2 physical processes in solar and stellar flares eric hilton general exam march 17th, 2008

1

Physical Processes in

Solar and Stellar Flares

Eric Hilton

General Exam

March 17th, 2008

2

Outline

• Overview and Flare Observations

• Physical Processes on the Sun– Standard Two-ribbon Model – Magnetic Reconnection– Particle Acceleration

• Stellar Comparison

• Summary

3

The Sun

Magnetic loopsTRACE image

Footprints

~ 109 cm

5

Flare basics

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

6

Light curves

QuickTime™ and a decompressor

are needed to see this picture.

Kane et al., 1985

Time

White light

Radio

X-rays

7



Moving Footprints

8



Moore et. al, 2001

Sigmoid model

9

Data of Sigmoid



Moore et. al, 2001

RHESSI data

attribute

10

QuickTime™ and aYUV420 codec decompressor


12

Where does the energy come from?

A typical Solar flare emits about 1032 ergs total.

The typical size is

L ~ 3x109 cm, H ~ 2x109 cm, leading to V ~ 2x1028 cm3

Thermal energy?

In the chromosphere, the column density, col is ~0.01 g/cm2 and T~ 1x104 K.

In the corona, it’s 3x10-6 g/cm2, 3x106 K

Eth ≈ 3 colkTL2/mH ≈ 2x1029 ergs for chromosphere

≈ 2x1028 ergs for corona.

Not cutting it.

13

Nuclear power?



The corona doesn’t have the temperature or density, unless…

14

No, magnetic energy

EB = VB2/(2o) so , for B = 300-1000 G, you’re at 1x1032-33 erg.

Now, how is the energy released quickly enough?

t ~ L2 o ~ 5x1011 seconds for diffusion, way too long

So, do it quickly in a current sheet

15

Outline


• Physical Processes on the Sun– Standard Two-ribbon Model– Magnetic Reconnection– Particle Acceleration


• Summary

16

“Magnetic event”(reconnection)

Two-Ribbon Flare Model

Martins & Kuin, 1990

17

Flow and impaction of

current sheet


Gyrosynchrotron radio emission

Brehmsstrahlung hard X-ray& optical emission


18

Chromospheric

evaporation &

condensation


Blue-shifted UV (≈100s km/s)

Red-shifted optical (≈10s km/s)



19

Two-Ribbon Flare ModelSoft X-ray

Optical

Corona become optically

thick


20

Post flare emission(quiescent)



Optical


21

This model explains…

• the relationship to CMEs

• the Neupert effect

• Sunquakes

• Radio observations

22

Outline




• Summary

23

Magnetic Reconnection

Sweet-Parker

(1958,1957)



B-field lines

vinflow

• Material flows in

• v x B gives current into the page

• called a ‘current sheet’ or ‘neutral sheet’

•current dissipation heats the plasma

24

Magnetic Reconnection

Sweet-Parker

(1958,1957)



B-field lines

vinflow

vout

Pressure is higher in the reconnection region, so flows out the ends

25

Petschek mechanism



Priest & Forbes, 2002

26

Reconnection Inflow



Narukage & Shibata, 2006

27

Outline




• Summary

28

Particle acceleration

1. DC from E-fields ~ 103 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.

29

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.

30

Displaced ion and electron beams

Hurford et al.,20062003, Oct 28th flare

4th with measured gamma rays - all showing displacement between - and hard X-rays. This is first to show both footprints

31

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.

32

Gamma-ray movie



Soft X-Rays Hard X-Rays Gamma Rays

33

New model for particle acceleration

Fletcher & Hudson, 2008 (RHESSI Nugget #68, Feb 4th, 2008)

34

Outline




• Summary

35

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?

36

Osten et al., 2005

Stellar Flares

37



Big stellar flares



Hawley & Pettersen, 1991

38

Flare - quietData courtesy of Marcel Agüeros

39

X-ray/microwave ratio



Benz & Gudel, 1994

40

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.

41

Solar vs. StellarAschwanden, 2007

42

Mullan et al.,2006

Magnetic loop lengths

V-I

L/R

43

EUVE Flare rates

Audard et al., 2000

44

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.

45

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.

46

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, & Shaypublished March 3rd, 2008

47

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!

48

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.

49

The End

50

Radio Flares Osten et al.,2005

51

X-ray flares

52

Stats of RHESSI flares



Su, Gan, & Li, 2006

53

Stats con’t



Su, Gan, & Li, 2006

54

Stats con’td



Su, Gan, & Li, 2006

55

Stats con’td



Su, Gan, & Li, 2006

56

Statistical motivation for Avalanche



Charbonneau et al., 2001

57

Downward motion of centroid

Sui, Homan, & Dennis, 2004

6-12keV *0.5

25-50keV

58

Disproving nano-flare heating

Aschwanden, 2008

59

Reconnection & X-ray flux

Jing et al., 2005

60

Correlations

Jing et al., 2005

61

CMEs

Jing et al., 2005

62

Neupert Effect

Veronig et al., 2005

63

Gamma-ray spectroscopy

Smith et al., 2003

64

Movie-time



66

Loop lengths in active stars

Mullan et al.,2006

67

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

68

The model









69

Model results




70

Model results




71

SOC-Cascades of Loops



Hughes, et al.,2003

72

Cassak et. al, 2008

73

Peak values during the flares



Benz & Gudel, 1994

74

Sunquakes

QuickTime™ and aCinepak decompressor


75

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.

76

More sigmoid



Moore et. al, 2001

77

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 - 102 Lbol)

Quiescent: steady emission that persists over long periods, typically line flux only in optical (L ≈ 10-6 - 10-3 Lbol)

Liebert et al. (1999)

78

Twisted field lines



Priest & Forbes, 2002

79

Solar to stellar - scaling laws

Aschwanden, 2007

80

Veronig et al., 2005

2 physical processes in solar and stellar flares eric hilton general exam march 17th, 2008

Documents

plasma slide

sigmoid model slide

current sheet slide

rhessi data attribute

magnetic event reconnection

typical solar flare

magnetic energy e b

post flare emission