek mechanism Priest & Forbes, 2002

27 Reconnection Inflow Narukage & Shibata, 2006

28 Outline Overview and Flare Observations Physical Processes on the Sun –Standard Two-ribbon Model –Magnetic Reconnection –Particle Acceleration Stellar Comparison Summary

29 Particle acceleration 1.DC from E-fields ~ 10 3 Vm -1 during reconnection 2.MHD shocks - accelerate more particles more slowly - can explain the main phase 3.Highly turbulent environment may give rise to stochastic acceleration - ie fast- mode Alfven-waves.

30 Ion beam 2.223 MeV is a neutron capture line - ions collide with atmosphere, producing fast neutrons. These neutrons thermalize for ~100 sec before being captured by Hydrogen. Hydrogen is turned into Deuterium, releasing a  -ray Time profiles (with 100 sec delay) suggest beams happen at same time.

31 Displaced ion and electron beams Hurford et al.,2006 2003, Oct 28th flare 4th with measured gamma rays - all showing displacement between  - and hard X-rays. This is first to show both footprints

32 Ion and electron beam displacement Possible displacement caused by drift of electrons and ions with different sign of charge. This effect is 2 orders of magnitude too small. Currently, it’s not known why there is displacement.

33 Gamma-ray movie Soft X-Rays Hard X-Rays Gamma Rays

34 New model for particle acceleration Fletcher & Hudson, 2008 (RHESSI Nugget #68, Feb 4th, 2008)

35 Outline Overview and Flare Observations Physical Processes on the Sun –Standard Two-ribbon Model –Magnetic Reconnection –Particle Acceleration Stellar Comparison Summary

36 Stellar comparison When we look at a star, we lose all spatial resolution, lots of photons, and continuous monitoring. We can’t observe hard X-rays, and only observe limited soft X-rays We gain new regimes of temperature, magnetic field generation and configuration, plasma density, etc. We can adopt the Solar analogy, but is it valid? What observations can we make?

37 Osten et al., 2005 Stellar Flares

38 Big stellar flares Hawley & Pettersen, 1991

39 Flare - quiet Data courtesy of Marcel Agüeros

40 X-ray/microwave ratio Benz & Gudel, 1994

41 The Sun is not a Flare Star! Although some parts of the analogy clearly hold, we would not see flares on the Sun if it were further away. Are the flares we see fundamentally different? We are biased to detecting only the largest flares, so must be cautious about extrapolating to rates of smaller flares.

42 Solar vs. Stellar Aschwanden, 2007

43 Mullan et al.,2006 Magnetic loop lengths V-I L/R

44 EUVE Flare rates Audard et al., 2000

45 My Thesis I will make hundreds of hours of new observations of M dwarfs to determine flare rates I am creating model galaxy simulations to predict flare rates on a Galactic scale that includes spectral type and activity level. We can ‘observe’ this model to predict what LSST will see.

46 Summary of Solar Flares Magnetic loops become entangled by motions of the footprints, storing magnetic energy This energy is released through rapid magnetic reconnection that accelerates particles. Flares emit in all wavelength regimes. The general theory is well-established, but the details continue to be very complex.

47 The Sun, in closing “Coronal dynamics remains an active research area. Details of the eruption process including how magnetic energy is stored, how eruptions onset, and how the stored energy is converted to other forms are still open questions.” - Cassak, Mullan, & Shay published March 3rd, 2008

48 Summary of Stellar Flares Many aspects of the Solar model seem to be true on stars as well. Observations have revealed inconsistencies that have not yet been resolved. Flares are the coolest!

49 Thanks Thanks to my committee, esp. Mihalis for coming all the way from Ireland on St. Patrick’s Day. Thanks to my fellow grad students for feedback on my practice talk.

50 The End

51 Radio Flares Osten et al.,2005

52 X-ray flares

53 Stats of RHESSI flares Su, Gan, & Li, 2006

54 Stats con’t Su, Gan, & Li, 2006

55 Stats con’td Su, Gan, & Li, 2006

56 Stats con’td Su, Gan, & Li, 2006

57 Statistical motivation for Avalanche Charbonneau et al., 2001

58 Downward motion of centroid Sui, Homan, & Dennis, 2004 6-12keV *0.5 25-50keV

59 Disproving nano-flare heating Aschwanden, 2008

60 Reconnection & X-ray flux Jing et al., 2005

61 Correlations Jing et al., 2005

62 CMEs Jing et al., 2005

63 Neupert Effect Veronig et al., 2005

64 Gamma-ray spectroscopy Smith et al., 2003

65 Movie-time

66

67 Loop lengths in active stars Mullan et al.,2006

68 Avalanche Such models, although a priori far removed from the physics of magnetic reconnection and magnetohydrodynamical evolution of coronal structures, nonetheless reproduce quite well the observed statistical distribution of flare characteristics. - Belanger, Vincent, & Charbonneau, 2007

69 The model

70 Model results Charbonneau et al., 2001

71 Model results Charbonneau et al., 2001

72 SOC-Cascades of Loops Hughes, et al.,2003

73 Cassak et. al, 2008

74 Peak values during the flares Benz & Gudel, 1994

75 Sunquakes

76 More complicated reconnection Petschek is just a special case of almost-uniform reconnection There are also non-uniform models with separatrix jets. In some cases, the sheet tears, and enters the regime of impulsive bursty reconnection The 3D models are very complicated 3D MHD.

77 More sigmoid Moore et. al, 2001

78 Types of Magnetic Emission Flaring: strong, impulsive emission that decays rapidly (minutes to hours), both line and continuum flux may be detected (L ≈ 10 -3 - 10 2 L bol ) Quiescent: steady emission that persists over long periods, typically line flux only in optical (L ≈ 10 -6 - 10 -3 L bol ) Liebert et al. (1999)

79 Twisted field lines Priest & Forbes, 2002

80 Solar to stellar - scaling laws Aschwanden, 2007

81 Veronig et al., 2005

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